Bone and fat: a relationship of different shades

Bone Marrow Adipocytes as Novel Regulators of Metabolic Homeostasis: Clinical Consequences of Bone Marrow Adiposity

PURPOSE OF REVIEW

Bone marrow adipose tissue is a distinctive fat depot located within the skeleton, with the potential to influence both local and systemic metabolic processes. Although significant strides have been made in understanding bone marrow adipose tissue over the past decade, many questions remain regarding their precise lineage and functional roles.

RECENT FINDINGS

Recent studies have highlighted bone marrow adipose tissue's involvement in continuous cross-talk with other organs and systems, exerting both endocrine and paracrine functions that play a crucial role in metabolic homeostasis, skeletal remodeling, hematopoiesis, and the progression of bone metastases. The advancement of imaging techniques, particularly cross-sectional imaging, has profoundly expanded our understanding of the complexities beyond the traditional view of bone marrow adipose tissue as an inert depot. Notably, marrow adipocytes are anatomically and functionally distinct from brown, beige, and classic white adipocytes. Emerging evidence suggests that bone marrow adipocytes, bone marrow adipose tissue originate from the differentiation of bone marrow mesenchymal stromal cells; however, they appear to be a heterogeneous population with varying metabolic profiles, lipid compositions, secretory properties, and functional responses depending on their specific location within the bone marrow. This review provides an up-to-date synthesis of current knowledge on bone marrow adipocytes, emphasizing the relationships between bone marrow adipogenesis and factors such as aging, osteoporosis, obesity, and bone marrow tumors or metastases, thereby elucidating the mechanisms underlying musculoskeletal pathophysiology.

Bone Marrow Adipose Tissue Is Not Required for Reconstitution of the Immune System Following Irradiation in Male Mice

Bone marrow adipose tissue (BMAT) is hypothesized to serve as an expandable/contractible fat depot which functions, in part, to minimize energy requirements for sustaining optimal hematopoiesis. We investigated whether BMAT is required for immune reconstitution following injury. Male wild type (WBB6F1, WT) and BMAT-deficient WBB6F1/J-KitW/KitW-v/J (KitW/W-v) mice were lethally irradiated. Irradiation was followed by adoptive transfer of 1000 purified WT hematopoietic stem cells (HSCs). The extent of immune reconstitution in blood, bone marrow, and lymph nodes in the irradiated mice was determined using HSCs from green fluorescent protein (GFP)-expressing mice. We also evaluated skeletal response to treatment. Detection of GFP-positive B and T cells in peripheral blood at 4 and 9 weeks following adoptive transfer and in bone marrow and lymph nodes following necropsy revealed excellent immune reconstitution in both WT and BMAT-deficient mice. Adipocytes were numerous in the distal femur of WT mice but absent or rare in KitW/W-v mice. Bone parameters, including length, mass, density, bone volume, microarchitecture, and turnover balance, exhibited few differences between WT and BMAT-deficient mice. The minimal differences suggest that BMAT is not required for reconstitution of the immune system following lethal radiation and is not a major contributor to the skeletal phenotypes of kit signaling-deficient mice.

Regulation of base excision repair during adipogenesis and osteogenesis of bone marrow-derived mesenchymal stem cells

Bone marrow-derived human mesenchymal stem cells (hMSCs) can differentiate into various lineages, such as chondrocytes, adipocytes, osteoblasts, and neuronal lineages. It has been shown that the high-efficiency DNA-repair capacity of hMSCs is decreased during their differentiation. However, the underlying its mechanism during adipogenesis and osteogenesis is unknown. Herein, we investigated how alkyl-damage repair is modulated during adipogenic and osteogenic differentiation, especially focusing on the base excision repair (BER) pathway. Response to an alkylation agent was assessed via quantification of the double-strand break (DSB) foci and activities of BER-related enzymes during differentiation in hMSCs. Adipocytes showed high resistance against methyl methanesulfonate (MMS)-induced alkyl damage, whereas osteoblasts were more sensitive than hMSCs. During the differentiation, activities, and protein levels of uracil-DNA glycosylase were found to be regulated. In addition, ligation-related proteins, such as X-ray repair cross-complementing protein 1 (XRCC1) and DNA polymerase β, were upregulated in adipocytes, whereas their levels and recruitment declined during osteogenesis. These modulations of BER enzyme activity during differentiation influenced DNA repair efficiency and the accumulation of DSBs as repair intermediates in the nucleus. Taken together, we suggest that BER enzymatic activity is regulated in adipogenic and osteogenic differentiation and these alterations in the BER pathway led to different responses to alkyl damage from those in hMSCs.

Global and local fat effects on bone mass and quality in obesity

Aims

The aim of this study was to investigate the global and local impact of fat on bone in obesity by using the diet-induced obese (DIO) mouse model.

Methods

In this study, we generated a diet-induced mouse model of obesity to conduct lipidomic and 3D imaging assessments of bone marrow fat, and evaluated the correlated bone adaptation indices and bone mechanical properties.

Results

Our results indicated that bone mass was reduced and bone mechanical properties were impaired in DIO mice. Lipidomic sequencing and bioinformatic analysis identified 373 differential lipids, 176 of which were upregulated and 197 downregulated. Functional enrichment analysis revealed a significant downregulation of the pathways: fat digestion and absorption (ko04975) and lipolysis regulation in adipocytes (ko04923) in DIO mice, leading to local fat accumulation. The use of 3D imaging confirmed the increase in fat accumulation within the bone marrow cavity of obese mice.

Conclusion

Our study sheds light on the intricate interplay between fat and bone, and provides a non-toxic and non-invasive method for measuring marrow adipose tissue.

