Marrow adiposity recovery after early zoledronic acid treatment of glucocorticoid-induced bone loss in rabbits assessed by magnetic resonance spectroscopy

Α Humanized RANKL Transgenic Mouse Model of Progestin-Induced Mammary Carcinogenesis for Evaluation of Novel Therapeutics

Receptor activator of nuclear factor-κB ligand (RANKL) is critically involved in mammary gland pathophysiology, while its pharmaceutical inhibition is being currently investigated in breast cancer. Herein, we investigated whether the overexpression of human RANKL in transgenic mice affects hormone-induced mammary carcinogenesis, and evaluated the efficacy of anti-RANKL treatments, such as OPG-Fc targeting both human and mouse RANKL or Denosumab against human RANKL. We established novel MPA/DMBA-driven mammary carcinogenesis models in TgRANKL mice that express both human and mouse RANKL, as well as in humanized humTgRANKL mice expressing only human RANKL, and compared them to MPA/DMBA-treated wild-type (WT) mice. Our results show that TgRANKL and WT mice have similar levels of susceptibility to mammary carcinogenesis, while OPG-Fc treatment restored mammary ductal density, and prevented ductal branching and the formation of neoplastic foci in both genotypes. humTgRANKL mice also developed MPA/DMBA-induced tumors with similar incidence and burden to those of WT and TgRANKL mice. The prophylactic treatment of humTgRANKL mice with Denosumab significantly prevented the rate of appearance of mammary tumors from 86.7% to 15.4% and the early stages of carcinogenesis, whereas therapeutic treatment did not lead to any significant attenuation of tumor incidence or tumor burden compared to control mice, suggesting the importance of RANKL primarily in the initial stages of tumorigenesis. Overall, we provide unique genetic tools for investigating the involvement of RANKL in breast carcinogenesis, and allow the preclinical evaluation of novel therapeutics that target hormone-related breast cancers.

mDIXON-Quant technique diagnostic accuracy for assessing bone mineral density in male adult population

BACKGROUND

To investigate the diagnostic efficacy of mDIXON-Quant technique for prediction of bone loss in male adults.

METHODS

One hundred thirty-eight male adults were divided into normal, osteopenia, and osteoporosis groups based on DXA and QCT for the lumbar spine. Differences in mDIXON-Quant parameters [fat fraction (FF) and T2^* value] among three groups, as well as the correlation of mDIXON-Quant parameters and bone mineral density (BMD) were analyzed. The areas under the curves (AUCs) for mDIXON-Quant parameters for prediction of low bone mass were calculated.

RESULTS

According to DXA standard, FF and T2^* value were significantly increased in osteoporosis group compared with normal group (P = 0.012 and P < 0.001). According to QCT standard, FF was significantly increased in osteopenia and osteoporosis groups compared with normal group (both P < 0.001). T2^* values were significantly different among three groups (all P < 0.05). After correction for age and body mass index, FF was negatively correlated with areal BMD and volumetric BMD (r = -0.205 and -0.604, respectively; both P < 0.05), and so was T2^* value (r = -0.324 and -0.444, respectively; both P < 0.05). The AUCs for predicting low bone mass according to DXA and QCT standards were 0.642 and 0.898 for FF, 0.648 and 0.740 for T2^* value, and 0.677 and 0.920 for both combined, respectively.

CONCLUSIONS

FF combined with T2^* value has a better diagnostic efficacy than FF or T2^* value alone in prediction of low bone mass in male adults, which is expected to be a promising MRI method for the screening of bone quality.

TRIAL REGISTRATION

ChiCTR1900024511 (Registered 13-07-2019).

Osteoporosis and Bone Marrow Adipose Tissue

PURPOSE OF REVIEW

This review focuses on the recent findings regarding bone marrow adipose tissue (BMAT) concerning bone health. We summarize the variations in BMAT in relation to age, sex, and skeletal sites, and provide an update on noninvasive imaging techniques to quantify human BMAT. Next, we discuss the role of BMAT in patients with osteoporosis and interventions that affect BMAT.

