Structure and Function of Bone Marrow Adipocytes

Ectopic fat in muscle and poor glycemic control are negatively associated with trabecular bone score in type 2 diabetes

INTRODUCTION

Type 2 Diabetes (T2D) is associated with fractures, despite preserved Bone Mineral Density (BMD). This study aimed to evaluate the relationship between BMD and trabecular bone score (TBS) with the reallocation of fat within muscle in individuals with eutrophy, obesity, and T2D.

METHODS

The subjects were divided into three groups: eutrophic controls paired by age and sex with the T2D group (n = 23), controls diagnosed with obesity paired by age, sex, and body mass index with the T2D group (n = 27), and the T2D group (n = 29). BMD and body fat percentage were determined using dual-energy X-Ray absorptiometry. TBS was determined using TBS iNsight software. Intra and extramyocellular lipids in the soleus were measured using proton magnetic resonance spectroscopy.

RESULTS

TBS was lower in the T2D group than in the other two groups. Glycated hemoglobin (A1c) was negatively associated with TBS. Body fat percentage was negatively associated with TBS and Total Hip (TH) BMD. TH BMD was positively associated with intramuscular lipids. A trend of negative association was observed between intramuscular lipids and TBS.

CONCLUSION

This study showed for the first time that the reallocation of lipids within muscle has a negative association with TBS. Moreover, these results are consistent with previous studies showing a negative association between a parameter related to insulin resistance (intramuscular lipids) and TBS.

Obesity, bone marrow adiposity, and leukemia: Time to act

Obesity has taken the face of a pandemic with less direct concern among the general population and scientific community. However, obesity is considered a low-grade systemic inflammation that impacts multiple organs. Chronic inflammation is also associated with different solid and blood cancers. In addition, emerging evidence demonstrates that individuals with obesity are at higher risk of developing blood cancers and have poorer clinical outcomes than individuals in a normal weight range. The bone marrow is critical for hematopoiesis, lymphopoiesis, and myelopoiesis. Therefore, it is vital to understand the mechanisms by which obesity-associated changes in BM adiposity impact leukemia development. BM adipocytes are critical to maintain homeostasis via different means, including immune regulation. However, obesity increases BM adiposity and creates a pro-inflammatory environment to upregulate clonal hematopoiesis and a leukemia-supportive environment. Obesity further alters lymphopoiesis and myelopoiesis via different mechanisms, which dysregulate myeloid and lymphoid immune cell functions mentioned in the text under different sequentially discussed sections. The altered immune cell function during obesity alters hematological malignancies and leukemia susceptibility. Therefore, obesity-induced altered BM adiposity, immune cell generation, and function impact an individual's predisposition and severity of leukemia, which should be considered a critical factor in leukemia patients.

The Effects of Corticosteroid Administration and Treadmill Exercise on Marrow Adipose Tissue and Trabecular Bone after Anterior Cruciate Ligament Reconstruction in Rats

We aimed to investigate the effects of short-term corticosteroid administration after anterior cruciate ligament (ACL) reconstruction on marrow adipose tissue (MAT) and trabecular bone mass, as well as to examine whether treadmill exercise can mitigate MAT increase and trabecular bone deterioration caused by corticosteroid. ACL-reconstructed rats were divided into groups: no intervention, daily treadmill exercise (60 min/day), administration of the steroidal drug dexamethasone (250 μg/kg on days 0-5, 7, and 9 post-operatively), or dexamethasone administration combined with treadmill exercise. Untreated rats were served as controls. At day 10 or 30 post-operatively, histological assessments were performed in the proximal tibial epiphysis. MAT accumulation and trabecular bone loss were observed after ACL reconstruction. Dexamethasone promoted MAT accumulation at day 10 post-operatively but did not affect the trabecular bone loss. The MAT accumulation caused by dexamethasone reversed within 21 days after discontinuation. Treadmill exercise did not influence the changes in the MAT and trabecular bone areas. Short-term corticosteroid administration after ACL reconstruction promoted MAT accumulation while not affecting trabecular bone area. The MAT accumulation resulting from corticosteroid administration was reversible after discontinuation. Treadmill exercise could not mitigate the accumulation of MAT caused by corticosteroid administration and did not affect trabecular bone area.

