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

The unique role of bone marrow adipose tissue in ovariectomy-induced bone loss in mice

BACKGROUND

Accumulation of bone marrow adipose tissue (BMAT) is always seen in osteoporosis induced by estrogen deficiency. Herein, we aimed to investigate the mechanisms and consequences of this phenomenon by establishing a mouse model of osteoporosis caused by ovariectomy (OVX)-mimicked estrogen deficiency.

METHODS

Micro-CT, osmium tetroxide staining, and histological analyses were performed to examine the changes in bone microstructure, BMAT and white adipose tissue (WAT) in OVX mice compared to sham mice. The osteogenesis and adipogenesis of primary bone marrow stromal cells (BMSCs) isolated from sham and OVX mice were compared in vitro. The molecular phenotypes of BMAT and WAT were determined and compared by quantitative PCR (qPCR). Bone marrow adipocyte-conditioned medium (BMA CM) was prepared from sham or OVX mice for coculture assays, and BMSCs or bone marrow monocytes/macrophages (BMMs) were isolated and subjected to osteoblast and osteoclast differentiation, respectively. Cell staining and qPCR were used to assess the effects of BMAT on bone metabolism.

RESULTS

OVX-induced estrogen deficiency induced reductions in both cortical and trabecular bone mass along with an expansion of BMAT volume. At the cellular level, loss of estrogen inhibited BMSC osteogenesis and promoted BMSC adipogenesis, whereas addition of estradiol exerted the opposite effects. In response to estrogen deficiency, despite the common proinflammatory molecular phenotype observed in both fat depots, BMAT, unlike WAT, unexpectedly exhibited an increase in adipocyte differentiation and lipolytic activity as well as the maintenance of insulin sensitivity. Importantly, BMAT, but not WAT, presented increased mRNA levels of both BMP receptor inhibitors (Grem1, Chrdl1) and Rankl following OVX. In addition, treatment with BMA CM, especially from OVX mice, suppressed the osteoblast differentiation of BMSCs while favoring the osteoclast differentiation of BMMs.

CONCLUSION

Our study illustrates that OVX-induced estrogen deficiency results in bone loss and BMAT expansion by triggering imbalance between the osteogenesis and adipogenesis of BMSCs. Furthermore, expanded BMAT, unlike typical WAT, may negatively regulate bone homeostasis through paracrine inhibition of osteoblast-mediated bone formation and promotion of osteoclast-mediated bone resorption.

Leptin and melatonin's effects on OVX rodents' bone metabolism

Purpose

This study aims to examine the effects of leptin and melatonin intervention on bone metabolism in ovariectomize (OVX) rodents, as well as their potential mechanisms of action.

Methods

Prepare an OVX model of osteoporosis in rodents and validate the model by collecting bilateral tibia samples for Micro-CT scanning and histological analysis. A control group of normal size, the OVX group, the OVX+Sema4D (Semaphorin 4D) group, the OVX+Sema4D+Leptin group, the OVX+Sema4D+ Melatonin(MT) group and the OVX+Sema4D+Leptin+ MT group were the experimental groups. Adenovirus vector construction and tibial medullary injection validation were conducted in accordance with the aforementioned experimental groups. Four groups of rats were injected with the Sema4D overexpression adenovirus vector into the tibial medullary cavity, and two groups were injected with the Leptin overexpression adenovirus vector. The repair of osteoporosis was observed using micro-CT and histological analysis. Immunohistochemical detection of bone morphogenetic protein-2 (BMP-2) expression in bone tissue was employed to ascertain the amount of osteoclasts in the upper tibial metaphysis, utilizing TRAP(tartrate-resistant acid phosphatase) staining.

