Injections of leptin into rat ventromedial hypothalamus increase adipocyte apoptosis in peripheral fat and in bone marrow

Central activation of catecholamine-independent lipolysis drives the end-stage catabolism of all adipose tissues

Several adipose depots, including constitutive bone marrow adipose tissue (cBMAT), resist conventional lipolytic cues, making them metabolically non-responsive. However, under starvation, wasting, or cachexia, the body can eventually catabolize these stable adipocytes through unknown mechanisms. To study this, we developed a mouse model of brain-evoked depletion of all fat, including cBMAT, independent of food intake. Genetic, surgical, and chemical approaches demonstrated that depletion of stable fat required adipose triglyceride lipase-dependent lipolysis but was independent of local nerves, the sympathetic nervous system, and catecholamines. Instead, concurrent hypoglycemia and hypoinsulinemia activated a potent catabolic state by suppressing lipid storage and increasing catecholamine-independent lipolysis via downregulation of cell-autonomous lipolytic inhibitors Acvr1c, G0s2, and Npr3. This was also sufficient to delipidate classical adipose depots. Overall, this work defines unique adaptations of stable adipocytes to resist lipolysis in healthy states while isolating a potent in vivo neurosystemic pathway by which the body can rapidly catabolize all adipose tissues.

Function and Regulation of Bone Marrow Adipose Tissue in Health and Disease: State of the Field and Clinical Considerations

Bone marrow adipose tissue (BMAT) is a metabolically and clinically relevant fat depot that exists within bone. Two subtypes of BMAT, regulated and constitutive, reside in hematopoietic-rich red marrow and fatty yellow marrow, respectively, and exhibit distinct characteristics compared to peripheral fat such as white and brown adipose tissues. Bone marrow adipocytes (BMAds) are evolutionally preserved in most vertebrates, start development after birth and expand throughout life, and originate from unique progenitor populations that control bone formation and hematopoiesis. Mature BMAds also interact closely with other cellular components of the bone marrow niche, serving as a nearby energy reservoir to support the skeletal system, a signaling hub that contributes to both local and systemic homeostasis, and a final fuel reserve for survival during starvation. Though BMAT and bone are often inversely correlated, more BMAT does not always mean less bone, and the prevention of BMAT expansion as a strategy to prevent bone loss remains questionable. BMAT adipogenesis and lipid metabolism are regulated by the nervous systems and a variety of circulating hormones. This contributes to the plasticity of BMAT, including BMAT expansion in common physiological or pathological conditions, and BMAT catabolism under certain extreme circumstances, which are often associated with malnutrition and/or systemic inflammation. Altogether, this article provides a comprehensive overview of the local and systemic functions of BMAT and discusses the regulation and plasticity of this unique adipose tissue depot in health and disease. © 2024 American Physiological Society. Compr Physiol 14:5521-5579, 2024.

Long-duration leptin transgene expression in dorsal vagal complex does not alter bone parameters in female Sprague Dawley rats

The hypothalamus and dorsal vagal complex (DVC) are both important for integration of signals that regulate energy balance. Increased leptin transgene expression in either the hypothalamus or DVC of female rats was shown to decrease white adipose tissue and circulating levels of leptin and adiponectin. However, in contrast to hypothalamus, leptin transgene expression in the DVC had no effect on food intake, circulating insulin, ghrelin and glucose, nor on thermogenic energy expenditure. These findings imply different roles for hypothalamus and DVC in leptin signaling. Leptin signaling is required for normal bone accrual and turnover. Leptin transgene expression in the hypothalamus normalized the skeletal phenotype of leptin-deficient ob/ob mice but had no long-duration (≥10 weeks) effects on the skeleton of leptin-replete rats. The goal of this investigation was to determine the long-duration effects of leptin transgene expression in the DVC on the skeleton of leptin-replete rats. To accomplish this goal, we analyzed bone from three-month-old female rats that were microinjected with recombinant adeno-associated virus encoding either rat leptin (rAAV-Leptin, n = 6) or green fluorescent protein (rAAV-GFP, control, n = 5) gene. Representative bones from the appendicular (femur) and axial (3rd lumbar vertebra) skeleton were evaluated following 10 weeks of treatment. Selectively increasing leptin transgene expression in the DVC had no effect on femur cortical or cancellous bone microarchitecture. Additionally, increasing leptin transgene expression had no effect on vertebral osteoblast-lined or osteoclast-lined bone perimeter or marrow adiposity. Taken together, the findings suggest that activation of leptin receptors in the DVC has minimal specific effects on the skeleton of leptin-replete female rats.

Minireview: Glucocorticoid-Leptin Crosstalk: Role of Glucocorticoid-Leptin Counterregulation in Metabolic Homeostasis and Normal Development

Glucocorticoids and leptin are two important hormones that regulate metabolic homeostasis by controlling appetite and energy expenditure in adult mammals. Also, glucocorticoids and leptin strongly counterregulate each other, such that chronic stress-induced glucocorticoids upregulate the production of leptin and leptin suppresses glucocorticoid production directly via action on endocrine organs and indirectly via action on food intake. Altered glucocorticoid or leptin levels during development can impair organ development and increase the risk of chronic diseases in adults, but there are limited studies depicting the significance of glucocorticoid-leptin interaction during development and its impact on developmental programming. In mammals, leptin-induced suppression of glucocorticoid production is critical during development, where leptin prevents stress-induced glucocorticoid production by inducing a period of short-hyporesponsiveness when the adrenal glands fail to respond to certain mild to moderate stressors. Conversely, reduced or absent leptin signaling increases glucocorticoid levels beyond what is appropriate for normal organogenesis. The counterregulatory interactions between leptin and glucocorticoids suggest the potential significant involvement of leptin in disorders that occur from stress during development.

Leptin as a key regulator of the adipose organ

Leptin is a hormone primarily produced by the adipose tissue in proportion to the size of fat stores, with a primary function in the control of lipid reserves. Besides adipose tissue, leptin is also produced by other tissues, such as the stomach, placenta, and mammary gland. Altogether, leptin exerts a broad spectrum of short, medium, and long-term regulatory actions at the central and peripheral levels, including metabolic programming effects that condition the proper development and function of the adipose organ, which are relevant for its main role in energy homeostasis. Comprehending how leptin regulates adipose tissue may provide important clues to understand the pathophysiology of obesity and related diseases, such as type 2 diabetes, as well as its prevention and treatment. This review focuses on the physiological and long-lasting regulatory effects of leptin on adipose tissue, the mechanisms and pathways involved, its main outcomes on whole-body physiological homeostasis, and its consequences on chronic diseases.