Progress in the contrary effects of glucagon-like peptide-1 and chemerin on obesity development

Glucagon-like peptide-1 (GLP-1), secreted by intestinal L-cells, plays a pivotal role in the modulation of β-cell insulin secretion in a glucose-dependent manner, concurrently promoting β-cell survival and β-cell mass. Notably, GLP-1 has emerged as an effective second-line treatment for type 2 diabetes mellitus, gaining further prominence for its pronounced impact on body weight reduction, positioning it as a potent antiobesity agent. However, the mechanism by which GLP-1 improves obesity remains unclear. Some reports suggest that this mechanism may be associated with the regulation of adipokine synthesis within adipose tissue. Chemerin, a multifunctional adipokine and chemokine, has been identified as a pivotal player in adipocyte differentiation and the propagation of systemic inflammation, a hallmark of obesity. This review provides a comprehensive overview of the mechanisms by which GLP-1 and chemerin play crucial roles in obesity and obesity-related diseases. It discusses well-established aspects, such as their effects on food intake and glycolipid metabolism, as well as recent insights, including their influence on macrophage polarization and adipose tissue thermogenesis. GLP-1 has been shown to increase the population of anti-inflammatory M2 macrophages, promote brown adipose tissue thermogenesis, and induce the browning of white adipose tissue. In contrast, chemerin exhibits opposite effects in these processes. In addition, recent research findings have demonstrated the promising potential of GLP-1-based therapies in directly or indirectly regulating chemerin expression. In an intriguing reciprocal relationship, chemerin has also been newly identified as a negative regulator of GLP-1 in vivo. This review delineates the intricate interplay between GLP-1 and chemerin, unraveling their mutual inhibitory interactions. To the best of our knowledge, no previous reviews have focused on this specific topic, making this review particularly valuable in expanding our understanding of the endocrine mechanisms of obesity and providing potential strategies for the treatment of obesity and related diseases.

Effects of joint immobilization and treadmill exercise on marrow adipose tissue and trabecular bone after anterior cruciate ligament reconstruction in the rat proximal tibial epiphysis

Marrow adipose tissue (MAT) adversely affects bone metabolism under certain conditions. Although mechanical stress is an important factor in regulating MAT and bone mass, how stress from different rehabilitation protocols after anterior cruciate ligament (ACL) reconstruction affects trabecular bone and MAT is unclear. We aimed to examine the effects of joint immobilization and treadmill exercise on trabecular bone and MAT after ACL reconstruction. Rats received unilateral knee ACL transection and reconstruction surgery. After surgery, rats were reared without intervention, with joint immobilization, or with treadmill exercise (12 m/min, 60 min/day, six days/week), with untreated rats as controls. At two or four weeks after starting experiments, we examined histological changes in trabecular bone and MAT in the proximal tibial epiphysis. After ACL reconstruction, there were no significant changes in trabecular bone area and MAT area at both time points. Joint immobilization after ACL reconstruction resulted in reduced trabecular bone area and MAT accumulation due to adipocyte hyperplasia and hypertrophy within four weeks. Treadmill exercise after ACL reconstruction did not affect any parameters in trabecular bone and MAT. We detected a moderate negative correlation between trabecular bone area and MAT area. Therefore, MAT accumulation induced by joint immobilization may contribute, at least in part, to reducing trabecular bone area. To minimize trabecular bone loss and MAT accumulation, joint immobilization after ACL reconstruction should be minimized. Exercise after ACL reconstruction did not alter trabecular bone and MAT.

Diabetes and bone

Globally, one in 11 adults has diabetes mellitus of which 90% have type 2 diabetes. The numbers for osteoporosis are no less staggering: 1 in 3 women has a fracture after menopause, and the same is true for 1 in 5 men after the age of 50 years. Aging is associated with several physiological changes that cause insulin resistance and impaired insulin secretion, which in turn lead to hyperglycemia. The negative balance between bone resorption and formation is a natural process that appears after the fourth decade of life and lasts for the following decades, eroding the bone structure and increasing the risk of fractures. Not incidentally, it has been acknowledged that diabetes mellitus, regardless of whether type 1 or 2, is associated with an increased risk of fracture. The nuances that differentiate bone damage in the two main forms of diabetes are part of the intrinsic heterogeneity of diabetes, which is enhanced when associated with a condition as complex as osteoporosis. This narrative review addresses the main parameters related to the increased risk of fractures in individuals with diabetes, and the mutual factors affecting the treatment of diabetes mellitus and osteoporosis.