Results

Increased levels of BV/TV, Tb.N, BMD, and BMC were seen in the OVX+ Sema4D+Leptin, OVX+ Sema4D+MT, and OVX+ Sema4D+Leptin+ MT groups compared to the OVX group, whereas Tb. Sp levels were lowered. When compared to the Sema4D overexpression group, the trabecular bone structure of the OVX + Sema4D + Leptin, OVX + Sema4D + MT, and OVX + Sema4D + Leptin + MT groups is largely intact, tends to be closer, and the amount of trabecular bone increases. The OVX + Sema4D + Leptin + MT group in particular.The expression of BMP-2 was dramatically upregulated (p<0.05), the number of TRAP-stained osteoclasts was significantly reduced (p<0.05), and BALP(bone-derived alkaline phosphatase) and TRAP-5b(tartrate-resistant acid phosphatase-5b) activities were significantly downregulated (p<0.05).

Conclusion

In rats with osteoporosis, leptin and melatonin can be seen to augment the trabecular microstructure of the bone, augment bone growth, diminish trabecular harm, and mend the bone. The combined effect is more powerful.

Common Regulators of Lipid Metabolism and Bone Marrow Adiposity in Postmenopausal Women

A variety of metabolic disorders are associated with a decrease in estradiol (E2) during natural or surgical menopause. Postmenopausal women are prone to excessive fat accumulation in skeletal muscle and adipose tissue due to the loss of E2 via abnormalities in lipid metabolism and serum lipid levels. In skeletal muscle and adipose tissue, genes related to energy metabolism and fatty acid oxidation, such as those encoding peroxisome proliferator-activated receptor-γ coactivator 1α (PGC-1α) and estrogen-related receptor alpha (ERRα), are downregulated, leading to increased fat synthesis and lipid metabolite accumulation. The same genes regulate lipid metabolism abnormalities in the bone marrow. In this review, abnormalities in lipid metabolism caused by E2 deficiency were investigated, with a focus on genes able to simultaneously regulate not only skeletal muscle and adipose tissue but also bone metabolism (e.g., genes encoding PGC-1α and ERRα). In addition, the mechanisms through which mesenchymal stem cells lead to adipocyte differentiation in the bone marrow as well as metabolic processes related to bone marrow adiposity, bone loss, and osteoporosis were evaluated, focusing on the loss of E2 and lipid metabolic alterations. The work reviewed here suggests that genes underlying lipid metabolism and bone marrow adiposity are candidate therapeutic targets for bone loss and osteoporosis in postmenopausal women.

Spatiotemporal correlation between HIF-1α and bone regeneration

Hypoxia-inducible factors (HIFs) are core regulators of the hypoxia response. HIF signaling is activated in the local physiological and pathological hypoxic environment, acting on downstream target genes to synthesize the corresponding proteins and regulate the hypoxic stress response. HIFs belong to the hypoxia-activated transcription family and contain two heterodimeric transcription factors, HIF-α and HIF-β. Under hypoxia, the dimer formed by HIF-α binding to HIF-β translocates into the nucleus and binds to the hypoxia response element (HRE) to induce transcription of a series of genes. HIF-1α plays an important role in innate bone development and acquired bone regeneration. HIF-1α promotes bone regeneration mainly through the following two pathways: (1) By regulating angiogenesis-osteoblast coupling to promote bone regeneration; and (2) by inducing metabolic reprogramming in osteoblasts, promoting cellular anaerobic glycolysis, ensuring the energy supply of osteoblasts under hypoxic conditions, and further promoting bone regeneration and repair. This article reviews recent basic research on HIF-1α and its role in promoting osteogenesis, discusses the possible molecular mechanisms, introduces the hypoxia-independent role of HIF-1α and reviews the application prospects of HIF-1α in tissue engineering.