Neural regulation of bone marrow adipose tissue

Bone marrow adipose tissue (BMAT) is an important cellular component of the skeleton. Understanding how it is regulated by the nervous system is crucial to the study of bone and bone marrow related diseases. BMAT is innervated by sympathetic and sensory axons in bone and fluctuations in local nerve density and function may contribute to its distinct physiologic adaptations at various skeletal sites. BMAT is directly responsive to adrenergic signals. In addition, neural regulation of surrounding cells may modify BMAT-specific responses, providing many potential avenues for both direct and indirect neural regulation of BMAT metabolism. Lastly, BMAT and peripheral adipose tissues share the same autonomic pathways across the central neuraxis and regulation of BMAT may occur in diverse clinical settings of neurologic and metabolic disease. This review will highlight what is known and unknown about the neural regulation of BMAT and discuss opportunities for future research in the field.

Bone Marrow Adipocytes-Role in Physiology and Various Nutritional Conditions in Human and Animal Models

In recent years, adipose tissue has attracted a lot of attention. It is not only an energy reservoir but also plays important immune, paracrine and endocrine roles. BMAT (bone marrow adipose tissue) is a heterogeneous tissue, found mostly in the medullary canal of the long bones (tibia, femur and humerus), in the vertebrae and iliac crest. Adipogenesis in bone marrow cavities is a consequence of ageing or may accompany pathologies like diabetes mellitus type 1 (T1DM), T2DM, anorexia nervosa, oestrogen and growth hormone deficiencies or impaired haematopoiesis and osteoporosis. This paper focuses on studies concerning BMAT and its physiology in dietary interventions, like obesity in humans and high fat diet in rodent studies; and opposite: anorexia nervosa and calorie restriction in animal models.

Lipids in the Bone Marrow: An Evolving Perspective

Because of heavy energy demands to maintain bone homeostasis, the skeletal system is closely tied to whole-body metabolism via neuronal and hormonal mediators. Glucose, amino acids, and fatty acids are the chief fuel sources for bone resident cells during its remodeling. Lipids, which can be mobilized from intracellular depots in the bone marrow, can be a potent source of fatty acids. Thus, while it has been suggested that adipocytes in the bone marrow act as "filler" and are detrimental to skeletal homeostasis, we propose that marrow lipids are, in fact, essential for proper bone functioning. As such, we examine the prevailing evidence regarding the storage, use, and export of lipids within the skeletal niche, including from both in vitro and in vivo model systems. We also highlight the numerous challenges that remain to fully appreciate the relationship of lipid turnover to skeletal homeostasis.

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

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

Hydrogen Sulfide, Adipose Tissue and Diabetes Mellitus

Hydrogen sulfide (H₂S) is now increasingly considered to be the third gasotransmitter alongside other gaseous signaling molecules, nitric oxide (NO) and carbon monoxide (CO). H₂S is produced by a variety of endogenous enzymatic and non-enzymatic pathways and acts as a modulator of the physiological and pathological events of the body. Adipocytes express the cystathionine γ lyase (CSE)/H₂S system, which modulates a variety of biological activities in adipose tissue (AT), including inflammation, apoptosis, insulin resistance, adipokine secretion and adipocyte differentiation. Abnormalities in the physiological functions of AT play an important role in the process of diabetes mellitus. Therefore, this review provides an overview of the general aspects of H₂S biochemistry, the effect of H₂S on AT function and diabetes mellitus and its molecular signalling mechanisms as well as the potential application of H₂S in pharmacotherapy.

Shared Autonomic Pathways Connect Bone Marrow and Peripheral Adipose Tissues Across the Central Neuraxis

Bone marrow adipose tissue (BMAT) is increased in both obesity and anorexia. This is unique relative to white adipose tissue (WAT), which is generally more attuned to metabolic demand. It suggests that there may be regulatory pathways that are common to both BMAT and WAT and also those that are specific to BMAT alone. The central nervous system (CNS) is a key mediator of adipose tissue function through sympathetic adrenergic neurons. Thus, we hypothesized that central autonomic pathways may be involved in BMAT regulation. To test this, we first quantified the innervation of BMAT by tyrosine hydroxylase (TH) positive nerves within the metaphysis and diaphysis of the tibia of B6 and C3H mice. We found that many of the TH+ axons were concentrated around central blood vessels in the bone marrow. However, there were also areas of free nerve endings which terminated in regions of BMAT adipocytes. Overall, the proportion of nerve-associated BMAT adipocytes increased from proximal to distal along the length of the tibia (from ~3-5 to ~14-24%), regardless of mouse strain. To identify the central pathways involved in BMAT innervation and compare to peripheral WAT, we then performed retrograde viral tract tracing with an attenuated pseudorabies virus (PRV) to infect efferent nerves from the tibial metaphysis (inclusive of BMAT) and inguinal WAT (iWAT) of C3H mice. PRV positive neurons were identified consistently from both injection sites in the intermediolateral horn of the spinal cord, reticular formation, rostroventral medulla, solitary tract, periaqueductal gray, locus coeruleus, subcoeruleus, Barrington's nucleus, and hypothalamus. We also observed dual-PRV infected neurons within the majority of these regions. Similar tracings were observed in pons, midbrain, and hypothalamic regions from B6 femur and tibia, demonstrating that these results persist across mouse strains and between skeletal sites. Altogether, this is the first quantitative report of BMAT autonomic innervation and reveals common central neuroanatomic pathways, including putative "command" neurons, involved in coordinating multiple aspects of sympathetic output and facilitation of parallel processing between bone marrow/BMAT and peripheral adipose tissue.

Effects of Leptin on the Skeleton

Leptin originates in adipocytes, including those in bone marrow, and circulates in concentrations 20 to 90 times higher than those in the cerebrospinal fluid. It has direct anabolic effects on osteoblasts and chondrocytes, but it also influences bone indirectly, via the hypothalamus and sympathetic nervous system, via changes in body weight, and via effects on the production of other hormones (e.g., pituitary). Leptin's role in bone physiology is determined by the balance of these conflicting effects. Reflecting this inconsistency, the leptin-deficient mouse has reduced length and bone mineral content of long bones but increased vertebral trabecular bone. A consistent bone phenotype in human leptin deficiency has not been established. Systemic leptin administration in animals and humans usually exerts a positive effect on bone mass, and leptin administration into the cerebral ventricles usually normalizes the bone phenotype in leptin-deficient mice. Reflecting the role of the sympathetic nervous system in mediating the central catabolic effects of leptin on the skeleton, β-adrenergic agonists and antagonists have major effects on bone in mice, but this is not consistently seen in humans. The balance of the central and peripheral effects of leptin on bone remains an area of substantial controversy and might vary between species and according to other factors such as body weight, baseline circulating leptin levels, and the presence of specific pathologies. In humans, leptin is likely to contribute to the positive relationship observed between adiposity and bone density, which allows the skeleton to respond appropriately to changes in soft tissue mass.