Reconstitution of the host holobiont in germ-free born male rats acutely increases bone growth and affects marrow cellular content

Integration of microbiota in a host begins at birth and progresses during adolescence, forming a multidirectional system of physiological interactions. Here, we present an instantaneous effect of natural, bacterial gut colonization on the acceleration of longitudinal and radial bone growth in germ-free born, 7-wk-old male rats. Changes in bone mass and structure were analyzed after 10 days following the onset of colonization through cohousing with conventional rats and revealed unprecedented acceleration of bone accrual in cortical and trabecular compartments, increased bone tissue mineral density, improved proliferation and hypertrophy of growth plate chondrocytes, bone lengthening, and preferential deposition of periosteal bone in the tibia diaphysis. In addition, the number of small in size adipocytes increased, whereas the number of megakaryocytes decreased, in the bone marrow of conventionalized germ-free rats indicating that not only bone mass but also bone marrow environment is under control of gut microbiota signaling. The changes in bone status paralleled with a positive shift in microbiota composition toward short-chain fatty acids (SCFA)-producing microbes and a considerable increase in cecal SCFA concentrations, specifically butyrate. Furthermore, reconstitution of the host holobiont increased hepatic expression of IGF-1 and its circulating levels. Elevated serum levels of 25-hydroxy vitamin D and alkaline phosphatase pointed toward an active process of bone formation. The acute stimulatory effect on bone growth occurred independently of body mass increase. Overall, the presented model of conventionalized germ-free rats could be used to study microbiota-based therapeutics for combatting dysbiosis-related bone disorders.

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.

Single cell transcriptomics identifies a unique adipose lineage cell population that regulates bone marrow environment

Bone marrow mesenchymal lineage cells are a heterogeneous cell population involved in bone homeostasis and diseases such as osteoporosis. While it is long postulated that they originate from mesenchymal stem cells, the true identity of progenitors and their in vivo bifurcated differentiation routes into osteoblasts and adipocytes remain poorly understood. Here, by employing large scale single cell transcriptome analysis, we computationally defined mesenchymal progenitors at different stages and delineated their bi-lineage differentiation paths in young, adult and aging mice. One identified subpopulation is a unique cell type that expresses adipocyte markers but contains no lipid droplets. As non-proliferative precursors for adipocytes, they exist abundantly as pericytes and stromal cells that form a ubiquitous 3D network inside the marrow cavity. Functionally they play critical roles in maintaining marrow vasculature and suppressing bone formation. Therefore, we name them marrow adipogenic lineage precursors (MALPs) and conclude that they are a newly identified component of marrow adipose tissue.

Serum lipidomics for exploring biomarkers of bortezomib therapy in patients with multiple myeloma

Although the proteasome inhibitor bortezomib (BTZ) shows excellent efficacy in multiple myeloma (MM), a fraction of patients has a suboptimal or no response to this agent. In addition, BTZ-induced peripheral neuropathy (BiPN), a frequent side-effect of this therapy, limits its use in some patients. This study aimed to explore serum lipid biomarker candidates to predict the response to BTZ and the severity of BiPN. Fifty-nine serum samples were collected from patients with MM prior to receiving BTZ plus low-dose dexamethasone therapy. Serum levels of phospholipids, sphingolipids, neutral lipids, and polyunsaturated fatty acids and their oxidation products were measured by a comprehensive lipidomic study. Overall, 385 lipid metabolites were identified in patients' sera; lower levels of several glycerophospholipids, sphingolipids, and cholesteryl esters were associated with a poor treatment response. Metabolites related to platelet-activating factor biosynthesis and cholesterol metabolism appeared particularly relevant. Furthermore, several lysophosphatidylcholines, phosphatidylcholines, ceramides, neutral lipids, and oxidative fatty acids were significantly increased or decreased in patients with BiPN grades ranging from G0 to G3. Among these compounds, mediators reportedly inducing myelin breakdown and stimulating inflammatory responses were prominent. Although further study is necessary to validate these biomarker candidates, our results contribute to the development of predictive biomarkers for response to BTZ treatment, or ensuing severe BiPN, in patients with MM.

Bone Marrow Fat Physiology in Relation to Skeletal Metabolism and Cardiometabolic Disease Risk in Children With Cerebral Palsy

Individuals with cerebral palsy exhibit neuromuscular complications and low physical activity levels. Adults with cerebral palsy exhibit a high prevalence of chronic diseases, which is associated with musculoskeletal deficits. Children with cerebral palsy have poor musculoskeletal accretion accompanied by excess bone marrow fat, which may lead to weaker bones. Mechanistic studies to determine the role of bone marrow fat on skeletal growth and maintenance and how it relates to systemic energy metabolism among individuals with cerebral palsy are lacking. In this review, we highlight the skeletal status in children with cerebral palsy and analyze the existing literature on the interactions among bone marrow fat, skeletal health, and cardiometabolic disease risk in the general population. Clinically vital questions are proposed, including the following: (1) Is the bone marrow fat in children with cerebral palsy metabolically distinct from typically developing children in terms of its lipid and inflammatory composition? (2) Does the bone marrow fat suppress skeletal acquisition? (3) Or, does it accelerate chronic disease development in children with cerebral palsy? (4) If so, what are the mechanisms? In conclusion, although inadequate mechanical loading may initiate poor skeletal development, subsequent expansion of bone marrow fat may further impede skeletal acquisition and increase cardiometabolic disease risk in those with cerebral palsy.