RECENT FINDINGS

There are wide individual variations with region-specific fluctuation and age- and gender-specific differences in BMAT content and composition. The Bone Marrow Adiposity Society (BMAS) recommendations aim to standardize imaging protocols to increase comparability across studies and sites. Water-fat imaging (WFI) seems an accurate and efficient alternative for spectroscopy (¹H-MRS). Most studies indicate that greater BMAT is associated with lower bone mineral density (BMD) and a higher prevalence of vertebral fractures. The proton density fat fraction (PDFF) and changes in lipid composition have been associated with an increased risk of fractures independently of BMD. Therefore, PDFF and lipid composition could potentially be future imaging biomarkers for assessing fracture risk. Evidence of the inhibitory effect of osteoporosis treatments on BMAT is still limited to a few randomized controlled trials. Moreover, results from the FRAME biopsy sub-study highlight contradictory findings on the effect of the sclerostin antibody romosozumab on BMAT. Further understanding of the role(s) of BMAT will provide insight into the pathogenesis of osteoporosis and may lead to targeted preventive and therapeutic strategies.

Bone Evaluation by High-Resolution Peripheral Quantitative Computed Tomography in Patients With Adrenal Incidentaloma

CONTEXT

Data regarding high-resolution peripheral quantitative computed tomography (HR-pQCT) in patients with adrenal incidentaloma (AI) are unknown.

PURPOSE

To evaluate the areal bone mineral density (aBMD), microstructure, and fractures in patients with nonfunctioning AI (NFAI) and autonomous cortisol secretion (ACS).

METHODS

We evaluated 45 patients with NFAI (1 mg dexamethasone suppression test [DST] ≤1.8 µg/dL) and 30 patients with ACS (1 mg DST 1.9-5.0 µg/dL). aBMD was measured using dual-energy X-ray absorptiometry; vertebral fracture by spine X-ray; and bone geometry, volumetric bone mineral density (vBMD), and microstructure by HR-pQCT.

RESULTS

Patients with ACS showed lower aBMD values at the spine, femoral neck, and radius 33% than those with NFAI. Osteoporosis was frequent in both groups: NFAI (64.9%) and ACS (75%). Parameters at the distal radius by HR-pQCT were decreased in patients with ACS compared to those with NFAI: trabecular vBMD (Tb.vBMD, P = 0.03), inner zone of the trabecular region (Inn.Tb.vBMD, P = 0.01), the bone volume/tissue volume ratio (BV/TV, P = 0.03) and trabecular thickness (P = 0.04). As consequence, a higher ratio of the outer zone of the trabecular region/inner zone vBMD (Meta/Inn.vBMD, P = 0.003) was observed. A correlation between the cortisol levels after 1 mg DST and Meta/Inn.vBMD ratio was found (r = 0.29; P = 0.01). The fracture frequency was 73.7% in patients with ACS vs 55.6% in patients with NFAI (P = 0.24).

CONCLUSION

Our findings point to an association between trabecular bone microarchitectural derangement at the distal radius and ACS. Our data suggest that AI have a negative impact on bone when assessed by HR-pQCT, probably associated to subclinical hypercortisolism.

Fat and bone: the multiperspective analysis of a close relationship

The study of bone has for many years been focused on the study of its mineralized component, and one of the main objects of study as radiology developed as a medical specialty. The assessment has until recently been almost limited to its role as principal component of the scaffolding of the human body. Bone is a very active tissue, in continuous cross-talk with other organs and systems, with functions that are endocrine and paracrine and that have an important involvement in metabolism, ageing and health in general. Bone is also the continent for the bone marrow, in the form of "yellow marrow" (mainly adipocytes) or "red marrow" (hematopoietic cells and adipocytes). Recently, numerous studies have focused on these adipocytes contained in the bone marrow, often referred to as marrow adipose tissue (MAT). Bone marrow adipocytes do not only work as storage tissue, but are also endocrine and paracrine cells, with the potential to contribute to local bone homeostasis and systemic metabolism. Many metabolic disorders (osteoporosis, obesity, diabetes) have a complex and still not well-established relationship with MAT. The development of imaging methods, in particular the development of cross-sectional imaging has helped us to understand how much more laid beyond our classical way to look at bone. The impact on the mineralized component of bone in some cases (e.g., osteoporosis) is well-established, and has been extensively analyzed and quantified through different radiological methods. The application of advanced magnetic resonance techniques has unlocked the possibility to access the detailed study, characterization and quantification of the bone marrow components in a non-invasive way. In this review, we will address what is the evidence on the physiological role of MAT in normal skeletal health (interaction with the other bone components), during the process of normal aging and in the context of some metabolic disorders, highlighting the role that imaging methods play in helping with quantification and diagnosis.