Novel leptin receptor signaling mutants identify location and sex-dependent modulation of bone density, adiposity, and growth

Leptin, a hormone primarily produced by adipocytes, contributes to the regulation of bone health by modulating bone density, growth and adiposity. Upon leptin binding, multiple sites of the long form of the leptin receptor (LepRb) are phosphorylated to trigger activation of downstream signaling pathways. To address the role of LepRb-signaling pathways in bone health, we compared the effects of three LepRb mutations on bone density, adiposity, and growth in male and female mice. The ∆65 mutation, which lacks the known tyrosine phosphorylation sites, caused obesity and the most dramatic bone phenotype marked by excessive bone adiposity, osteoporosis, and decreased growth, consistent with the phenotype of db/db and ob/ob mice that fully lack leptin receptor signaling. Mutation of LepRb Tyr ₁₁₃₈ , which results in an inability to recruit and phosphorylate signal transducer and activator of transcription 3, also caused obesity, but bone loss and adiposity were more dominant in male mice and no growth defect was observed. In contrast, mutation of LepRb Tyr ₉₈₅ , which blocks SHP2/SOCS3 recruitment to LepRb and contributes to leptin hypersensitivity, promoted increased femur bone density only in male mice, while marrow adiposity and bone growth were not affected. Additional analyses of vertebral trabecular bone volume indicate that only the Tyr ₁₁₃₈ mutant mice exhibit bone loss in vertebrae. Together, our findings suggest that the phosphorylation status of specific sites of the LepRb contribute to the sex- and location-dependent bone responses to leptin. Unraveling the mechanisms by which leptin responses are sex- and location-dependent can contribute to the development of uniquely targeted osteoporosis therapies.

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

OBJECTIVE

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

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.

Evolution of the Marrow Adipose Tissue Microenvironment

Adipocytes of the marrow adipose tissue (MAT) are distributed throughout the skeleton, are embedded in extracellular matrix, and are surrounded by cells of the hematopoietic and osteogenic lineages. MAT is a persistent component of the skeletal microenvironment and has the potential to impact local processes including bone accrual and hematopoietic function. In this review, we discuss the initial evolution of MAT in vertebrate lineages while emphasizing comparisons to the development of peripheral adipose, hematopoietic, and skeletal tissues. We then apply these evolutionary clues to define putative functions of MAT. Lastly, we explore the regulation of MAT by two major components of its microenvironment, the extracellular matrix and the nerves embedded within. The extracellular matrix and nerves contribute to both rapid and continuous modification of the MAT niche and may help to explain evolutionary conserved mechanisms underlying the coordinated regulation of blood, bone, and MAT within the skeleton.

Association of serum leptin with bone mineral density in postmenopausal osteoporotic females

OBJECTIVE

Present study was designed to find out whether leptin is a predictor of bone mass density (BMD) in premenopausal women (PMW) and postmenopausal osteoporotic women (PMOPW) or it has no association with BMD.

METHODS

One hundred and ninety two women (98 PMOPW and 94 PMW) were recruited for this study. The control group was BMI matched with osteoporotic subjects. BMD assessment was done on calcaneus by peripheral ultrasound bone densitometry and T scores were determined. Serum leptin levels were measured by enzyme-linked immunosorbent assay (ELISA).

RESULTS

Serum leptin and BMD values were significantly different in both groups (leptin, 18.56 ± 8.65 ng/ml versus 21.64 ± 9.80 ng/ml, p = 0.02) and (BMD, -0.70 ± 0.19 versus -3.17 ± 0.59, p = 0.000), respectively. In PMOPW serum leptin and BMD were considerably correlated with weight (lep, r = 0.53, p = <0.001; BMD, r = -0.21, p = 0.02), BMI (lep, r = 0.52, p = <0.001; BMD, r = -0.27, p = 0.005), waist circumference (lep, r = 0.61, p = <0.001; BMD, r = 0.18, p = 0.04), hip circumference (lep, r = 0.58, p = <0.001). Multivariate linear stepwise regression analysis showed that weight and BMI in PMW and PMOPW were independent predictors of BMD. Serum leptin level was not found to be the predictor of BMD in both groups.

CONCLUSION

The present results indicate that body weight and BMI have an impact on BMD while serum leptin is not associated with BMD in PMW and PMOPW.

Role of the Cytokine-like Hormone Leptin in Muscle-bone Crosstalk with Aging

The cytokine-like hormone leptin is a classic adipokine that is secreted by adipocytes, increases with weight gain, and decreases with weight loss. Additional studies have, however, shown that leptin is also produced by skeletal muscle, and leptin receptors are abundant in both skeletal muscle and bone-derived mesenchymal (stromal) stem cells. These findings suggest that leptin may play an important role in muscle-bone crosstalk. Leptin treatment in vitro increases the expression of myogenic genes in primary myoblasts, and leptin treatment in vivo increases the expression of microRNAs involved in myogenesis. Bone marrow adipogenesis is associated with low bone mass in humans and rodents, and leptin can reduce marrow adipogenesis centrally through its receptors in the hypothalamus as well as directly via its receptors in bone marrow stem cells. Yet, central leptin resistance can increase with age, and low circulating levels of leptin have been observed among the frail elderly. Thus, aging appears to significantly alter leptin-mediated crosstalk among various organs and tissues. Aging is associated with bone loss and muscle atrophy, contributing to frailty, postural instability, and the incidence of falls. Therapeutic interventions such as protein and amino acid supplementation that can increase muscle mass and muscle-derived leptin may have multiple benefits for the elderly that can potentially reduce the incidence of falls and fractures.

Daily leptin blunts marrow fat but does not impact bone mass in calorie-restricted mice

Starvation induces low bone mass and high bone marrow adiposity in humans, but the underlying mechanisms are poorly understood. The adipokine leptin falls in starvation, suggesting that hypoleptinemia may be a link between negative energy balance, bone marrow fat accumulation, and impaired skeletal acquisition. In that case, treating mice with leptin during caloric restriction (CR) should reduce marrow adipose tissue (MAT) and improve bone mass. To test this hypothesis, female C57Bl/6J mice were fed a 30% CR or normal (N) diet from 5 to 10 weeks of age, with daily injections of vehicle (VEH), 1mg/kg leptin (LEP1), or 2mg/kg leptin (LEP2) (N=6-8/group). Outcomes included body mass, body fat percentage, and whole-body bone mineral density (BMD) via peripheral dual-energy X-ray absorptiometry, cortical and trabecular microarchitecture via microcomputed tomography (μCT), and MAT volume via μCT of osmium tetroxide-stained bones. Overall, CR mice had lower body mass, body fat percentage, BMD, and cortical bone area fraction, but more connected trabeculae, vs N mice (P<0.05 for all). Most significantly, although MAT was elevated in CR vs N overall, leptin treatment blunted MAT formation in CR mice by 50% vs VEH (P<0.05 for both leptin doses). CR LEP2 mice weighed less vs CR VEH mice at 9-10 weeks of age (P<0.05), but leptin treatment did not affect body fat percentage, BMD, or bone microarchitecture within either diet. These data demonstrate that once daily leptin bolus during CR inhibits bone marrow adipose expansion without affecting bone mass acquisition, suggesting that leptin has distinct effects on starvation-induced bone marrow fat formation and skeletal acquisition.