Leptin and environmental temperature as determinants of bone marrow adiposity in female mice

Bone marrow adipose tissue (BMAT) levels are higher in distal femur metaphysis of female mice housed at thermoneutral (32°C) than in mice housed at 22°C, as are abdominal white adipose tissue (WAT) mass, and serum leptin levels. We performed two experiments to explore the role of increased leptin in temperature-enhanced accrual of BMAT. First, we supplemented 6-week-old female C57BL/6J (B6) mice with leptin for 2 weeks at 10 µg/d using a subcutaneously implanted osmotic pump. Controls consisted of ad libitum (ad lib) fed mice and mice pair fed to match food intake of leptin-supplemented mice. The mice were maintained at 32°C for the duration of treatment. At necropsy, serum leptin in leptin-supplemented mice did not differ from ad lib mice, suggesting suppression of endogenous leptin production. In support, Ucp1 expression in BAT, percent body fat, and abdominal WAT mass were lower in leptin-supplemented mice. Leptin-supplemented mice also had lower BMAT and higher bone formation in distal femur metaphysis compared to the ad lib group, changes not replicated by pair-feeding. In the second experiment, BMAT response was evaluated in 6-week-old female B6 wild type (WT), leptin-deficient ob/ob and leptin-treated (0.3 μg/d) ob/ob mice housed at 32°C for the 2-week duration of the treatment. Compared to mice sacrificed at baseline (22°C), BMAT increased in ob/ob mice as well as WT mice, indicating a leptin independent response to increased temperature. However, infusion of ob/ob mice with leptin, at a dose rate having negligible effects on either energy metabolism or serum leptin levels, attenuated the increase in BMAT. In summary, increased housing temperature and increased leptin have independent but opposing effects on BMAT in mice.

Obesity and Bone Health: A Complex Link

So far, the connections between obesity and skeleton have been extensively explored, but the results are inconsistent. Obesity is thought to affect bone health through a variety of mechanisms, including body weight, fat volume, bone formation/resorption, proinflammatory cytokines together with bone marrow microenvironment. In this review, we will mainly describe the effects of adipokines secreted by white adipose tissue on bone cells, as well as the interaction between brown adipose tissue, bone marrow adipose tissue, and bone metabolism. Meanwhile, this review also reviews the evidence for the effects of adipose tissue and its distribution on bone mass and bone-related diseases, along with the correlation between different populations with obesity and bone health. And we describe changes in bone metabolism in patients with anorexia nervosa or type 2 diabetes. In summary, all of these findings show that the response of skeleton to obesity is complex and depends on diversified factors, such as mechanical loading, obesity type, the location of adipose tissue, gender, age, bone sites, and secreted cytokines, and that these factors may exert a primary function in bone health.

Critical Assessment of In Vitro and In Vivo Models to Study Marrow Adipose Tissue

PURPOSE OF THE REVIEW

The purpose of this review is to describe the in vitro and in vivo methods that researchers use to model and investigate bone marrow adipocytes (BMAds).

RECENT FINDINGS

The bone marrow (BM) niche is one of the most interesting and dynamic tissues of the human body. Relatively little is understood about BMAds, perhaps in part because these cells do not easily survive flow cytometry and histology processing and hence have been overlooked. Recently, researchers have developed in vitro and in vivo models to study normal function and dysfunction in the BM niche. Using these models, scientists and clinicians have noticed that BMAds, which form bone marrow adipose tissue (BMAT), are able to respond to numerous signals and stimuli, and communicate with local cells and distant tissues in the body. This review provides an overview of how BMAds are modeled and studied in vitro and in vivo.

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.

Bone Marrow Adiposity: Basic and Clinical Implications

The presence of adipocytes in mammalian bone marrow (BM) has been recognized histologically for decades, yet, until recently, these cells have received little attention from the research community. Advancements in mouse transgenics and imaging methods, particularly in the last 10 years, have permitted more detailed examinations of marrow adipocytes than ever before and yielded data that show these cells are critical regulators of the BM microenvironment and whole-body metabolism. Indeed, marrow adipocytes are anatomically and functionally separate from brown, beige, and classic white adipocytes. Thus, areas of BM space populated by adipocytes can be considered distinct fat depots and are collectively referred to as marrow adipose tissue (MAT) in this review. In the proceeding text, we focus on the developmental origin and physiologic functions of MAT. We also discuss the signals that cause the accumulation and loss of marrow adipocytes and the ability of these cells to regulate other cell lineages in the BM. Last, we consider roles for MAT in human physiology and disease.