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.

Marrow adiposity and bone: Review of clinical implications

There is growing interest in the relationship between bone marrow fat (BMF) and skeletal health. Progress in clinical studies of BMF and skeletal health has been greatly enhanced by recent technical advances in our ability to measure BMF non-invasively. Magnetic resonance imagery (MRI) with or without spectroscopy is currently the standard technique for evaluating BMF content and composition in humans. This review focuses on clinical studies of marrow fat and its relationship with bone. The amount of marrow fat is associated with bone mineral density (BMD). Several studies have reported a significant negative association between marrow fat content and BMD in both healthy and osteoporotic populations. There may also be a relationship between marrow fat and fracture (mostly vertebral fracture), but data are scarce and further studies are needed. Furthermore, a few studies suggest that a lower proportion of unsaturated lipids in vertebral BMF may be associated with reduced BMD and greater prevalence of fracture. Marrow fat might be influenced by metabolic diseases associated with bone loss and fractures, such as diabetes mellitus, obesity and anorexia nervosa. An intriguing aspect of bariatric (weight loss) surgery is that it induces bone loss and fractures, but with different impacts on marrow fat depending on diabetic status. In daily practice, the usefulness for clinicians of assessing marrow fat using MRI is still limited. However, the perspectives are exciting, particularly in terms of improving the diagnosis and management of osteoporosis. Further studies are needed to better understand the regulators involved in the marrow fat-bone relationship and the links between marrow fat, other fat depots and energy metabolism.

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.

Omentum and bone marrow: how adipocyte-rich organs create tumour microenvironments conducive for metastatic progression

A number of clinical studies have linked adiposity with increased cancer incidence, progression and metastasis, and adipose tissue is now being credited with both systemic and local effects on tumour development and survival. Adipocytes, a major component of benign adipose tissue, represent a significant source of lipids, cytokines and adipokines, and their presence in the tumour microenvironment substantially affects cellular trafficking, signalling and metabolism. Cancers that have a high predisposition to metastasize to the adipocyte-rich host organs are likely to be particularly affected by the presence of adipocytes. Although our understanding of how adipocytes influence tumour progression has grown significantly over the last several years, the mechanisms by which adipocytes regulate the metastatic niche are not well-understood. In this review, we focus on the omentum, a visceral white adipose tissue depot, and the bone, a depot for marrow adipose tissue, as two distinct adipocyte-rich organs that share common characteristic: they are both sites of significant metastatic growth. We highlight major differences in origin and function of each of these adipose depots and reveal potential common characteristics that make them environments that are attractive and conducive to secondary tumour growth. Special attention is given to how omental and marrow adipocytes modulate the tumour microenvironment by promoting angiogenesis, affecting immune cells and altering metabolism to support growth and survival of metastatic cancer cells.

The novel bisphosphonate disodium dihydrogen-4-[(methylthio) phenylthio] methanebisphosphonate increases bone mass in post-ovariectomy rats

The novel bisphosphonate (BP) disodium dihydrogen-4-[(methylthio) phenylthio] methanebisphosphonate (MPMBP) is a non-nitrogen-containing BP with an antioxidant side chain that possesses anti-inflammatory properties. We investigated the systemic effects of this compound on bone loss induced by ovariectomy (OVX) in adult rats. Micro-computed tomography revealed that MPMBP increased bone mass and density in both the metaphysis and diaphysis, and improved the structural properties important for mechanical strength of osteoporotic bone. Sequential bone labeling with tetracycline and calcein indicated that MPMBP decreased longitudinal growth of the primary spongiosa (PS), but stimulated cortical bone formation in the diaphysis. MPMBP increased type I collagen accumulation in the PS, and decreased the number and size of adipocytes in the bone marrow, suggesting inhibition of increased bone marrow adipogenesis induced by OVX. Furthermore, MPMBP reduced the number of bone resorbing cathepsin K-positive osteoclasts induced by OVX. These results suggest that MPMBP could improve bone loss induced by estrogen deficiency. Both stimulation of bone formation and inhibition of bone resorption might play a role in the increase in bone mass and bone density after MPMBP treatment.