Marrow Adipose Tissue: Trimming the Fat

Marrow adipose tissue (MAT) is a unique fat depot, located in the skeleton, that has the potential to contribute to both local and systemic metabolic processes. In this review we highlight several recent conceptual developments pertaining to the origin and function of MAT adipocytes; consider the relationship of MAT to beige, brown, and white adipose depots; explore MAT expansion and turnover in humans and rodents; and discuss future directions for MAT research in the context of endocrine function and metabolic disease. MAT has the potential to exert both local and systemic effects on metabolic homeostasis, skeletal remodeling, hematopoiesis, and the development of bone metastases. The diversity of these functions highlights the breadth of the potential impact of MAT on health and disease.

Expansion of Bone Marrow Adipose Tissue During Caloric Restriction Is Associated With Increased Circulating Glucocorticoids and Not With Hypoleptinemia

Bone marrow adipose tissue (MAT) accounts for up to 70% of bone marrow volume in healthy adults and increases further in clinical conditions of altered skeletal or metabolic function. Perhaps most strikingly, and in stark contrast to white adipose tissue, MAT has been found to increase during caloric restriction (CR) in humans and many other species. Hypoleptinemia may drive MAT expansion during CR but this has not been demonstrated conclusively. Indeed, MAT formation and function are poorly understood; hence, the physiological and pathological roles of MAT remain elusive. We recently revealed that MAT contributes to hyperadiponectinemia and systemic adaptations to CR. To further these observations, we have now performed CR studies in rabbits to determine whether CR affects adiponectin production by MAT. Moderate or extensive CR decreased bone mass, white adipose tissue mass, and circulating leptin but, surprisingly, did not cause hyperadiponectinemia or MAT expansion. Although this unexpected finding limited our subsequent MAT characterization, it demonstrates that during CR, bone loss can occur independently of MAT expansion; increased MAT may be required for hyperadiponectinemia; and hypoleptinemia is not sufficient for MAT expansion. We further investigated this relationship in mice. In females, CR increased MAT without decreasing circulating leptin, suggesting that hypoleptinemia is also not necessary for MAT expansion. Finally, circulating glucocorticoids increased during CR in mice but not rabbits, suggesting that glucocorticoids might drive MAT expansion during CR. These observations provide insights into the causes and consequences of CR-associated MAT expansion, knowledge with potential relevance to health and disease.

Hypothalamic Leptin Gene Therapy Reduces Bone Marrow Adiposity in ob/ob Mice Fed Regular and High-Fat Diets

Low bone mass is often associated with elevated bone marrow adiposity. Since osteoblasts and adipocytes are derived from the same mesenchymal stem cell (MSC) progenitor, adipocyte formation may increase at the expense of osteoblast formation. Leptin is an adipocyte-derived hormone known to regulate energy and bone metabolism. Leptin deficiency and high-fat diet-induced obesity are associated with increased marrow adipose tissue (MAT) and reduced bone formation. Short-duration studies suggest that leptin treatment reduces MAT and increases bone formation in leptin-deficient ob/ob mice fed a regular diet. Here, we determined the long-duration impact of increased hypothalamic leptin on marrow adipocytes and osteoblasts in ob/ob mice following recombinant adeno-associated virus (rAAV) gene therapy. Eight- to 10-week-old male ob/ob mice were randomized into four groups: (1) untreated, (2) rAAV-Lep, (3) rAAV-green fluorescent protein (rAAV-GFP), or (4) pair-fed to rAAV-Lep. For vector administration, mice were injected intracerebroventricularly with either rAAV-leptin gene therapy (rAAV-Lep) or rAAV-GFP (9 × 10(7) particles) and maintained for 30 weeks. In a second study, the impact of increased hypothalamic leptin levels on MAT was determined in mice fed high-fat diets; ob/ob mice were randomized into two groups and treated with either rAAV-Lep or rAAV-GFP. At 7 weeks post-vector administration, half the mice in each group were switched to a high-fat diet for 8 weeks. Wild-type (WT) controls included age-matched mice fed regular or high-fat diet. High-fat diet resulted in a threefold increase in MAT in WT mice, whereas MAT was increased by leptin deficiency up to 50-fold. Hypothalamic leptin gene therapy increased osteoblast perimeter and osteoclast perimeter with minor change in cancellous bone architecture. The gene therapy decreased MAT levels in ob/ob mice fed regular or high-fat diet to values similar to WT mice fed regular diet. These findings suggest that leptin plays an important role in regulating the differentiation of MSCs to adipocytes and osteoblasts, a process that may be dysregulated by high-fat diet. However, the results also illustrate that reducing MAT by increasing leptin levels does not necessarily result in increased bone mass.

Lipid metabolism disorders and bone dysfunction--interrelated and mutually regulated (review)

The association between lipid and bone metabolism has become an increasing focus of interest in recent years, and accumulating evidence has shown that atherosclerosis (AS) and osteoporosis (OP), a disorder of bone metabolism, frequently co-exist. Fat and bone are known to share a common progenitor cell: Multipotent mesenchymal stem cells (MSC) in the bone marrow (BM), which are able to differentiate into various cell phenotypes, including osteoblasts, adipocytes and chondrocytes. Laboratory-based and clinical trials have shown that increasing adipocytes are accompanied by a decrease in bone mineral density (BMD) and bone mass. Statins, lipid-lowering drugs used to treat hyperlipidemia, also provide benefit in the treatment of OP. There is thus evidence that the metabolism of lipids is correlated with that of bone, and that the two are mutually regulated. The present review primarily focuses on the potential association between lipid metabolism disturbance and OP, based on biological metabolism, pathophysiological processes, results from clinical and experimental animal studies, processes involved in the differentiation of adipocytes and osteoblasts, as well as pharmacological treatments of these diseases.

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

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

What's the matter with MAT? Marrow adipose tissue, metabolism, and skeletal health

Marrow adipose tissue (MAT) is functionally distinct from both white and brown adipose tissue and can contribute to systemic and skeletal metabolism. MAT formation is a spatially and temporally defined developmental event, suggesting that MAT is an organ that serves important functions and, like other organs, can undergo pathologic change. The well-documented inverse relationship between MAT and bone mineral density has been interpreted to mean that MAT removal is a possible therapeutic target for osteoporosis. However, the bone and metabolic phenotypes of patients with lipodystrophy argues that retention of MAT may actually be beneficial in some circumstances. Furthermore, MAT may exist in two forms, regulated and constitutive, with divergent responses to hematopoietic and nutritional demands. In this review, we discuss the role of MAT in lipodystrophy, bone loss, and metabolism, and highlight our current understanding of this unique adipose tissue depot.