Bone, fat, and muscle interactions in health and disease

Energy metabolism is a point of integration among the various organs and tissues of the human body, not only in terms of consumption of energy substrates but also because it concentrates a wide interconnected network controlled by endocrine factors. Thus, not only do tissues consume substrates, but they also participate in modulating energy metabolism. Soft mesenchymal tissues, in particular, play a key role in this process. The recognition that high energy consumption is involved in bone remodeling has been accompanied by evidence showing that osteoblasts and osteocytes produce factors that influence, for example, insulin sensitivity and appetite. Additionally, there are significant interactions between muscle, adipose, and bone tissues to control mutual tissue trophism. Not by chance, trophic and functional changes in these tissues go hand in hand from the beginning of an individual's development until aging. Likewise, metabolic and nutritional diseases deeply affect the musculoskeletal system and adipose tissue. The present narrative review highlights the importance of the interaction of the mesenchymal tissues for bone development and maintenance and the impact on bone from diseases marked by functional and trophic disorders of adipose and muscle tissues.

The role of the Wnt signalling pathway in the energy metabolism of bone remodelling

OBJECTIVES

Bone remodelling is necessary to repair old and impaired bone caused by aging and its effects. Injury in the process of bone remodelling generally leads to the development of various bone diseases. Energy metabolism plays crucial roles in bone cell formation and function, the disorder of which will disrupt the balance between bone formation and bone resorption.

MATERIALS AND METHODS

Here, we review the intrinsic interactions between bone remodelling and energy metabolism and the role of the Wnt signalling pathway.

RESULTS

We found a close interplay between metabolic pathways and bone homeostasis, demonstrating that bone plays an important role in the regulation of energy balance. We also discovered that Wnt signalling is associated with multiple biological processes regulating energy metabolism in bone cells.

CONCLUSIONS

Thus, targeted regulation of Wnt signalling and the recovery of the energy metabolism function of bone cells are key means for the treatment of metabolic bone diseases.

Marrow adipose tissue accumulation and dysgenesis of the trabecular bone after anterior cruciate ligament transection and reconstruction in the rat proximal tibial epiphysis

The accumulation of marrow adipose tissue (MAT) is frequently associated with bone loss. Although anterior cruciate ligament (ACL) injury induces bone loss, MAT accumulation after ACL injury has not been evaluated. In addition, no information about changes in MAT after ACL reconstruction is available. In this study, we aimed to examine (1) the effects of ACL transection on the amounts of trabecular bone and MAT present, and (2) whether ACL reconstruction inhibits the changes in the trabecular bone and MAT that are induced by ACL transection. ACL transection alone or with immediate reconstruction was performed on the right knees of rats. Untreated left knees were used as controls. Histomorphological changes in the trabecular bone and MAT in the proximal tibial epiphysis were examined prior to surgery and at one, four, and 12 weeks postsurgery. The trabecular bone area on the untreated side increased in a time-dependent manner. However, after ACL transection, the trabecular bone area did not increase during the experimental period, indicating dysgenesis of the bone (bone loss). Dysgenesis of the trabecular bone after ACL transection was attenuated by ACL reconstruction. MAT accumulation due to adipocyte hyperplasia and hypertrophy had been induced by ACL transection by four weeks postsurgery. This ACL transection-induced MAT accumulation was not prevented by ACL reconstruction. Based on these results, we conclude that (1) dysgenesis of the bone in the proximal tibia following ACL transection is accompanied by MAT accumulation, and (2) ACL reconstruction attenuates dysgenesis of the trabecular bone but cannot prevent MAT accumulation.

Official position of the Brazilian Association of Bone Assessment and Metabolism (ABRASSO) on the evaluation of body composition by densitometry: part I (technical aspects)—general concepts, indications, acquisition, and analysis

Objective

To review the technical aspects of body composition assessment by dual-energy X-ray absorptiometry (DXA) and other methods based on the most recent scientific evidence.

Materials and methods

This Official Position is a result of efforts by the Scientific Committee of the Brazilian Association of Bone Assessment and Metabolism (Associação Brasileira de Avaliação Óssea e Osteometabolismo, ABRASSO) and health care professionals with expertise in body composition assessment who were invited to contribute to the preparation of this document. The authors searched current databases for relevant publications. In this first part of the Official Position, the authors discuss the different methods and parameters used for body composition assessment, general principles of DXA, and aspects of the acquisition and analysis of DXA scans.

