Leptin predicts BMD and bone resorption in older women but not older men: the Rancho Bernardo study

Crosstalk Between the Neuroendocrine System and Bone Homeostasis

The homeostasis of bone microenvironment is the foundation of bone health and comprises 2 concerted events: bone formation by osteoblasts and bone resorption by osteoclasts. In the early 21st century, leptin, an adipocytes-derived hormone, was found to affect bone homeostasis through hypothalamic relay and the sympathetic nervous system, involving neurotransmitters like serotonin and norepinephrine. This discovery has provided a new perspective regarding the synergistic effects of endocrine and nervous systems on skeletal homeostasis. Since then, more studies have been conducted, gradually uncovering the complex neuroendocrine regulation underlying bone homeostasis. Intriguingly, bone is also considered as an endocrine organ that can produce regulatory factors that in turn exert effects on neuroendocrine activities. After decades of exploration into bone regulation mechanisms, separate bioactive factors have been extensively investigated, whereas few studies have systematically shown a global view of bone homeostasis regulation. Therefore, we summarized the previously studied regulatory patterns from the nervous system and endocrine system to bone. This review will provide readers with a panoramic view of the intimate relationship between the neuroendocrine system and bone, compensating for the current understanding of the regulation patterns of bone homeostasis, and probably developing new therapeutic strategies for its related disorders.

Determinants of bone mass in older adults with normal- and overweight derived from the crosstalk with muscle and adipose tissue

Lower bone mass in older adults may be mediated by the endocrine crosstalk between muscle, adipose tissue and bone. In 150 community-dwelling adults (59-86 years, BMI 17-37 kg/m²; 58.7% female), skeletal muscle mass index, adipose tissue and fat mass index (FMI) were determined. Levels of myokines, adipokines, osteokines, inflammation markers and insulin were measured as potential determinants of bone mineral content (BMC) and density (BMD). FMI was negatively associated with BMC and BMD after adjustment for mechanical loading effects of body weight (r-values between -0.37 and -0.71, all p < 0.05). Higher FMI was associated with higher leptin levels in both sexes, with higher hsCRP in women and with lower adiponectin levels in men. In addition to weight and FMI, sclerostin, osteocalcin, leptin × sex and adiponectin were independent predictors of BMC in a stepwise multiple regression analysis. Muscle mass, but not myokines, showed positive correlations with bone parameters that were weakened after adjusting for body weight (r-values between 0.27 and 0.58, all p < 0.01). Whereas the anabolic effect of muscle mass on bone in older adults may be partly explained by mechanical loading, the adverse effect of obesity on bone is possibly mediated by low-grade inflammation, higher leptin and lower adiponectin levels.

Effect of adipokine and ghrelin levels on BMD and fracture risk: an updated systematic review and meta-analysis

Context

Circulating adipokines and ghrelin affect bone remodeling by regulating the activation and differentiation of osteoblasts and osteoclasts. Although the correlation between adipokines, ghrelin, and bone mineral density (BMD) has been studied over the decades, its correlations are still controversial. Accordingly, an updated meta-analysis with new findings is needed.

Objective

This study aimed to explore the impact of serum adipokine and ghrelin levels on BMD and osteoporotic fractures through a meta-analysis.

Data sources

Studies published till October 2020 in Medline, Embase, and the Cochrane Library were reviewed.

Study selection

We included studies that measured at least one serum adipokine level and BMD or fracture risk in healthy individuals. We excluded studies with one or more of the following: patients less than 18 years old, patients with comorbidities, who had undergone metabolic treatment, obese patients, patients with high physical activities, and a study that did not distinguish sex or menopausal status.

Data extraction

We extracted the data that include the correlation coefficient between adipokines (leptin, adiponectin, and resistin) and ghrelin and BMD, fracture risk by osteoporotic status from eligible studies.

Data synthesis

A meta-analysis of the pooled correlations between adipokines and BMD was performed, demonstrating that the correlation between leptin and BMD was prominent in postmenopausal women. In most cases, adiponectin levels were inversely correlated with BMD. A meta-analysis was conducted by pooling the mean differences in adipokine levels according to the osteoporotic status. In postmenopausal women, significantly lower leptin (SMD = -0.88) and higher adiponectin (SMD = 0.94) levels were seen in the osteoporosis group than in the control group. By predicting fracture risk, higher leptin levels were associated with lower fracture risk (HR = 0.68), whereas higher adiponectin levels were associated with an increased fracture risk in men (HR = 1.94) and incident vertebral fracture in postmenopausal women (HR = 1.18).