Multiple Myeloma and Fatty Acid Metabolism

Multiple myeloma (MM) accounts for 13% to 15% of all blood cancers1 and is characterized by the proliferation of malignant cells within the bone marrow (BM). Despite important advances in treatment, most patients become refractory and relapse with the disease. As MM tumors grow in the BM, they disrupt hematopoiesis, create monoclonal protein spikes in the blood, initiate systemic organ and immune system shutdown,2 and induce painful osteolytic lesions caused by overactive osteoclasts and inhibited osteoblasts.3, 4 MM cells are also extremely dependent on the BM niche, and targeting the BM niche has been clinically transformative for inhibiting the positive-feedback "vicious cycle" between MM cells and osteoclasts that leads to bone resorption and tumor proliferation.5, 6, 7, 8 Bone marrow adipocytes (BMAs) are dynamic, secretory cells that have complex effects on osteoblasts and tumor cells, but their role in modifying the MM cell phenotype is relatively unexplored.9, 10, 11, 12, 13 Given their active endocrine function, capacity for direct cell-cell communication, correlation with aging and obesity (both MM risk factors), potential roles in bone disease, and physical proximity to MM cells, it appears that BMAs support MM cells.14, 15, 16, 17 This supposition is based on research from many laboratories, including our own. Therapeutically targeting the BMA may prove to be equally transformative in the clinic if the pathways through which BMAs affect MM cells can be determined. In this review, we discuss the potential for BMAs to provide free fatty acids to myeloma cells to support their growth and evolution. We highlight certain proteins in MM cells responsible for fatty acid uptake and oxidation and discuss the potential for therapeutically targeting fatty acid metabolism or BMAs from where they may be derived. © 2019 The Authors. JBMR Plus published by Wiley Periodicals, Inc. on behalf of American Society for Bone and Mineral Research.

Lack of Adipocytes Alters Hematopoiesis in Lipodystrophic Mice

Adult hematopoiesis takes place in the perivascular zone of the bone cavity, where endothelial cells, mesenchymal stromal/stem cells and their derivatives such as osteoblasts are key components of bone marrow (BM) niches. Defining the contribution of BM adipocytes to the hematopoietic stem cell niche remains controversial. While an excess of medullar adiposity is generally considered deleterious for hematopoiesis, an active role for adipocytes in shaping the niche has also been proposed. We thus investigated the consequences of total adipocyte deletion, including in the BM niche, on adult hematopoiesis using mice carrying a constitutive deletion of the gene coding for the nuclear receptor peroxisome proliferator-activated receptor-γ (PPARγ). We show that Pparg ^Δ/Δ lipodystrophic mice exhibit severe extramedullary hematopoiesis (EMH), which we found to be non-cell autonomous, as it is reproduced when wild-type donor BM cells are transferred into Pparg ^Δ/Δ recipients. This phenotype is not due to a specific alteration linked to Pparg deletion, such as chronic inflammation, since it is also found in AZIP^tg/+ mice, another lipodystrophic mouse model with normal PPARγ expression, that display only very moderate levels of inflammation. In both models, the lack of adipocytes alters subpopulations of both myeloid and lymphoid cells. The CXCL12/CXCR4 axis in the BM is also dysregulated in an adipocyte deprived environment supporting the hypothesis that adipocytes are required for normal hematopoietic stem cell mobilization or retention. Altogether, these data suggest an important role for adipocytes, and possibly for the molecular interactions they provide within the BM, in maintaining the appropriate microenvironment for hematopoietic homeostasis.

The adipose organ and multiple myeloma: Impact of adipokines on tumor growth and potential sites for therapeutic intervention

In addition to its capacity to store lipids the adipose tissue is now identified as a real organ with both endocrine and metabolic roles. Preclinical results indicate that modifying adipose tissue and bone marrow adipose tissue (BMAT) could be a successful multiple myeloma (MM) therapy. BMAT interrelates with bone marrow cells and other immune cells, and may influence MM disease progression. The BM adipocytes may have a role in MM progression, bone homing, chemoresistance, and relapse, due to local endocrine, paracrine, or metabolic factors. BM adipocytes isolated from MM subjects have been shown to increase myeloma growth in vitro and may preserve cells from chemotherapy-induced apoptosis. By producing free fatty acids and emitting signaling molecules such as growth factors and adipokines, BM adipocytes are both an energy font and an endocrine signaling factory. This review should suggest future research approaches toward developing novel treatments to target MM by targeting BMAT and its products.

The effects of Exendin-4 on bone marrow-derived mesenchymal cells

PURPOSE

GLP-1 receptor agonists are antidiabetic drugs currently used in the therapy of type 2 diabetes. Despite several in vitro and in vivo animal studies suggesting a beneficial effect of GLP-1 analogues on bone, in humans their skeletal effects are not clear and clinical studies report conflicting results.

METHODS

We differentiated human mesenchymal stromal cells (hMSC) toward the adipogenic and the osteoblastic lineages, analysing the effect of Exendin-4 (EXE) before, during and after specific differentiations.

RESULTS

We showed EXE ability to act selectively on a sub-population of hMSC characterised by a more stem potential, shifting them from G1 to S/M phase of cell cycle. We observed that EXE pre-treatment promotes both adipogenic and osteoblastic differentiations, possibly determined by an increased number of uncommitted progenitors. In fully differentiated cells, EXE affects mature adipocytes by increasing lipolysis, otherwise not altering osteoblasts metabolic activity. Moreover, the increased expression of osteoprotegerin, a modulator of the RANK/RANKL system, observed during osteogenic induction in presence of EXE, could negatively modulate osteoclastogenesis.

CONCLUSIONS

Our data suggest a complex action of EXE on bone, targeting the proliferation of mesenchymal progenitors, the metabolism of mature adipocytes and the modulation of osteoclastogenesis. Thus, an overall positive effect of this molecule on bone quality might be hypothesised.

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.

Metabolic Coupling Between Bone Marrow Adipose Tissue and Hematopoiesis

PURPOSE OF REVIEW

Mesenchymal stem cells (MSCs) located in the bone marrow have the capacity to differentiate into multiple cell lineages, including osteoblast and adipocyte. Adipocyte density within marrow is inversely associated with bone mass during aging and in some pathological conditions, contributing to the prevailing view that marrow adipocytes play a largely negative role in bone metabolism. However, a negative association between marrow adipocytes and bone balance is not universal. Although MAT levels appear tightly regulated, establishing the precise physiological significance of MAT has proven elusive. Here, we review recent literature aimed at delineating the function of MAT.