Delayed bone regeneration is linked to chronic inflammation in murine muscular dystrophy

Duchenne muscular dystrophy (DMD) patients exhibit skeletal muscle weakness with continuous cycles of muscle fiber degeneration/regeneration, chronic inflammation, low bone mineral density, and increased risks of fracture. Fragility fractures and associated complications are considered as a consequence of the osteoporotic condition in these patients. Here, we aimed to establish the relationship between muscular dystrophy and fracture healing by assessing bone regeneration in mdx mice, a model of DMD with absence of osteoporosis. Our results illustrate that muscle defects in mdx mice impact the process of bone regeneration at various levels. In mdx fracture calluses, both cartilage and bone deposition were delayed followed by a delay in cartilage and bone remodeling. Vascularization of mdx fracture calluses was also decreased during the early stages of repair. Dystrophic muscles are known to contain elevated numbers of macrophages contributing to muscle degeneration. Accordingly, we observed increased macrophage recruitment in the mdx fracture calluses and abnormal macrophage accumulation throughout the process of bone regeneration. These changes in the inflammatory environment subsequently had an impact on the recruitment of osteoclasts and the remodeling phase of repair. Further damage to the mdx muscles, using a novel model of muscle trauma, amplified both the chronic inflammatory response and the delay in bone regeneration. In addition, PLX3397 treatment of mdx mice, a cFMS (colony stimulating factor receptor 1) inhibitor in monocytes, partially rescued the bone repair defect through increasing cartilage deposition and decreasing the number of macrophages. In conclusion, chronic inflammation in mdx mice contributes to the fracture healing delay and is associated with a decrease in angiogenesis and a transient delay in osteoclast recruitment. By revealing the role of dystrophic muscle in regulating the inflammatory response during bone repair, our results emphasize the implication of muscle in the normal bone repair process and may lead to improved treatment of fragility fractures in DMD patients.

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

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

Leptin in human physiology and pathophysiology

Leptin, discovered through positional cloning 15 years ago, is an adipocyte-secreted hormone with pleiotropic effects in the physiology and pathophysiology of energy homeostasis, endocrinology, and metabolism. Studies in vitro and in animal models highlight the potential for leptin to regulate a number of physiological functions. Available evidence from human studies indicates that leptin has a mainly permissive role, with leptin administration being effective in states of leptin deficiency, less effective in states of leptin adequacy, and largely ineffective in states of leptin excess. Results from interventional studies in humans demonstrate that leptin administration in subjects with congenital complete leptin deficiency or subjects with partial leptin deficiency (subjects with lipoatrophy, congenital or related to HIV infection, and women with hypothalamic amenorrhea) reverses the energy homeostasis and neuroendocrine and metabolic abnormalities associated with these conditions. More specifically, in women with hypothalamic amenorrhea, leptin helps restore abnormalities in hypothalamic-pituitary-peripheral axes including the gonadal, thyroid, growth hormone, and to a lesser extent adrenal axes. Furthermore, leptin results in resumption of menses in the majority of these subjects and, in the long term, may increase bone mineral content and density, especially at the lumbar spine. In patients with congenital or HIV-related lipoatrophy, leptin treatment is also associated with improvements in insulin sensitivity and lipid profile, concomitant with reduced visceral and ectopic fat deposition. In contrast, leptin's effects are largely absent in the obese hyperleptinemic state, probably due to leptin resistance or tolerance. Hence, another emerging area of research pertains to the discovery and/or usefulness of leptin sensitizers. Results from ongoing studies are expected to further increase our understanding of the role of leptin and the potential clinical applications of leptin or its analogs in human therapeutics.

Long-term metreleptin treatment increases bone mineral density and content at the lumbar spine of lean hypoleptinemic women

Strenuously exercising young women with hypothalamic amenorrhea are hypoleptinemic and have low bone mineral density (BMD) and content (BMC), which predispose them to increased fracture risk. Short-term leptin replacement in these women corrects many neuroendocrine abnormalities and increases circulating levels of bone formation markers. Whether treatment with recombinant methionyl human leptin (metreleptin) for a long period improves BMD and BMC remains unknown. We studied 20 strenuously exercising young women with hypoleptinemia (leptin concentration <5 ng/mL) and hypothalamic amenorrhea of at least 6 months' duration. Eleven were randomized to metreleptin (initial dose, 0.08 mg/[kg·d] for 3 months; altered thereafter to 0.12 mg/kg for lack of efficacy or 0.04 mg/[kg d] for more than 5% weight loss) and 9 were randomized to placebo for 9 months. After a 3-month washout period, subjects were reexamined at the 1-year time point. Six subjects elected to continue on open-label metreleptin treatment for another 12 months. Two subjects dropped out after 18 months, and 4 completed the entire 2-year study. The BMD and BMC of the total body, lumbar spine (L1-L4), hip, and radius were assessed by using dual-energy x-ray absorptiometry at baseline and at 3, 6, 9, 12, 18, and 24 months of treatment. Metabolic and hormonal parameters and bone markers were measured in blood and urine. Metreleptin significantly increased BMC (P = .034) and tended to increase BMD (P = .069) at the lumbar spine at 9 months in the entire study group (intention-to-treat analysis). In subjects who completed the entire 2-year study (n = 4), metreleptin significantly increased BMD (P = .024) and BMC (P = .049) at the lumbar spine by 4% to 6%. Changes were not significant at the whole body, hip, and radius. Changes in hormonal and metabolic parameters and bone markers were moderate during the first year of treatment, but metreleptin further increased insulin-like growth factor 1 and decreased cortisol and cross-linked C-terminal telopeptide of type 1 collagen concentrations in serum during the second year of treatment (P < .05). The incremental area under the estradiol concentration curve over the 2-year course of the study correlated positively with the corresponding increase in lumbar spine BMD (ρ = 0.42, P = .039). Long-term metreleptin administration in strenuously exercising young women with hypothalamic amenorrhea and hypoleptinemia increases lumbar spine BMD and BMC and alters bone remodeling milieu to favor bone accretion. Results from this pilot study should be confirmed by future, larger clinical trials and need to be extended by studying bone microarchitecture and fracture risk.

Central (ICV) leptin injection increases bone formation, bone mineral density, muscle mass, serum IGF-1, and the expression of osteogenic genes in leptin-deficient ob/ob mice

Both central and peripheral leptin administrations reduce body weight, food intake, and adiposity in ob/ob mice. In this study we compared effects of intracerebroventricular (ICV) and subcutaneous (SC) administration of leptin on bone metabolism in the appendicular and axial skeleton and adipose tissue gene expression and determined the effects of ICV leptin on bone marrow gene expression in ob/ob mice. In experiment 1, leptin (1.5 or 0.38 µg/d) or control was continuously injected ICV for 12 days. Gene expression analysis of femoral bone marrow stromal cells showed that expression of genes associated with osteogenesis was increased after ICV injection, whereas those associated with osteoclastogenesis, adipogenesis, and adipocyte lipid storage were decreased. In experiment 2, leptin was injected continuously ICV (0.0 or 1.5 µg/d) or SC (0.0 or 10 µg/d) for 12 days. In both experiments, regardless of mode of administration, leptin decreased body weight, food intake, and body fat and increased muscle mass, bone mineral density, bone mineral content, bone area, marrow adipocyte number, and mineral apposition rate. Serum insulin was decreased, whereas serum osteocalcin, insulin-like growth factor 1, osteoprotegerin, pyridinoline, and receptor activator of nuclear factor κB ligand concentrations were increased. In experiment 2, expression of genes in adipose tissue associated with apoptosis, lipid mobilization, insulin sensitivity, and thermogenesis was increased, whereas expression of genes associated with cell differentiation and maturation was decreased regardless of mode of administration. Thus ICV injection of leptin promotes expression of pro-osteogenic factors in bone marrow, leading to enhanced bone formation in ob/ob mice.