Conclusions

Serum adipokines levels can utilize to predict osteoporotic status and fracture risk of patients.

Systematic review registration

https://www.crd.york.ac.uk/prospero/display_record.php?ID=CRD42021224855, identifier CRD42021224855.

Bariatric surgery and skeletal health: A narrative review and position statement for management by the European Calcified Tissue Society (ECTS)

CONTEXT

Numerous studies have demonstrated detrimental skeletal consequences following bariatric surgery.

METHODS

A working group of the European Calcified Tissue Society (ECTS) performed an updated review of existing literature on changes of bone turnover markers (BTMs), bone mineral density (BMD), and fracture risk following bariatric surgery and provided advice on management based on expert opinion.

LITERATURE REVIEW

Based on observational studies, bariatric surgery is associated with a 21-44% higher risk of all fractures. Fracture risk is time-dependent and increases approximately 3 years after bariatric surgery. The bariatric procedures that have a malabsorptive component (including Roux-en-Y Gastric bypass (RYGB) and biliopancreatic diversion (BPD)) have clearly been associated with the highest risk of fracture. The extent of high-turnover bone loss suggests a severe skeletal insult. This is associated with diminished bone strength and compromised microarchitecture. RYGB was the most performed bariatric procedure worldwide until very recently, when sleeve gastrectomy (SG) became more prominent. There is growing evidence that RYGB is associated with greater reduction in BMD, greater increase in BTMs, and higher risk of fractures compared with SG but RCTs on optimal management are still lacking.

EXPERT OPINION

In all patients, it is mandatory to treat vitamin D deficiency, to achieve adequate daily calcium and protein intake and to promote physical activity before and following bariatric surgery. In post-menopausal women and men older than 50 years, osteoporosis treatment would be reasonable in the presence of any of the following criteria: i) history of recent fragility fracture after 40 years of age, ii) BMD T-score ≤ -2 at hip or spine, iii) FRAX score with femoral neck BMD exceeding 20% for the 10-year major osteoporotic fracture probability or exceeding 3% for hip fracture. Zoledronate as first choice should be preferred due to intolerance of oral formulations and malabsorption. Zoledronate should be used with caution due to hypocemia risk. It is recommended to ensure adequate 25-OH vitamin D level and calcium supplementation before administering zoledronate.

CONCLUSIONS

The bariatric procedures that have a malabsorptive component have been associated with the highest turnover bone loss and risk of fracture. There is a knowledge gap on osteoporosis treatment in patients undergoing bariatric surgery. More research is necessary to direct and support guidelines.

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.

Insights into the Role of Circadian Rhythms in Bone Metabolism: A Promising Intervention Target?

Numerous physiological processes of mammals, including bone metabolism, are regulated by the circadian clock system, which consists of a central regulator, the suprachiasmatic nucleus (SCN), and the peripheral oscillators of the BMAL1/CLOCK-PERs/CRYs system. Various bone turnover markers and bone metabolism-regulating hormones such as melatonin and parathyroid hormone (PTH) display diurnal rhythmicity. According to previous research, disruption of the circadian clock due to shift work, sleep restriction, or clock gene knockout is associated with osteoporosis or other abnormal bone metabolism, showing the importance of the circadian clock system for maintaining homeostasis of bone metabolism. Moreover, common causes of osteoporosis, including postmenopausal status and aging, are associated with changes in the circadian clock. In our previous research, we found that agonism of the circadian regulators REV-ERBs inhibits osteoclast differentiation and ameliorates ovariectomy-induced bone loss in mice, suggesting that clock genes may be promising intervention targets for abnormal bone metabolism. Moreover, osteoporosis interventions at different time points can provide varying degrees of bone protection, showing the importance of accounting for circadian rhythms for optimal curative effects in clinical treatment of osteoporosis. In this review, we summarize current knowledge about circadian rhythms and bone metabolism.

Retinol-binding protein 4 is positively associated with bone mineral density in patients with type 2 diabetes and osteopenia or osteoporosis

OBJECTIVE

This study intends to study the association between serum retinol binding protein 4 (RBP4), bone mineral density (BMD) and other bone metabolic-related parameters in type 2 diabetic patients older than 50 years, with or without osteopenia or osteoporosis.

METHODS

Patients (n = 274 cases) with type 2 diabetes, hospitalized in the Endocrinology Department of Yantai Yuhuangding Hospital from December 2015 to March 2017, were enrolled in the study. The bone mineral density (BMD) was recorded by the dual-energy X-ray absorptiometer, and patients were divided into normal bone mineral density (148 cases), osteopenia (93 cases) and osteoporosis (33 cases) groups. The serum adipokine RBP4 and other biomarkers were determined accordingly.