RECENT FINDINGS

An important physiological function of MAT may be to provide an expandable/contractible fat depot, which is critical for minimization of energy requirements for sustaining optimal hematopoiesis. Because the energy requirements for storing fat are negligible compared to those required to maintain hematopoiesis, even small reductions in hematopoietic tissue volume to match a reduced requirement for hematopoiesis could represent an important reduction in energy cost. Such a physiological function would require tight coupling between hematopoietic stem cells and MSCs to regulate the balance between MAT and hematopoiesis. Kit-ligand, an important regulator of proliferation, differentiation, and survival of hematopoietic cells, may function as a prototypic factor coupling MAT and hematopoiesis. Crosstalk between hematopoietic and mesenchymal cells in the bone marrow may contribute to establishing the balance between MAT levels and hematopoiesis.

IKKβ is a β-catenin kinase that regulates mesenchymal stem cell differentiation

Mesenchymal stem cells (MSCs) can give rise to both adipocytes and osteoblasts, but the molecular mechanisms underlying MSC fate determination remain poorly understood. IκB kinase β (IKKβ), a central coordinator of inflammation and immune responses through activation of NF-κB, has been implicated as a critical molecular link between obesity and metabolic disorders. Here, we show that IKKβ can reciprocally regulate adipocyte and osteoblast differentiation of murine and human MSCs through an NF-κB-independent mechanism. IKKβ is a β-catenin kinase that phosphorylates the conserved degron motif of β-catenin to prime it for β-TrCP-mediated ubiquitination and degradation, thereby increasing adipogenesis and inhibiting osteogenesis in MSCs. Animal studies demonstrated that deficiency of IKKβ in BM mesenchymal stromal cells increased bone mass and decreased BM adipocyte formation in adult mice. In humans, IKKβ expression in adipose tissue was also positively associated with increased adiposity and elevated β-catenin phosphorylation. These findings suggest IKKβ as a key molecular switch that regulates MSC fate, and they provide potentially novel mechanistic insights into the understanding of the cross-regulation between the evolutionarily conserved IKKβ and Wnt/β-catenin signaling pathways. The IKKβ-Wnt axis we uncovered may also have important implications for development, homeostasis, and disease pathogenesis.

Exercise Decreases Marrow Adipose Tissue Through ß-Oxidation in Obese Running Mice

The relationship between marrow adipose tissue (MAT) and bone health is poorly understood. We used running exercise to ask whether obesity-associated MAT can be attenuated via exercise and whether this correlates with gains in bone quantity and quality. C57BL/6 mice were divided into diet-induced obesity (DIO, n = 14) versus low-fat diet (LFD, n = 14). After 3 months, 16-week-old mice were allocated to an exercise intervention (LFD-E, DIO-E) or a control group (LFD, DIO) for 6 weeks (4 groups, n = 7/group). Marrow adipocyte area was 44% higher with obesity (p < 0.0001) and after exercise 33% lower in LFD (p < 0.0001) and 39% lower in DIO (p < 0.0001). In LFD, exercise did not affect adipocyte number; however, in DIO, the adipocyte number was 56% lower (p < 0.0001). MAT was 44% higher in DIO measured by osmium-μCT, whereas exercise associated with reduced MAT (-23% in LFD, -48% in DIO, p < 0.05). MAT was additionally quantified by 9.4TMRI, and correlated with osmium-µCT (r = 0.645; p < 0.01). Consistent with higher lipid beta oxidation, perilipin 3 (PLIN3) rose with exercise in tibial mRNA (+92% in LFD, +60% in DIO, p < 0.05). Tibial µCT-derived trabecular bone volume (BV/TV) was not influenced by DIO but responded to exercise with an increase of 19% (p < 0.001). DIO was associated with higher cortical periosteal and endosteal volumes of 15% (p = 0.012) and 35% (p < 0.01), respectively, but Ct.Ar/Tt.Ar was lower by 2.4% (p < 0.05). There was a trend for higher stiffness (N/m) in DIO, and exercise augmented this further. In conclusion, obesity associated with increases in marrow lipid-measured by osmium-μCT and MRI-and partially due to an increase in adipocyte size, suggesting increased lipid uptake into preexisting adipocytes. Exercise associated with smaller adipocytes and less bone lipid, likely invoking increased ß-oxidation and basal lipolysis as evidenced by higher levels of PLIN3. © 2017 American Society for Bone and Mineral Research.

ToF-SIMS study of differentiation of human bone-derived stromal cells: new insights into osteoporosis

Lipids have numerous important functions in the human body, as they form the cells' plasma membranes and play a key role in many disease states, presumably also in osteoporosis. Here, the fatty acid composition of the outer plasma membranes of cells differentiated into the osteogenic and adipogenic direction is studied with surface-sensitive time-of-flight secondary ion mass spectrometry (ToF-SIMS). For data evaluation, principal component analysis (PCA) is applied. Human (bone-derived) mesenchymal stromal cells (hMSCs) from an osteoporotic donor and a control donor are compared to reveal differences in the fatty acid composition of the membranes. The chemical information is correlated to staining and real-time quantitative polymerase chain reaction (rt-qPCR) results to provide insight into the gene expression of several differentiation markers on the RNA level. Adipogenic differentiation of hMSCs from a non-osteoporotic donor correlates with increased relative intensities of all fatty acids under investigation. After osteogenic differentiation of non-osteoporotic cells, the relative mass signal intensities of unsaturated fatty acids such as oleic and linoleic acids are increased. However, the osteoporotic cells show increased levels of palmitic acid in the plasma membrane after exposure to osteogenic differentiation conditions, which correlates to an immature differentiation state relative to non-osteoporotic osteogenic cells. This immature differentiation state is confirmed by increased early osteogenic differentiation factor Runx2 on RNA level and by less calcium mineralization spots seen in von Kossa staining and ToF-SIMS images. Graphical abstract Time-of-flight secondary ion mass spectrometry is applied to analyze the fatty acid composition of the outer plasma membranes of cells differentiated into the adipogenic and osteogenic direction. Cells from an osteoporotic and a control donor are compared to reveal differences due to differentiation and disease stage of the cells.