Effects of increased hypothalamic leptin gene expression on ovariectomy-induced bone loss in rats

Estrogen deficiency results in accelerated bone turnover with a net increase in bone resorption. Subcutaneous administration of leptin attenuates bone loss in ovariectomized (ovx) rats by reducing bone resorption. However, in addition to its direct beneficial effects, leptin has been reported to have indirect (central nervous system-mediated) antiosteogenic effects on bone, which may limit the efficacy of elevated serum leptin to prevent estrogen deficiency-associated bone loss. The present study evaluated the long-term effects of increased hypothalamic leptin transgene expression, using recombinant adeno-associated virus-leptin (rAAV-Lep) gene therapy, on bone mass, architecture, and cellular endpoints in sexually mature ovx Sprague-Dawley rats. Ovx rats were implanted with cannulae in the 3rd ventricle of the hypothalamus and injected with either rAAV-Lep or rAAV-GFP (control vector encoding green fluorescent protein) and maintained for 10 weeks. Additional controls consisted of ovary-intact rats and ovx rats pair-fed to rAAV-Lep rats. Lumbar vertebrae were analyzed by micro-computed tomography and tibiae by histomorphometry. Cancellous bone volume was lower and osteoclast perimeter, osteoblast perimeter, and bone marrow adipocyte density were greater in ovx rats compared to ovary-intact controls. In contrast, differences among ovx groups were not detected for any endpoint evaluated. In conclusion, whereas estrogen deficiency resulted in marked cancellous osteopenia, increased bone turnover and marrow adiposity, increasing hypothalamic leptin transgene expression in ovx rats had neither detrimental nor beneficial effects on bone mass, architecture, or cellular endpoints. These findings demonstrate that the antiresorptive effects of subcutaneous leptin administration in ovx rats are mediated through leptin targets in the periphery.

Why does starvation make bones fat?

Body fat, or adipose tissue, is a crucial energetic buffer against starvation in humans and other mammals, and reserves of white adipose tissue (WAT) rise and fall in parallel with food intake. Much less is known about the function of bone marrow adipose tissue (BMAT), which are fat cells found in bone marrow. BMAT mass actually increases during starvation, even as other fat depots are being mobilized for energy. This review considers several possible reasons for this poorly understood phenomenon. Is BMAT a passive filler that occupies spaces left by dying bone cells, a pathological consequence of suppressed bone formation, or potentially an adaptation for surviving starvation? These possibilities are evaluated in terms of the effects of starvation on the body, particularly the skeleton, and the mechanisms involved in storing and metabolizing BMAT during negative energy balance.

Energy Balance, Myostatin, and GILZ: Factors Regulating Adipocyte Differentiation in Belly and Bone

Peroxisome proliferator-activated receptor gamma (PPAR-gamma) belongs to the nuclear hormone receptor subfamily of transcription factors. PPARs are expressed in key target tissues such as liver, fat, and muscle and thus they play a major role in the regulation of energy balance. Because of PPAR-gamma's role in energy balance, signals originating from the gut (e.g., GIP), fat (e.g., leptin), muscle (e.g., myostatin), or bone (e.g., GILZ) can in turn modulate PPAR expression and/or function. Of the two PPAR-gamma isoforms, PPAR-gamma2 is the key regulator of adipogenesis and also plays a role in bone development. Activation of this receptor favors adipocyte differentiation of mesenchymal stem cells, while inhibition of PPAR-gamma2 expression shifts the commitment towards the osteoblastogenic pathway. Clinically, activation of this receptor by antidiabetic agents of the thiazolidinedione class results in lower bone mass and increased fracture rates. We propose that inhibition of PPAR-gamma2 expression in mesenchymal stem cells by use of some of the hormones/factors mentioned above may be a useful therapeutic strategy to favor bone formation.

Hypothalamic leptin gene therapy prevents weight gain without long-term detrimental effects on bone in growing and skeletally mature female rats

Hypothalamic leptin gene therapy normalizes the mosaic skeletal phenotype of leptin-deficient ob/ob mice. However, it is not clear whether increased hypothalamic leptin alters bone metabolism in animals already producing the hormone. The objective of this study was to evaluate the long duration effects of recombinant adeno-associated virus-rat leptin (rAAV-Lep) hypothalamic gene therapy on weight gain and bone metabolism in growing and skeletally mature leptin-replete female Sprague-Dawley rats. Rats were either unoperated or implanted with cannulas in the third ventricle of the hypothalamus and injected with either rAAV-Lep or rAAV-GFP (control vector encoding green fluorescent protein) and maintained on standard rat chow fed ad libitum for either 5 or 10 weeks (starting at 3 months of age) or 18 weeks (starting at 9 months of age). Tibias, femurs, or lumbar vertebrae were analyzed by micro-computed tomography and/or histomorphometry. In comparison with age-matched rAAV-GFP rats, rAAV-Lep rats maintained a lower body weight for the duration of studies. At 5 weeks after vector administration, rAAV-Lep rats had lower cancellous bone volume and bone marrow adiposity but higher osteoblast perimeter compared with nonoperated controls. However, these values did not differ between the two groups at 10 weeks after vector administration. Differences in cancellous bone volume and architecture were not detected between the rAAV-Lep and rAAV-GFP groups at either time point. Also, rAAV-Lep had no negative effects on bone in the 9-month-old skeletally mature rats at 18 weeks after vector administration. We hypothesize that the transient reductions in bone mass and bone marrow adiposity at 5 weeks after vector administration were due to hypothalamic surgery. We conclude that increased hypothalamic leptin, sufficient to prevent weight gain, has minimal specific effects (rAAV-Lep versus rAAV-GFP) on bone metabolism in normal female rats.

Central insulin and leptin-mediated autonomic control of glucose homeostasis

Largely as a result of rising obesity rates, the incidence of type 2 diabetes is escalating rapidly. Type 2 diabetes results from multi-organ dysfunctional glucose metabolism. Recent publications have highlighted hypothalamic insulin- and adipokine-sensing as a major determinant of peripheral glucose and insulin responsiveness. The preponderance of evidence indicates that the brain is the master regulator of glucose homeostasis, and that hypothalamic insulin and leptin signaling in particular play a crucial role in the development of insulin resistance. This review discusses the neuronal crosstalk between the hypothalamus, autonomic nervous system, and tissues associated with the pathogenesis of type 2 diabetes, and how hypothalamic insulin and leptin signaling are integral to maintaining normal glucose homeostasis.