Conclusion

Considering aspects of accuracy, precision, cost, duration, and ability to evaluate all three compartments, DXA is considered the gold-standard method for body composition assessment, particularly for the evaluation of fat mass. In order to ensure reliable, adequate, and reproducible DXA reports, great attention is required regarding quality control procedures, preparation, removal of external artifacts, imaging acquisition, and data analysis and interpretation.

Hashtag bone: detrimental effects on bone contrast with metabolic benefits one and five years after Roux-en-Y gastric bypass

Bone loss is a potential adverse consequence of rapid and sustained weight loss after bariatric surgery. The aim of the present study was to evaluate the bone mass, body fat distribution, and metabolic parameters in women submitted to Roux-en-Y gastric bypass (RYGB). The study included the following three groups: one group of lean women (control [C] group) and two groups of obese women, one evaluated one year (B1) and the other five years (B5) after RYGB. Dual-energy X-ray absorptiometry and magnetic resonance imaging were used to determine bone mineral density (BMD; lumbar spine, total hip, and femoral neck) and abdominal fat content (subcutaneous [SAT] and visceral [VAT] adipose tissues, and intrahepatic lipids [IHL]). The BMD/body mass index ratio was lower in the B5 compared with the C group at all sites. Serum C-terminal telopeptide of type I collagen (CTX) levels were higher in the B1 and B5 groups compared with the C group. Individuals submitted to RYGB showed greater SAT but similar VAT and IHL values compared with those in the C group. However, the B5 group had higher mean parathyroid hormone levels compared with the other two groups. Individuals submitted to RYGB presented increased levels of CTX and low BMD for body weight than those in the C group, suggesting that bone catabolism is a persistent alteration associated with RYGB. In conclusion, the long-lasting metabolic benefits obtained with RYGB in obesity are counterbalanced by a persistent catabolic effect of the procedure on bone and mineral metabolism.

Physiologically decreased F-18 fluorodeoxyglucose uptake in the lower vertebrae associated with daily drinking habit in Japanese men with alcohol flushing reaction

Alcohol flushing reaction (AFR) is known as one of the risks for esophageal squamous cell cancer, and scientists have been elucidating this issue. However, little attention has been given to relevant imaging features. This study aims to investigate whether physiological ¹⁸F-fluorodeoxyglucose (FDG) uptake patterns in vertebrae are associated with drinking habits or AFR. Japanese male patients who underwent FDG positron emission computed tomography for evaluation of their known or suspected malignancies or inflammatory diseases were asked about their drinking habits and AFR. Altogether, 192 patients, 139 every-day drinkers and 53 non-drinkers were evaluated. Comparing the FDG uptake between that in the thoracic region and that in the lumbar region, vertebral uptake was visually classified into four patterns: Ld, dominant in lumbar region; TL, almost equal in both regions; BL, slightly higher in thoracic region (borderline pattern); Td, dominant in thoracic region. The uptake patterns were evaluated according to drinking habit (every-day drinker or non-drinker), AFR (flusher or non-flusher), and the combination of these two factors (habit/reaction: every-day drinker/flusher, every-day drinker/non-flusher, non-drinker/flusher, or non-drinker/non-flusher). There were 95 flushers (51 every-day drinkers and 44 non-drinkers) and 97 non-flushers (88 every-day drinkers and 9 non-drinkers). Ld, TL, BL, and Td patterns were observed in 0, 109 (56.8%), 31 (16.1%), and 52 (27.1%) patients, respectively. Td and BL patterns were more frequently observed in every-day drinkers compared with non-drinkers (p = 0.0467). Though the uptake patterns did not differ between flushers and non-flushers (p = 0.116), the Td pattern was more frequently observed in every-day drinkers/flushers (51%) compared with every-day drinkers/non-flushers (20.5%), non-drinkers/flushers (13.6%), and non-drinkers/non-flushers (22.2%) (p = 0.0014). The Td pattern was observed in patients with various diseases, with higher frequency in esophageal cancer, head and neck cancer, and lung cancer compared with other diseases. In conclusion, drinking habits and AFR were related to the vertebral uptake pattern with decreased uptake in the lumbar region in Japanese male patients.