Adipose, Bone, and Myeloma: Contributions from the Microenvironment

Researchers globally are working towards finding a cure for multiple myeloma (MM), a destructive blood cancer diagnosed yearly in ~750,000 people worldwide (Podar et al. in Expert Opin Emerg Drugs 14:99-127, 2009). Although MM targets multiple organ systems, it is the devastating skeletal destruction experienced by over 90 % of patients that often most severely impacts patient morbidity, pain, and quality of life. Preventing bone disease is therefore a priority in MM treatment, and understanding how and why myeloma cells target the bone marrow (BM) is fundamental to this process. This review focuses on a key area of MM research: the contributions of the bone microenvironment to disease origins, progression, and drug resistance. We describe some of the key cell types in the BM niche: osteoclasts, osteoblasts, osteocytes, adipocytes, and mesenchymal stem cells. We then focus on how these key cellular players are, or could be, regulating a range of disease-related processes spanning MM growth, drug resistance, and bone disease (including osteolysis, fracture, and hypercalcemia). We summarize the literature regarding MM-bone cell and MM-adipocyte relationships and subsequent phenotypic changes or adaptations in MM cells, with the aim of providing a deeper understanding of how myeloma cells grow in the skeleton to cause bone destruction. We identify avenues and therapies that intervene in these networks to stop tumor growth and/or induce bone regeneration. Overall, we aim to illustrate how novel therapeutic target molecules, proteins, and cellular mediators may offer new avenues to attack this disease while reviewing currently utilized therapies.

High-density lipoprotein (HDL) metabolism and bone mass

It is well appreciated that high-density lipoprotein (HDL) and bone physiology and pathology are tightly linked. Studies, primarily in mouse models, have shown that dysfunctional and/or disturbed HDL can affect bone mass through many different ways. Specifically, reduced HDL levels have been associated with the development of an inflammatory microenvironment that affects the differentiation and function of osteoblasts. In addition, perturbation in metabolic pathways of HDL favors adipoblastic differentiation and restrains osteoblastic differentiation through, among others, the modification of specific bone-related chemokines and signaling cascades. Increased bone marrow adiposity also deteriorates bone osteoblastic function and thus bone synthesis, leading to reduced bone mass. In this review, we present the current knowledge and the future directions with regard to the HDL-bone mass connection. Unraveling the molecular phenomena that underline this connection will promote the deeper understanding of the pathophysiology of bone-related pathologies, such as osteoporosis or bone metastasis, and pave the way toward the development of novel and more effective therapies against these conditions.

Formononetin, an isoflavone, activates AMP-activated protein kinase/β-catenin signalling to inhibit adipogenesis and rescues C57BL/6 mice from high-fat diet-induced obesity and bone loss

Balance between adipocyte and osteoblast differentiation is the key link of disease progression in obesity and osteoporosis. We have previously reported that formononetin (FNT), an isoflavone extracted from Butea monosperma, stimulates osteoblast formation and protects against postmenopausal bone loss. The inverse relationship between osteoblasts and adipocytes prompted us to analyse the effect of FNT on adipogenesis and in vivo bone loss, triggered by high-fat diet (HFD)-induced obesity. The anti-obesity effect and mechanism of action of FNT was determined in 3T3-L1 cells and HFD-induced obese male mice. Our findings show that FNT suppresses the adipogenic differentiation of 3T3-L1 fibroblasts, through down-regulation of key adipogenic markers such as PPARγ, CCAAT/enhancer-binding protein alpha (C/EBPα) and sterol regulatory element-binding protein (SREBP) and inhibits intracellular TAG accumulation. Increased intracellular reactive oxygen species levels and AMP-activated protein kinase (AMPK) activation accompanied by stabilisation of β-catenin were attributed to the anti-adipogenic action of FNT. In vivo, 12 weeks of FNT treatment inhibited the development of obesity in mice by attenuating HFD-induced body weight gain and visceral fat accumulation. The anti-obesity effect of FNT results from increased energy expenditure. FNT also protects against HFD-induced dyslipidaemia and rescues deterioration of trabecular bone volume by increasing bone formation and decreasing bone resorbtion caused by HFD. FNT's rescuing action against obesity-induced osteoporosis commenced at the level of progenitors, as bone marrow progenitor cells, obtained from the HFD mice group supplemented with FNT, showed increased osteogenic and decreased adipogenic potentials. Our findings suggest that FNT inhibits adipogenesis through AMPK/β-catenin signal transduction pathways and protects against HFD-induced obesity and bone loss.