Leptin is an effective treatment for hypothalamic amenorrhea

Hypothalamic amenorrhea (HA) is associated with dysfunction of the hypothalamic-pituitary-peripheral endocrine axes, leading to infertility and bone loss, and usually is caused by chronic energy deficiency secondary to strenuous exercise and/or decreased food intake. Energy deficiency also leads to hypoleptinemia, which has been proposed, on the basis of observational studies as well as an open-label study, to mediate the neuroendocrine abnormalities associated with this condition. To prove definitively a causal role of leptin in the pathogenesis of HA, we performed a randomized, double-blinded, placebo-controlled trial of human recombinant leptin (metreleptin) in replacement doses over 36 wk in women with HA. We assessed its effects on reproductive outcomes, neuroendocrine function, and bone metabolism. Leptin replacement resulted in recovery of menstruation and corrected the abnormalities in the gonadal, thyroid, growth hormone, and adrenal axes. We also demonstrated changes in markers of bone metabolism suggestive of bone formation, but no changes in bone mineral density were detected over the short duration of this study. If these data are confirmed, metreleptin administration in replacement doses to normalize circulating leptin levels may prove to be a safe and effective therapy for women with HA.

Anorexia nervosa: a unified neurological perspective

The roles of corticotrophin-releasing factor (CRF), opioid peptides, leptin and ghrelin in anorexia nervosa (AN) were discussed in this paper. CRF is the key mediator of the hypothalamo-pituitary-adrenal (HPA) axis and also acts at various other parts of the brain, such as the limbic system and the peripheral nervous system. CRF action is mediated through the CRF1 and CRF2 receptors, with both HPA axis-dependent and HPA axis-independent actions, where the latter shows nil involvement of the autonomic nervous system. CRF1 receptors mediate both the HPA axis-dependent and independent pathways through CRF, while the CRF2 receptors exclusively mediate the HPA axis-independent pathways through urocortin. Opioid peptides are involved in the adaptation and regulation of energy intake and utilization through reward-related behavior. Opioids play a role in the addictive component of AN, as described by the "auto-addiction opioids theory". Their interactions have demonstrated the psychological aspect of AN and have shown to prevent the functioning of the physiological homeostasis. Important opioids involved are β-lipotropin, β-endorphin and dynorphin, which interact with both µ and κ opioids receptors to regulate reward-mediated behavior and describe the higher incidence of AN seen in females. Moreover, ghrelin is known as the "hunger" hormone and helps stimulate growth hormone (GH) and hepatic insulin-like-growth-factor-1(IGF-1), maintaining anabolism and preserving a lean body mass. In AN, high levels of GH due to GH resistance along with low levels of IGF-1 are observed. Leptin plays a role in suppressing appetite through the inhibition of neuropeptide Y gene. Moreover, the CRF, opioid, leptin and ghrelin mechanisms operate collectively at the HPA axis and express the physiological and psychological components of AN. Fear conditioning is an intricate learning process occurring at the level of the hippocampus, amygdala, lateral septum and the dorsal raphe by involving three distinct pathways, the HPA axis-independent pathway, hypercortisolemia and ghrelin. Opioids mediate CRF through noradrenergic stimulation in association with the locus coeruleus. Furthermore, CRF's inhibitory effect on gonadotropin releasing hormone can be further explained by the direct relationship seen between CRF and opioids. Low levels of gonadotropin have been demonstrated in AN where only estrogen has shown to mediate energy intake. In addition, estrogen is involved in regulating µ receptor concentrations, but in turn both CRF and opioids regulate estrogen. Moreover, opioids and leptin are both an effect of AN, while many studies have demonstrated a causal relationship between CRF and anorexic behavior. Moreover, leptin, estrogen and ghrelin play a role as predictors of survival in starvation. Since both leptin and estrogen are associated with higher levels of bone marrow fat they represent a longer survival than those who favor the ghrelin pathway. Future studies should consider cohort studies involving prepubertal males and females with high CRF. This would help prevent the extrapolation of results from studies on mice and draw more meaningful conclusions in humans. Studies should also consider these mechanisms in post-AN patients, as well as look into what predisposes certain individuals to develop AN. Finally, due to its complex pathogenesis the treatment of AN should focus on both the pharmacological and behavioral perspectives.

Leptin in human physiology and therapeutics

Leptin regulates energy homeostasis and reproductive, neuroendocrine, immune, and metabolic functions. In this review, we describe the role of leptin in human physiology and review evidence from recent "proof of concept" clinical trials using recombinant human leptin in subjects with congenital leptin deficiency, hypoleptinemia associated with energy-deficient states, and hyperleptinemia associated with garden-variety obesity. Since most obese individuals are largely leptin-tolerant or -resistant, therapeutic uses of leptin are currently limited to patients with complete or partial leptin deficiency, including hypothalamic amenorrhea and lipoatrophy. Leptin administration in these energy-deficient states may help restore associated neuroendocrine, metabolic, and immune function and bone metabolism. Leptin treatment is currently available for individuals with congenital leptin deficiency and congenital lipoatrophy. The long-term efficacy and safety of leptin treatment in hypothalamic amenorrhea and acquired lipoatrophy are currently under investigation. Whether combination therapy with leptin and potential leptin sensitizers will prove effective in the treatment of garden-variety obesity and whether leptin may have a role in weight loss maintenance is being greatly anticipated.

Central leptin versus ghrelin: effects on bone marrow adiposity and gene expression

This study compared the central effects of ghrelin and leptin on body and bone marrow adiposity and gene expression in adipose tissue and bone marrow. Male Sprague-Dawley rats were injected intracerebroventricular (ICV) twice daily with control, 66 ng ghrelin (G66), 330 ng ghrelin (G330), or 5 μg leptin (L5) for 5 days. Food intake (FI) and body weight (BW) were measured daily. Gene expression in adipose tissue and bone marrow was assessed using RT-PCR. Leptin reduced FI (P < 0.05) and BW (P < 0.05), whereas ghrelin increased BW (P < 0.05) without affecting FI. Leptin decreased fat pad weights, whereas ghrelin (G330) increased fat pad weights (P < 0.05). In epididymal adipose tissue, leptin increased expression of lipolysis marker ADRB2 and thermogenesis marker MFN2 and decreased expression of adipogenic markers, FASN, SLC2A4, and SCD1, whereas ghrelin increased expression of FASN and SCD1. Leptin decreased bone marrow adipocyte size and number; however, ghrelin had no effect on these parameters. In whole bone marrow, leptin decreased expression of FASN and SCD1 and increased expression of DLK1, whereas ghrelin (G330) decreased expression of COL1A1. Thus, leptin induces similar changes in bone marrow and adipose tissue gene expression, reflecting the decreased adiposity in both compartments.

Exogenous leptin administration decreases sperm count and increases the fraction of abnormal sperm in adult rats

Daily intraperitoneal injection of 5-30 microg/kg body weight of leptin for 42 days to adult rats decreases sperm count and increases the fraction of abnormal sperm.