Jabuticaba peel extract modulates adipocyte and osteoblast differentiation of MSCs from healthy and osteoporotic rats

INTRODUCTION

The jabuticaba peel extract (JPE) contains bioactive compounds that regulate fat metabolism. Because the negative correlation between fat accumulation and bone formation in bone marrow, we hypothesized that JPE inhibits adipocyte as well as favors osteoblast differentiation of mesenchymal stromal cells (MSCs) under healthy and osteoporotic conditions, a disease that display an imbalance between adipocyte and osteoblast differentiation resulting in reduced bone mass.

MATERIAL AND METHODS

To test these hypotheses, bone marrow MSCs were harvested from healthy and osteoporotic rats and cultured in adipogenic and osteogenic media with three concentrations of JPE, 0.25, 5 and 10 µg/ml, and vehicle (control). After selecting the most efficient concentrations of JPE, we used them to evaluate adipocyte and osteoblast differentiation of MSCs from both sources.

RESULTS

We observed that, in general, JPE inhibited adipocyte differentiation of MSCs with more pronounced effects in cells from healthy than osteoporotic rats. In addition, JPE increased osteoblast differentiation, exhibiting a slightly higher osteogenic potential on MSCs from osteoporotic compared to healthy condition.

CONCLUSION

Our results demonstrated that JPE drives MSCs to inhibit adipocyte differentiation and toward osteoblast differentiation under healthy and osteoporotic conditions. These findings pave the way for further translational studies to investigate the therapeutic possibilities of JPE in both prevention and treatment of osteoporosis.

GAS5 protects against osteoporosis by targeting UPF1/SMAD7 axis in osteoblast differentiation

Osteoporosis is a common systemic skeletal disorder resulting in bone fragility and increased fracture risk. It is still necessary to explore its detailed mechanisms and identify novel targets for the treatment of osteoporosis. Previously, we found that a lncRNA named GAS5 in human could negatively regulate the lipoblast/adipocyte differentiation. However, it is still unclear whether GAS5 affects osteoblast differentiation and whether GAS5 is associated with osteoporosis. Our current research found that GAS5 was decreased in the bones and BMSCs, a major origin of osteoblast, of osteoporosis patients. Mechanistically, GAS5 promotes the osteoblast differentiation by interacting with UPF1 to degrade SMAD7 mRNA. Moreover, a decreased bone mass and impaired bone repair ability were observed in Gas5 heterozygous mice, manifesting in osteoporosis. The systemic supplement of Gas5-overexpressing adenoviruses significantly ameliorated bone loss in an osteoporosis mouse model. In conclusion, GAS5 promotes osteoblast differentiation by targeting the UPF1/SMAD7 axis and protects against osteoporosis.

Insulin resistance negatively affects bone quality not quantity: the relationship between bone and adipose tissue

The present study suggests that insulin resistance has no association with bone quantity, but quality.

INTRODUCTION

The literature has contradictory results concerning the influence of insulin resistance on bone. The present study sought to evaluate the association of insulin resistance and adipose tissue with either bone mineral density or the trabecular bone score.

METHODS

The study included 56 individuals (36 women and 20 men): age = 46.6 ± 14.2 years, weight = 67.8 ± 10.9 kg, height = 1.65 ± 0.10 m and BMI = 24.8 ± 3.9 kg/m². The investigational protocol included biochemical determinations and bone assessment by dual X-ray absorptiometry for evaluation of bone mineral density and trabecular bone score. Magnetic resonance was employed to estimate visceral, subcutaneous and bone marrow adipose tissues, as well as intrahepatic lipids.

RESULTS

The bone mineral density of the lumbar spine, femoral neck and total hip were not associated with insulin resistance-related parameters [visceral adipose tissue, intrahepatic lipids and homeostatic model assessment of insulin resistance (HOMA-IR)]. In contrast, there was a negative relationship between the trabecular bone score and all these components. The association between the trabecular bone score and HOMA-IR was reinforced after adjustment for age and BMI. Marrow adipose tissue was negatively associated with both bone mineral density and trabecular bone score.

CONCLUSIONS

The present study shows that the trabecular bone score is negatively associated with marrow adipose tissue, insulin resistance, visceral adipose tissue and intrahepatic lipid measurements. Additionally, there was a negative relationship between saturated lipids in marrow adipose tissue and the trabecular bone score. These results encourage further studies to investigate the role of the trabecular bone score exam in the clinical evaluation of osteoporosis in conditions of insulin resistance.