Vanadate Impedes Adipogenesis in Mesenchymal Stem Cells Derived from Different Depots within Bone

Glucocorticoid-induced osteoporosis (GIO) is associated with an increase in bone marrow adiposity, which skews the differentiation of mesenchymal stem cell (MSC) progenitors away from osteoblastogenesis and toward adipogenesis. We have previously found that vanadate, a non-specific protein tyrosine phosphatase inhibitor, prevents GIO in rats, but it was unclear whether vanadate directly influenced adipogenesis in bone-derived MSCs. For the present study, we investigated the effect of vanadate on adipogenesis in primary rat MSCs derived from bone marrow (bmMSCs) and from the proximal end of the femur (pfMSCs). By passage 3 after isolation, both cell populations expressed the MSC cell surface markers CD90 and CD106, but not the hematopoietic marker CD45. However, although variable, expression of the fibroblast marker CD26 was higher in pfMSCs than in bmMSCs. Differentiation studies using osteogenic and adipogenic induction media (OM and AM, respectively) demonstrated that pfMSCs rapidly accumulated lipid droplets within 1 week of exposure to AM, while bmMSCs isolated from the same femur only formed lipid droplets after 3 weeks of AM treatment. Conversely, pfMSCs exposed to OM produced mineralized extracellular matrix (ECM) after 3 weeks, compared to 1 week for OM-treated bmMSCs. Vanadate (10 μM) added to AM resulted in a significant reduction in AM-induced intracellular lipid accumulation and expression of adipogenic gene markers (PPARγ2, aP2, adipsin) in both pfMSCs and bmMSCs. Pharmacological concentrations of glucocorticoids (1 μM) alone did not induce lipid accumulation in either bmMSCs or pfMSCs, but resulted in significant cell death in pfMSCs. Our findings demonstrate the existence of at least two fundamentally different MSC depots within the femur and highlights the presence of MSCs capable of rapid adipogenesis within the proximal femur, an area prone to osteoporotic fractures. In addition, our results suggest that the increased bone marrow adiposity observed in GIO may not be solely due to direct effect of glucocorticoids on bone-derived MSCs, and that an increase in femur lipid content may also arise from increased adipogenesis in MSCs residing outside of the bone marrow niche.

MicroRNA-29a mitigates glucocorticoid induction of bone loss and fatty marrow by rescuing Runx2 acetylation

Glucocorticoid treatment reportedly increases the morbidity of osteoporotic or osteonecrotic disorders. Exacerbated bone acquisition and escalated marrow adipogenesis are prominent pathological features of glucocorticoid-mediated skeletal disorders. MicroRNAs reportedly modulate tissue metabolism and remodeling. This study was undertaken to investigate the biological roles of microRNA-29a (miR-29a) in skeletal and fat metabolism in the pathogenesis of glucocorticoid-induced osteoporosis. Transgenic mice overexpressing miR-29a precursor or wild-type mice were given methylprednisolone. Bone mass, microarchitecture and histology were assessed by dual energy X-ray absorptiometry, μCT and histomorphometry. Differential gene expression and signaling components were delineated by quantitative RT-PCR and immunoblotting. Glucocorticoid treatment accelerated bone loss and marrow fat accumulation in association with decreased miR-29a expression. The miR-29a transgenic mice had high bone mineral density, trabecular microarchitecture and cortical thickness. miR-29a overexpression mitigated the glucocorticoid-induced impediment of bone mass, skeletal microstructure integrity and mineralization reaction and attenuated fatty marrow histopathology. Ex vivo, miR-29a increased osteogenic differentiation capacity and alleviated the glucocorticoid-induced promotion of adipocyte formation in primary bone-marrow mesenchymal progenitor cell cultures. Through inhibition of histone deacetylase 4 (HDAC4) expression, miR-29a restored acetylated Runx2 and β-catenin abundances and reduced RANKL, leptin and glucocorticoid receptor expression in glucocorticoid-mediated osteoporosis bone tissues. Taken together, glucocorticoid suppression of miR-29a signaling disturbed the balances between osteogenic and adipogenic activities, and thereby interrupted bone formation and skeletal homeostasis. miR-29a inhibition of HDAC4 stabilized the acetylation state of Runx2 and β-catenin that ameliorated the detrimental effects of glucocorticoid on mineralization and lipogenesis reactions in bone tissue microenvironments. This study highlighted emerging skeletal-anabolic actions of miR-29a signaling in the progression of glucocorticoid-induced bone tissue destruction. Sustaining miR-29a actions is beneficial in protecting against glucocorticoid-mediated osteoporosis.