Leptin treatment prevents type I diabetic marrow adiposity but not bone loss in mice

Leptin is a hormone secreted by adipocytes that is implicated in the regulation of bone density. Serum leptin levels are decreased in rodent models of type 1 (T1-) diabetes and in diabetic patients. Whether leptin mediates diabetic bone changes is unclear. Therefore, we treated control and T1-diabetic mice with chronic (28 days) subcutaneous infusion of leptin or saline to elucidate the therapeutic potential of leptin for diabetic osteoporosis. Leptin prevented the increase of marrow adipocytes and the increased aP2 expression that we observed in vehicle-treated diabetic mice. However, leptin did not prevent T1-diabetic decreases in trabecular bone volume fraction or bone mineral density in tibia or vertebrae. Consistent with this finding, markers of bone formation (osteocalcin RNA and serum levels) in diabetic mice were not restored to normal levels with leptin treatment. Interestingly, markers of bone resorption (TRAP5 RNA and serum levels) were decreased in diabetic mice by leptin treatment. In summary, we have demonstrated a link between low leptin levels in T1-diabetes and marrow adiposity. However, leptin treatment alone was not successful in preventing bone loss.

Leptin and the sympathetic connection of fat to bone

Loss of body weight is associated with bone loss, and body weight gain is associated with increased bone formation. The molecular mechanisms linking body weight, body composition, and bone density are now better understood. Lean mass is likely to have a significant, local effect on bone modeling and remodeling through mechanotransduction pathways. In contrast to the local regulation of bone formation and resorption by muscle-derived stimuli, peripheral body fat appears to influence bone mass via secretion of systemic, endocrine factors that link body weight to bone density even in non-weight bearing regions (e.g., the forearm). The cytokine-like hormone leptin, which is secreted by fat cells, is an important candidate molecule linking changes in body composition with bone formation and bone resorption. Increases in body fat increase leptin levels and stimulate periosteal bone formation through its direct anabolic effects on osteoblasts, and through central (CNS) effects including the stimulation of the GH-IGF-1 axis and suppression of neuropeptide Y, a powerful inhibitor of bone formation. Stimulation of beta2-adrenergic receptors through central (hypothalamic) leptin receptors does, however, increase remodeling of trabecular bone, resulting in a lower cancellous bone volume that may be better adapted to a concomitantly larger cortical bone compartment. These findings suggest that body weight and body fat can regulate bone mass and structure through molecular pathways that are independent of load-bearing. Furthermore, pharmacological manipulation of the signaling pathways activated by leptin may have significant potential for the treatment and prevention of bone loss.

Leptin as an endocrine signal in bone

Leptin and its actions in bone came to prominence in 2000, with the publication of two landmark articles identifying a novel interaction between energy and bone homeostasis, as well as a novel hypothalamic circuit to the skeleton. However, they also revealed the dichotomous nature of leptin's effect on the skeleton. Subsequent research has increased understanding of the factors critical to interpretation of the leptin-bone signaling. These include opposing effects in cortical and cancellous bone, central and peripheral effects, involvement of other neural and endocrine factors, and leptin receptor polymorphisms in human populations. It is clear that leptin can markedly influence the regulation of bone mass, and that study of this pathway continues to increase our knowledge of the biology of skeletal tissue and its interactions with other tissues. However, this relationship is complex and requires careful interpretation.

Caloric restriction decreases cortical bone mass but spares trabecular bone in the mouse skeleton: implications for the regulation of bone mass by body weight

INTRODUCTION

Body weight is positively correlated with bone mass and density, and both muscle mass and body fat are thought to play a role in regulating bone metabolism. We examined bone metabolism in calorically restricted mice to determine how alterations in soft tissue mass affect bone mass, density, and strength.

MATERIALS AND METHODS

Caloric restriction (CR) was initiated in male mice at 14 wk of age at 10% restriction, increased to 25% restriction at 15 wk, and then increased to 40% restriction at 16 wk, where it was maintained until 24 wk of age when the study was terminated. Control mice were fed ad libitum (AL). Body composition, BMD, and BMC were measured by DXA, BMD and BMC in the femoral metaphysis were measured by pQCT, femora were tested in three-point bending, serum leptin and IGF-1 were measured using immunoassay, and osteoblast and osteoclast numbers were determined using histomorphometry.

RESULTS

Body weight, lean mass, fat mass, percent body fat, serum leptin, and serum IGF-1 were all significantly lower in CR mice than AL mice. Whole body BMC and BMD did not differ significantly between the two groups. Femur BMC, BMD, cortical thickness, and fracture strength decreased significantly in CR mice, but trabecular bone volume fraction in the femur did not change with food restriction. Vertebral cortical thickness also decreased with caloric restriction, whereas spine BMC, BMD, and trabecular bone volume fraction were significantly increased with caloric restriction.

CONCLUSIONS

Caloric restriction and its related weight reduction are associated with marked decreases in lean mass, fat mass, serum leptin and IGF-1, and cortical bone mass. Consistent with the opposite effects of leptin on cortical and cancellous bone, trabecular bone mass is spared during food restriction.

Age-related changes in bone morphology are accelerated in group VIA phospholipase A2 (iPLA2beta)-null mice

Phospholipases A(2) (PLA(2)) hydrolyze the sn-2 fatty acid substituent, such as arachidonic acid, from phospholipids, and arachidonate metabolites are recognized mediators of bone modeling. We have previously generated knockout (KO) mice lacking the group VIA PLA(2) (iPLA(2)beta), which participates in a variety of signaling events; iPLA(2)beta mRNA is expressed in bones of wild-type (WT) but not KO mice. Cortical bone size, trabecular bone volume, bone mineralizing surfaces, and bone strength are similar in WT and KO mice at 3 months and decline with age in both groups, but the decreases are more pronounced in KO mice. The lower bone mass phenotype observed in KO mice is not associated with an increase in osteoclast abundance/activity or a decrease in osteoblast density, but is accompanied by an increase in bone marrow fat. Relative to WT mice, undifferentiated bone marrow stromal cells (BMSCs) from KO mice express higher levels of PPAR-gamma and lower levels of Runx2 mRNA, and this correlates with increased adipogenesis and decreased osteogenesis in BMSCs from these mice. In summary, our studies indicate that age-related losses in bone mass and strength are accelerated in iPLA(2)beta-null mice. Because adipocytes and osteoblasts share a common mesenchymal stem cell origin, our findings suggest that absence of iPLA(2)beta causes abnormalities in osteoblast function and BMSC differentiation and identify a previously unrecognized role of iPLA(2)beta in bone formation.

Central nervous system regulation of adipocyte metabolism

The white adipose tissue was initially largely known only as an energy storage tissue. It is now well recognized that white adipose tissue is a major endocrine and secretory organ, which releases a wide range of protein signals and factors termed adipokines. The regulation of adipocyte metabolism is an important factor for the understanding of obesity, and some mechanisms are still unknown. Many homeostatic processes, including appetite and food intake, are controlled by neuroendocrine circuits involving the central nervous system. There is substantial evidence demonstrating that the central nervous system also directly regulates adipocyte metabolism. In this review, we discuss the central actions of some peptides with an important role in energy balance regulation on adipocyte metabolism and the physiological relevance of these actions.