Simvastatin inhibits the adipogenesis of bone marrow‑derived mesenchymal stem cells through the downregulation of chemerin/CMKLR1 signaling

Simvastatin is effective in the treatment of osteoporosis, partly through the inhibition of the adipogenesis of bone-marrow derived mesenchymal stem cells (BMSCs). The present study focused on the mechanisms responsible for the inhibitory effects of simvastatin on adipogenesis and examined the effects of simvastatin on the expression of peroxisome proliferator-activated receptor γ (PPARγ), chemerin, chemokine-like receptor 1 (CMKLR1), G protein-coupled receptor 1 (GPR1) and the adipocyte marker gene, adiponectin. BMSCs were isolated from 4-week-old female Sprague-Dawley (SD) rats, and adipogenesis was measured by the absorbance values at 490 nm of Oil Red O dye. The expression of each gene was evaluated by western blot analysis or reverse transcription-quantitative PCR (RT-qPCR). The expression of chemerin increased during adipogenesis, while CMKLR1 exhibited a trend towards a decreased expression. On days 7 and 14, the simvastatin-treated cells exhibited a down-regulated expression of chemerin, whereas the upregulated expression of its receptor, CMKLR1 was observed. The results also revealed that CMKLR1 is required for adipogenesis and the simvastatin-mediated inhibitory effect on adipogenesis. Simvastatin regulated adipogenesis by negatively modulating chemerin-CMKLR1 signaling. Importantly, simvastatin stimulation inhibited the upregulation of PPARγ and PPARγ-mediated chemerin expression to prevent adipogenesis. Treatment with the PPARγ agonist, rosiglitazone, partially reversed the negative regulatory effects of simvastatin. On the whole, the findings of the present study demonstrate that simvastatin inhibits the adipogenesis of BMSCs through the downregulation of PPARγ and subsequently prevents the PPARγ-mediated induction of chemerin/CMKLR1 signaling.

Osteoporosis and Hepatic Steatosis: 2 Closely Related Complications in Short-Bowel Syndrome

BACKGROUND

Osteoporosis has scarcely been prospectively investigated in short-bowel syndrome (SBS). This prospective study was designed to evaluate incretins, adipokines, bone mass, and lipid deposits from marrow adipose tissue (MAT), visceral adipose tissue (VAT), subcutaneous adipose tissue (SAT), and liver (IHLs).

METHODS

The study comprised 2 groups matched by gender, height, and age: the control group (CG) (9 males, 9 females) and the SBS group (SBSG) (6 males, 5 females). The SBSG was evaluated twice in an interval of 1 year (SBSG₀ and SBSG₁ ). The biochemical evaluation included incretins, leptin, and adiponectin. Dual-energy x-ray absorptiometry and magnetic resonance were, respectively, used to measure BMD and lipid deposits.

RESULTS

Bone mineral density (BMD) was lower in the SBSG than in the CG, but there was no difference between SBSG₀ and SBSG₁ . There was no difference in MAT, SAT, and VAT, but IHL was lower in CG than in SBSG₀ and SBSG₁ . A negative correlation between MAT and third lumbar vertebrae BMD was found in the CG but not in SBSG₀ or SBSG₁ . There was a negative association between IHL and bone mass considering all participants (CG and SBSG₀ ) (R² = 0.38; P < .05).

CONCLUSION

Appropriate nutrition assistance recovers body composition, reverts the relationship of bone mass and MAT, and mitigates bone loss in SBS. In spite of this, osteoporosis seems to be an early and persistent complication in SBS. Curiously, SBS seems to be a highly vulnerable condition for the development of hepatic steatosis and shows an association between bone mass and IHL.

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.

Marrow Adipocytes: Origin, Structure, and Function

The skeleton harbors an array of lineage cells that have an essential role in whole body homeostasis. Adipocytes start the colonization of marrow space early in postnatal life, expanding progressively and influencing other components of the bone marrow through paracrine signaling. In this unique, closed, and hypoxic environment close to the endosteal surface and adjacent to the microvascular space the marrow adipocyte can store or provide energy, secrete adipokines, and target neighboring bone cells. Adipocyte progenitors can also migrate from the bone marrow to populate white adipose tissue, a process that accelerates during weight gain. The marrow adipocyte also has an endocrine role in whole body homeostasis through its varied secretome that targets distant adipose depots, skeletal muscle, and the nervous system. Further insights into the biology of this unique and versatile cell will undoubtedly lead to novel therapeutic approaches to metabolic and age-related disorders such as osteoporosis and diabetes mellitus.