Marrow Adipose Tissue: Skeletal Location, Sexual Dimorphism, and Response to Sex Steroid Deficiency

Marrow adipose tissue (MAT) is unique with respect to origin, metabolism, and function. MAT is characterized with high heterogeneity which correlates with skeletal location and bone metabolism. This fat depot is also highly sensitive to various hormonal, environmental, and pharmacologic cues to which it responds with changes in volume and/or metabolic phenotype. We have demonstrated previously that MAT has characteristics of both white (WAT) and brown (BAT)-like or beige adipose tissue, and that beige phenotype is attenuated with aging and in diabetes. Here, we extended our analysis by comparing MAT phenotype in different locations within a tibia bone of mature C57BL/6 mice and with respect to the presence of sex steroids in males and females. We report that MAT juxtaposed to trabecular bone of proximal tibia (pMAT) is characterized by elevated expression of beige fat markers including Ucp1, HoxC9, Prdm16, Tbx1, and Dio2, when compared with MAT located in distal tibia (dMAT). There is also a difference in tissue organization with adipocytes in proximal tibia being dispersed between trabeculae, while adipocytes in distal tibia being densely packed. Higher trabecular bone mass (BV/TV) in males correlates with lower pMAT volume and higher expression of beige markers in the same location, when compared with females. However, there is no sexual divergence in the volume and transcriptional profile of dMAT. A removal of ovaries in females resulted in decreased cortical bone mass and increased volume of both pMAT and dMAT, as well as volume of gonadal WAT (gWAT). Increase in pMAT volume was associated with marked increase in Fabp4 and Adiponectin expression and relative decrease in beige fat gene markers. A removal of testes in males resulted in cortical and trabecular bone loss and the tendency to increased volume of both pMAT and dMAT, despite a loss of gWAT. Orchiectomy did not affect the expression of white and beige adipocyte gene markers. In conclusion, expression profile of beige adipocyte gene markers correlates with skeletal location of active bone remodeling and higher BV/TV, however bone loss resulted from sex steroid deficiency is not proportional to MAT expansion at the same skeletal location.

Skeletal Metabolism, Fracture Risk, and Fracture Outcomes in Type 1 and Type 2 Diabetes

Fracture risk is significantly increased in both type 1 and type 2 diabetes, and individuals with diabetes experience worse fracture outcomes than normoglycemic individuals. Factors that increase fracture risk include lower bone mass in type 1 diabetes and compromised skeletal quality and strength despite preserved bone density in type 2 diabetes, as well as the effects of comorbidities such as diabetic macro- and microvascular complications. In this Perspective, we assess the developing scientific knowledge regarding the epidemiology and pathophysiology of skeletal fragility in patients with diabetes and the emerging data on the prediction, treatment, and outcomes of fractures in individuals with type 1 and type 2 diabetes.

Increased Gs Signaling in Osteoblasts Reduces Bone Marrow and Whole-Body Adiposity in Male Mice

Bone is increasingly recognized as an endocrine organ that can regulate systemic hormones and metabolism through secreted factors. Although bone loss and increased adiposity appear to be linked clinically, whether conditions of increased bone formation can also change systemic metabolism remains unclear. In this study, we examined how increased osteogenesis affects metabolism by using an engineered G protein-coupled receptor, Rs1, to activate Gs signaling in osteoblastic cells in ColI(2.3)(+)/Rs1(+) transgenic mice. We previously showed that these mice have dramatically increased bone formation resembling fibrous dysplasia of the bone. We found that total body fat was significantly reduced starting at 3 weeks of age. Furthermore, ColI(2.3)(+)/Rs1(+) mice showed reduced O2 consumption and respiratory quotient measures without effects on food intake and energy expenditure. The mice had significantly decreased serum triacylglycerides, leptin, and adiponectin. Resting glucose and insulin levels were unchanged; however, glucose and insulin tolerance tests revealed increased sensitivity to insulin. The mice showed resistance to fat accumulation from a high-fat diet. Furthermore, ColI(2.3)(+)/Rs1(+) mouse bones had dramatically reduced mature adipocyte differentiation, increased Wingless/Int-1 (Wnt) signaling, and higher osteoblastic glucose utilization than controls. These findings suggest that osteoblasts can influence both local and peripheral adiposity in conditions of increased bone formation and suggest a role for osteoblasts in the regulation of whole-body adiposity and metabolic homeostasis.

p38α MAPK Regulates Lineage Commitment and OPG Synthesis of Bone Marrow Stromal Cells to Prevent Bone Loss under Physiological and Pathological Conditions

Bone marrow-derived mesenchymal stromal cells (BM-MSCs) are capable of differentiating into osteoblasts, chondrocytes, and adipocytes. Skewed differentiation of BM-MSCs contributes to the pathogenesis of osteoporosis. Yet how BM-MSC lineage commitment is regulated remains unclear. We show that ablation of p38α in Prx1+ BM-MSCs produced osteoporotic phenotypes, growth plate defects, and increased bone marrow fat, secondary to biased BM-MSC differentiation from osteoblast/chondrocyte to adipocyte and increased osteoclastogenesis and bone resorption. p38α regulates BM-MSC osteogenic commitment through TAK1-NF-κB signaling and osteoclastogenesis through osteoprotegerin (OPG) production by BM-MSCs. Estrogen activates p38α to maintain OPG expression in BM-MSCs to preserve the bone. Ablation of p38α in BM-MSCs positive for Dermo1, a later BM-MSC marker, only affected osteogenic differentiation. Thus, p38α mitogen-activated protein kinase (MAPK) in Prx1+ BM-MSCs acts to preserve the bone by promoting osteogenic lineage commitment and sustaining OPG production. This study thus unravels previously unidentified roles for p38α MAPK in skeletal development and bone remodeling.

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p38α deletion in Prx1+ BM-MSCs led to osteoporosis and cartilage anomaly

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p38α controls proliferation and tri-lineage differentiation of Prx1+ BM-MSCs

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p38α regulates osteoclastogenesis through OPG production by BM-MSCs

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The BM-MSC p38-OPG axis participates in estrogen deficiency-induced osteoporosis

Bone and high-density lipoprotein: The beginning of a beautiful friendship

There is a tight link between bone and lipid metabolic pathways. In this vein, several studies focused on the exploration of high-density lipoprotein (HDL) in the pathobiology of bone diseases, with emphasis to the osteoarthritis (OA) and osteoporosis, the most common bone pathologies. Indeed, epidemiological and in vitro data have connected reduced HDL levels or dysfunctional HDL with cartilage destruction and OA development. Recent studies uncovered functional links between HDL and OA fueling the interesting hypothesis that OA could be a chronic element of the metabolic syndrome. Other studies have linked HDL to bone mineral density. Even though at epidemiological levels the results are conflicting, studies in animals as well as in vitro experiments have shown that HDL facilitates osteoblastogensis and bone synthesis and most probably affects osteoclastogenesis and osteoclast bone resorption. Notably, reduced HDL levels result in increased bone marrow adiposity affecting bone cells function. Unveiling the mechanisms that connect HDL and bone/cartilage homeostasis may contribute to the design of novel therapeutic agents for the improvement of bone and cartilage quality and thus for the treatment of related pathological conditions.