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.

The Role of Bone Marrow Fat in Skeletal Health: Usefulness and Perspectives for Clinicians

CONTEXT

There is growing interest in the relationship between bone marrow fat (BMF), bone mineral density (BMD), and fractures. Moreover, BMF might be influenced by metabolic diseases associated with bone loss and fractures, such as type 2 diabetes mellitus (T2DM), anorexia nervosa (AN), and obesity.

METHODS

The primary-source literature for this review was acquired using a PubMed search for articles published between January 2000 and April 2015. Search terms included BMF, BMD, fractures, T2DM, AN, and obesity. The titles and abstracts of all articles were reviewed for relevant subjects.

RESULTS

Magnetic resonance imaging, with or without spectroscopy, was used to noninvasively quantify BMF in humans. A negative relationship was found between BMD and BMF in both healthy and osteopenic/osteoporotic populations. Data are lacking on the relationship between BMF and fractures. Studies in populations of individuals with metabolic diseases such as T2DM, AN, and obesity have shown BMF abnormalities.

CONCLUSIONS

We conclude that most human data demonstrate an inverse relationship between BMF and BMD, but data on the relationship with fractures are inconsistent and need further study. In daily practice, the usefulness for clinicians of assessing BMF using magnetic resonance imaging is still limited. However, the perspectives are exciting, particularly in terms of improving the diagnosis and management of osteoporosis.

Longitudinal assessment of oxytocin efficacy on bone and bone marrow fat masses in a rabbit osteoporosis model through 3.0-T magnetic resonance spectroscopy and micro-CT

This study aims to longitudinally assess the effect of oxytocin on bone and bone fat masses using micro-CT, in vivo magnetic resonance spectroscopy (MRS), and histopathological adipocyte quantification. Early in vivo oxytocin (OT) treatment to the osteoporosis (OP) rabbit model may reliably inhibit bone degeneration and reduce bone marrow fat accumulation by decreasing marrow adipocyte size and density.

INTRODUCTION

This study aims to longitudinally assess the effect of early OT treatment on bone and bone fat masses in a rabbit OP model by comparing the results of MRS and micro-CT with histopathological findings.

METHODS

Sixty 20-week-old female rabbits were randomly assigned into three groups. The control and OP groups were subjected to either sham surgery or bilateral ovariectomy (OVX). The OT group was subcutaneously injected with OT daily from the second week after OVX for 8 weeks. The left proximal femurs of the rabbits were evaluated through MRS, micro-CT, and histopathological examination at 0, 4, 8, 10, and 12 weeks after operation. Differences in fat fraction (FF) values, micro-CT parameters, and calculated pathological marrow adipocytes among three groups were analyzed.

RESULTS

The FF values of the OP group significantly increased (p = 0.019), but the tissue mineral density (TMD) decreased (p = 0.037) from eighth week compared with those of the control group. The FF values of the OT group significantly decreased (p = 0.044), but the TMD values increased (p = 0.042) from eighth week compared with those of the OP group. The adypocyte diameter of the OT group significantly decreased (p = 0.041) from eighth week and then adypocyte density did so too from tenth week, compared with those of the OP group at the same time point. No difference in adypocyte calculation was found between the OT and control groups until the 12th week after operation.