Prostate Tumor Cell-Derived IL1β Induces an Inflammatory Phenotype in Bone Marrow Adipocytes and Reduces Sensitivity to Docetaxel via Lipolysis-Dependent Mechanisms

Adipocyte-tumor cell cross-talk is one of the critical mediators of tumor progression and an emerging facilitator of therapy evasion. Tumor cells that metastasize to adipocyte-rich bone marrow take advantage of the interplay between metabolic and inflammatory pathways to activate prosurvival mechanisms that allow them to thrive and escape therapy. Using in vitro and in vivo models of marrow adiposity, we demonstrate that metastatic prostate carcinoma cells engage bone marrow adipocytes in a functional cross-talk that promotes IL1β expression in tumor cells. Tumor-supplied IL1β contributes to adipocyte lipolysis and regulates a proinflammatory phenotype in adipocytes via upregulation of COX-2 and MCP-1. We further show that the enhanced activity of the IL1β/COX-2/MCP-1 axis and a resulting increase in PGE₂ production by adipocytes coincide with augmented hypoxia signaling and activation of prosurvival pathways in tumor cells, revealing a potential mechanism of chemoresistance. The major consequence of this interplay is the reduced response of prostate cancer cells to docetaxel, a phenomenon sensitive to the inhibition of lipolysis. IMPLICATIONS: Studies presented herein highlight adipocyte lipolysis as a tumor-regulated metabolic event that engages proinflammatory cross-talk in the microenvironment to promote prostate cancer progression in bone. Understanding the impact of bone marrow adipose tissue on tumor adaptation, survival, and chemotherapy response is fundamentally important, as current treatment options for metastatic prostate cancer are palliative.

Bone marrow adipose tissue-derived stem cell factor mediates metabolic regulation of hematopoiesis

Hematopoiesis is dynamically regulated by metabolic cues in homeostatic and stressed conditions; however, the cellular and molecular mechanisms mediating the metabolic sensing and regulation remain largely obscure. Bone marrow adipose tissue remodels in various metabolic conditions and has been recently proposed as a niche for hematopoietic stem cells after irradiation. Here, we investigated the role of marrow adipose tissue-derived hematopoietic cytokine stem cell factor in unperturbed hematopoiesis by selectively ablating the Kitl gene from adipocytes and bone marrow stroma cells using Adipoq-Cre and Osx1-Cre, respectively. We found that both Adipoq-Kitl knockout (KO) and Osx1-Kitl KO mice diminished hematopoietic stem and progenitor cells and myeloid progenitors in the bone marrow and developed macrocytic anemia at the steady-state. The composition and differentiation of hematopoietic progenitor cells in the bone marrow dynamically responded to metabolic challenges including high fat diet, β3-adrenergic activation, thermoneutrality, and aging. However, such responses, particularly within the myeloid compartment, were largely impaired in Adipoq-Kitl KO mice. Our data demonstrate that marrow adipose tissue provides stem cell factor essentially for hematopoiesis both at the steady state and upon metabolic stresses.

The Lipid Side of Bone Marrow Adipocytes: How Tumor Cells Adapt and Survive in Bone

PURPOSE OF REVIEW

Bone marrow adipocytes have emerged in recent years as key contributors to metastatic progression in bone. In this review, we focus specifically on their role as the suppliers of lipids and discuss pro-survival pathways that are closely linked to lipid metabolism, affected by the adipocyte-tumor cell interactions, and likely impacting the ability of the tumor cell to thrive in bone marrow space and evade therapy.

RECENT FINDINGS

The combined in silico, pre-clinical, and clinical evidence shows that in adipocyte-rich tissues such as bone marrow, tumor cells rely on exogenous lipids for regulation of cellular energetics and adaptation to harsh metabolic conditions of the metastatic niche. Adipocyte-supplied lipids have a potential to alter the cell's metabolic decisions by regulating glycolysis and respiration, fatty acid oxidation, lipid desaturation, and PPAR signaling. The downstream effects of lipid signaling on mitochondrial homeostasis ultimately control life vs. death decisions, providing a mechanism for gaining survival advantage and reduced sensitivity to treatment. There is a need for future research directed towards identifying the key metabolic and signaling pathways that regulate tumor dependence on exogenous lipids and consequently drive the pro-survival behavior in the bone marrow niche.