Fat and Sucrose Intake Induces Obesity-Related Bone Metabolism Disturbances: Kinetic and Reversibility Studies in Growing and Adult Rats

Metabolic and bone effects were investigated in growing (G, n = 45) and mature (M, n = 45) rats fed a high-fat/high-sucrose diet (HFS) isocaloric to the chow diet of controls (C, n = 30 per group). At week 19, a subset of 15 rats in each group (HFS or C, at both ages) was analyzed. Then one-half of the remaining 30 HFS rats in each groups continued HFS and one-half were shifted to C until week 27. Although no serum or bone marrow inflammation was seen, HFS increased visceral fat, serum leptin and insulin at week 19 and induced further alterations in lipid profile, serum adiponectin, and TGFβ1, TIMP1, MMP2, and MMP9, suggesting a prediabetic phenotype and cardiovascular dysfunction at week 27 more pronounced in M than G. These events were associated with dramatic reduction of osteoclastic and osteoid surfaces with accelerated mineralizing surfaces in both HFS age groups. Mineral metabolism and its major regulators were disturbed, leading to hyperphosphatemia and hypocalcemia. These changes were associated with bone alterations in the weight-bearing tibia, not in the non-weight-bearing vertebra. Indeed in fat rats, tibia trabecular bone accrual increased in G whereas loss of trabecular bone in M was alleviated. At diaphysis cortical porosity increased in G and even more in M at week 27. After the diet switch, metabolic and bone cellular disturbances fully reversed in G, but not in M. Trabecular benefit of the obese was preserved in both age groups and in M the age-related bone loss was even lighter after the diet switch than in prolonged HFS. At the diaphysis, cortical porosity normalized in G but not in M. Hypocalcemia in G and M was irreversible. Thus, the mild metabolic syndrome induced by isocaloric HFS is able to alter bone cellular activities and mineral metabolism, reinforce trabecular bone, and affect cortical bone porosity in an irreversible manner in older rats.

Signaling Interplay between Bone Marrow Adipose Tissue and Multiple Myeloma cells

In the year 2000, Hanahan and Weinberg (1) defined the six Hallmarks of Cancer as: self-sufficiency in growth signals, evasion of apoptosis, insensitivity to antigrowth mechanisms, tissue invasion and metastasis, limitless replicative potential, and sustained angiogenesis. Eleven years later, two new Hallmarks were added to the list (avoiding immune destruction and reprograming energy metabolism) and two new tumor characteristics (tumor-promoting inflammation and genome instability and mutation) (2). In multiple myeloma (MM), a destructive cancer of the plasma cell that grows predominantly in the bone marrow (BM), it is clear that all these hallmarks and characteristics are in play, contributing to tumor initiation, drug resistance, disease progression, and relapse. Bone marrow adipose tissue (BMAT) is a newly recognized contributor to MM oncogenesis and disease progression, potentially affecting MM cell metabolism, immune action, inflammation, and influences on angiogenesis. In this review, we discuss the confirmed and hypothetical contributions of BMAT to MM development and disease progression. BMAT has been understudied due to technical challenges and a previous lack of appreciation for the endocrine function of this tissue. In this review, we define the dynamic, responsive, metabolically active BM adipocyte. We then describe how BMAT influences MM in terms of: lipids/metabolism, hypoxia/angiogenesis, paracrine or endocrine signaling, and bone disease. We then discuss the connection between BMAT and systemic inflammation and potential treatments to inhibit the feedback loops between BM adipocytes and MM cells that support MM progression. We aim for researchers to use this review to guide and help prioritize their experiments to develop better treatments or a cure for cancers, such as MM, that associate with and may depend on BMAT.

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.

Sulfuretin induces osteoblast differentiation through activation of TGF-β signaling

The identification and examination of potential determinants controlling the progression of cell fate toward osteoblasts can be intriguing subjects. In this study, the effects of sulfuretin, a major compound isolated from Rhus verniciflua Stokes, on osteoblast differentiation were investigated. Treatments of sulfuretin induced alkaline phosphatase (ALP) activity in mesenchymal C3H10T1/2 cells and mineralization in preosteoblast MC3T3-E1 cells. Pro-osteogenic effects of sulfuretin were consistently observed in freshly isolated primary bone marrow cells. In mechanical studies, sulfuretin specifically induced expression of TGF-β target genes, such as SMAD7 and PAI-1, but not other signaling pathway-related genes. Similar to the results of gene expression analysis, reporter assays further demonstrated TGF-β-specific induction by sulfuretin. Furthermore, disruption of TGF-β signaling using treatment with TGF-β-specific inhibitor, SB-431542, and introduction of SMAD2/3 small interfering RNA impaired the effects of sulfuretin in inducing ALP activity and expression of ALP mRNA. Together, these data indicate that the pro-osteogenic effects of sulfuretin are mediated through activation of TGF-β signaling, further supporting the potential of sulfuretin in the prevention of bone-related diseases such as bone fracture and osteoporosis.