CONCLUSION

Early in vivo OT treatment slowed down bone deterioration and reduced bone marrow adiposity accumulation in a rabbit OP model, which is consistent with pathologic findings. OT treatment is a promising preventive OP therapy.

The bone-fat interface: basic and clinical implications of marrow adiposity

Obesity and osteoporosis are two of the most common chronic disorders of the 21st century. Both are accompanied by significant morbidity. The only place in the mammalian organism where bone and fat lie adjacent to each other is in the bone marrow. Marrow adipose tissue is a dynamic depot that probably exists as both constitutive and regulated compartments. Adipocytes secrete cytokines and adipokines that either stimulate or inhibit adjacent osteoblasts. The relationship of marrow adipose tissue to other fat depots is complex and might play very distinct parts in modulation of metabolic homoeostasis, haemopoiesis, and osteogenesis. Understanding of the relationship between bone and fat cells that arise from the same progenitor within the bone marrow niche provides insight into the pathophysiology of age-related osteoporosis, diabetes, and obesity.

Influence of early zoledronic acid administration on bone marrow fat in ovariectomized rats

Although the primary target cell of bisphosphonates is the osteoclast, increasing attention is being given to other effector cells influenced by bisphosphonates, such as osteoblasts and marrow adipocytes. Early zoledronic acid (ZA) treatment to ovariectomized (OVX) rats has been found to fully preserve bone microarchitecture over time. However, little is known regarding the influence of ZA on marrow adipogenesis. The purpose of this study was to monitor the ability of early administration of ZA in restoring marrow adiposity in an estrogen-deficient rat model. Thirty female Sprague-Dawley rats were randomly divided into sham-operated (SHAM), OVX + vehicle, and OVX + ZA groups (n=10/group). Dual-energy x-ray absorptiometry and water/fat magnetic resonance imaging were performed at baseline, 6 weeks, and 12 weeks after treatment to assess bone mineral density and marrow fat fraction. Serum biochemical markers, bone remodeling, and marrow adipocyte parameters were analyzed using biochemistry, histomorphometry, and histopathology, respectively. The expression levels of osteoblast, adipocyte, and osteoclast-related genes in bone marrow were assessed using RT-PCR. The OVX rats showed marked bone loss, first detected at 12 weeks, but estrogen deficiency resulted in a remarked increase in marrow fat fraction, first detected at 6 weeks compared with the SHAM rats (all P < .001). Similarly, the OVX rats had a substantially larger percent adipocyte area (+163.0%), mean diameter (+29.5%), and higher density (+57.3%) relative to the SHAM rats. Bone histomorphometry, levels of osteoclast-related gene expression, and a serum resorption marker confirmed that ZA significantly suppressed bone resorption activities. Furthermore, ZA treatment returned adipocyte-related gene expression and marrow adipocyte parameters toward SHAM levels. These data suggest that a single dose of early ZA treatment acts to reverse marrow adipogenesis occurring during estrogen deficiency, which may contribute to its capacity to reduce bone loss.

Determinants of bone marrow adiposity: the modulation of peroxisome proliferator-activated receptor-γ2 activity as a central mechanism

Although the presence of adipocytes in the bone marrow is a normal physiological phenomenon, the role of these cells in bone homeostasis and during pathological states has not yet been fully delineated. As osteoblasts and adipocytes originate from a common progenitor, with an inverse relationship existing between osteoblastogenesis and adipogenesis, bone marrow adiposity often negatively correlates with osteoblast number and bone mineral density. Bone adiposity can be affected by several physiological and pathophysiological factors, with abnormal, elevated marrow fat resulting in a pathological state. This review focuses on the regulation of bone adiposity by physiological factors, including aging, mechanical loading and growth factor expression, as well as the pathophysiological factors, including diseases such as anorexia nervosa and dyslipidemia, and pharmacological agents such as thiazolidinediones and statins. Although these factors regulate bone marrow adiposity via a plethora of different intracellular signaling pathways, these diverse pathways often converge on the modulation of the expression and/or activity of the pro-adipogenic transcription factor peroxisome proliferator-activated receptor (PPAR)-γ2, suggesting that any factor that affects PPAR-γ2 may have an impact on the fat content of bone.