High bone mass in adult mice with diet-induced obesity results from a combination of initial increase in bone mass followed by attenuation in bone formation; implications for high bone mass and decreased bone quality in obesity

Obesity is generally recognized as a condition which positively influences bone mass and bone mineral density (BMD). Positive effect of high body mass index (BMI) on bone has been recognized as a result of increased mechanical loading exerted on the skeleton. However, epidemiologic studies indicate that obesity is associated with increased incidence of fractures. The results presented here offer a new perspective regarding the mechanisms which may be responsible for the increase of bone mass and concurrent decrease in bone quality. Two groups of 12 week old C57BL/6 males were fed either high fat diet (HFD) or regular diet (RD) for 11 weeks. Metabolic profile, bone parameters and gene expression were assessed in these groups at the end of the experiment. Additionally, bone status was evaluated in a third group of 12 week old animals corresponding to animals at the start of the feeding period. Administration of HFD resulted in development of a diet-induced obesity (DIO), glucose intolerance, alteration in energy metabolism, and impairment in WAT function, as compared to the age-matched control animals fed RD. The expression of adiponectin, FABP4/aP2, DIO2 and FoxC2 were decreased in WAT of DIO animals, as well as transcript levels for IGFBP2, the cytokine regulating both energy metabolism and bone mass. At the end of experiment, DIO mice had higher bone mass than both control groups on RD, however they had decreased bone formation, as assessed by calcein labeling, and increased marrow adipocyte content. This study suggests that the bone mass acquired in obesity is a result of a two-phase process. First phase would consist of either beneficial effect of fat expansion to increase bone mass by increased mechanical loading and/or increased production of bone anabolic adipokines and/or nutritional effect of fatty acids. This is followed by a second phase characterized by decreased bone formation and bone turnover resulting from development of metabolic impairment.

Monounsaturated fatty acids-rich diets in hypercholesterolemic-growing rats

The effects of replacing dietary saturated fat by different monounsaturated fatty acid (ω-9MUFA) sources on serum lipids, body fat and bone in growing hypercholesterolemic rats were studied. Rats received one of the six different diets: AIN-93G (control, C); extra virgin olive oil (OO) + C; high-oleic sunflower oil (HOSO) + C or atherogenic diet (AT) for 8 weeks; the remaining two groups received AT for 3 weeks and then, the saturated fat was replaced by an oil mixture of soybean oil added with OO or HOSO for 5 weeks. Rats consuming MUFA-rich diets showed the highest body fat, hepatic index and epididymal, intestinal and perirenal fat, and triglycerides. T-chol and non-HDL-chol were increased in HOSO rats but decreased in OO rats. Bone mineral content and density were higher in both OO and HOSO groups than in AT rats. This study casts caution to the generalization of the benefits of MUFA for the treatment of hypercholesterolemia.

Osteopetrosis and its relevance for the discovery of new functions associated with the skeleton

Osteopetrosis is a rare genetic disorder characterized by an increase of bone mass due to defective osteoclast function. Patients typically displayed spontaneous fractures, anemia, and in the most severe forms hepatosplenomegaly and compression of cranial facial nerves leading to deafness and blindness. Osteopetrosis comprises a heterogeneous group of diseases as several forms are known with different models of inheritance and severity from asymptomatic to lethal. This review summarizes the genetic and clinical features of osteopetrosis, emphasizing how recent studies of this disease have contributed to understanding the central role of the skeleton in the whole body physiology. In particular, the interplay of bone with the stomach, insulin metabolism, male fertility, the immune system, bone marrow, and fat is described.

Plasma fatty acid profile in multiple myeloma patients

New membrane formation in the proliferating tumor cells consequently results in hypermetabolism of fatty acids (FA), as seen in many cancer patients, including multiple myeloma (MM). The FA composition of plasma reflects both endogenous synthesis as well as the dietary supply of these compounds. Additionally, obesity is a risk factor for the development of MM. The aim of this study was to compare the FA composition of plasma in 60 MM patients and 60 healthy controls. We noted significant differences in the FA profile of plasma from patients with MM when compared to the control group. Increased levels of saturated and n-6 polyunsaturated fatty acids in MM patients suggest that there may be increased endogenous synthesis of these fatty acids, likely due to increased expression of desaturase and elongase. Furthermore, cluster analysis showed differences in the distribution of FA in plasma from MM patients compared to controls. Dietary fat and a deranged endogenous FA metabolism may contribute to cancer-associated inflammation through an abnormal arachidonic acid metabolism, caused by pro-inflammatory derivatives. Our study supports further research on the biochemistry of lipids in patients with MM.

Black rice (Oryza sativa L.) extracts induce osteoblast differentiation and protect against bone loss in ovariectomized rats

Osteoporosis, an age associated skeletal disease, exhibits increased adipogenesis at the expense of osteogenesis from common osteoporotic bone marrow cells. In this study, black rice (Oryza sativa L.) extracts (BRE) were identified as osteogenic inducers. BRE stimulated the alkaline phosphatase (ALP) activity in both C3H10T1/2 and primary bone marrow cells. Similarly, BRE increased mRNA expression of ALP and osterix. Oral administration of BRE in OVX rats prevented decreases in bone density and strength. By contrast, BRE inhibited adipocyte differentiation of mesenchymal C3H10T1/2 cells and prevented increases in body weight and fat mass in high fat diet fed obese mice, further suggesting the dual effects of BRE on anti-adipogenesis and pro-osteogenesis. UPLC analysis identified cyanidin-3-O-glucoside and peonidin-3-O-glucoside as main anti-adipogenic effectors but not for pro-osteogenic induction. In mechanism studies, BRE selectively stimulated Wnt-driven luciferase activities. BRE treatment also induced Wnt-specific target genes such as Axin2, WISP2, and Cyclin D1. Taken together, these data suggest that BRE is a potentially useful ingredient to protect against age related osteoporosis and diet induced obesity.