RESULTS

Serum RBP4, body weight, calcium and body mass index (BMI) demonstrated a positive correlation with BMD at all tested body sites in osteopenia and osteoporosis groups compared with normal bone mineral density group. In contrast, age, duration of diabetes and alkaline phosphatase (ALP) were inversely correlated with BMD at all tested body sites. In nonadjusted analyses, age, gender, duration of diabetes and ALP were inversely associated with BMD at the femoral neck, total hip and lumbar spine, while body weight, BMI and RBP4 were positively associated with BMD at all sites. In multiple regression analyses, adjusted for age, weight, BMI and other bone-related factors, a graded stepwise positive association between serum RBP4 and BMD was shown, at all sites.

CONCLUSION

Serum RBP4 was positively associated with BMD at all sites after adjustments for other factors in osteopenia and osteoporosis groups compared with normal bone mineral density group of type 2 diabetic patients.

Associations between circulating adipokines and bone mineral density in patients with knee osteoarthritis: a cross-sectional study

Background

Associations between adipokines and bone mineral density (BMD) in knee osteoarthritis (OA) remain indistinct. The aim of this study was to investigate the cross-sectional associations between serum levels of adipokines and BMD in patients with knee OA.

Methods

This study included 164 patients with symptomatic knee OA from the Anhui Osteoarthritis study. Serum levels of leptin, adiponectin, and resistin were measured using an enzyme-linked immunosorbent assay (ELISA). BMD at total body, spine, hip, and femur were measured by dual-energy X-ray absorptiometry (DXA).

Results

In multivariable analyses, serum levels of leptin were significantly associated with reduced BMD at total body, hip, total femur, femoral neck, and femoral shaft (β = − 0.019, 95% CI -0.034 to − 0.005; β = − 0.018, 95% CI -0.034 to − 0.003; β = − 0.018, 95% CI -0.034 to − 0.002; β = − 0.016, 95% CI -0.032 to 0.000; β = − 0.026, 95% CI -0.046 to − 0.006; respectively). Serum levels of adiponectin were significantly and negatively associated with BMD at total femur and femoral shaft (β = − 0.007, 95% CI -0.013 to 0.000; β = − 0.011, 95% CI -0.018 to − 0.003; respectively). However, no significant associations were found between serum levels of resistin and BMD at any site measured.

Conclusions

Serum levels of leptin and adiponectin were significantly and negatively associated with BMD, suggesting potentially detrimental effects of leptin and adiponectin on BMD in knee OA patients.

A cross-sectional study of the association between adipokine levels and bone mineral density according to obesity and menopausal status in Korean women

Results regarding the association between adipokine levels and bone mineral density (BMD) have been inconsistent; the effects of sex, menopause, and central obesity remain unknown. We evaluated the association between serum leptin, adiponectin, and high-molecular-weight (HMW) adiponectin levels and BMD according to menopause and central obesity status in Korean women. This cross-sectional study comprised 255 women undergoing examinations at the CHA Bundang Medical Center. Participants were divided according to menopause, and central obesity status. We measured serum adipokine levels and BMD using an enzyme-linked immunosorbent assay and dual-energy X-ray absorptiometry, respectively. After adjusting for age, body mass index, alkaline phosphatase levels and the Homeostasis Model Assessment index, leptin levels were negatively associated with non-vertebral BMD (total hip, β = -0.576, P = 0.006; femoral neck, β = -0.608, P = 0.007) in postmenopausal women without central obesity. Among women without central obesity, HMW adiponectin levels were positively associated with total hip BMD (β = 0.240, P = 0.010) in premenopausal women but negatively associated with BMD (lumbar, β = -0.436, P = 0.012; femoral neck, β = -0.468, P = 0.007) in postmenopausal women. Thus, the association between adipokine levels and BMD varies according to the menopause and central obesity status.

Metabolically healthy/unhealthy components may modify bone mineral density in obese people

Link between obesity and bone health is controversial. It seems that maybe the difference in metabolic status leads to this difference. We studied relation between metabolically healthy/unhealthy components with bone mineral density. Results showed metabolically unhealthy obesity (MUHO) phenotypes have better bone status at hip site than metabolically healthy obesity (MHO). Also, component metabolic can effect on BMD in different sites.

PURPOSE/INTRODUCTION

This cross-sectional study aimed to compare total BMD and L-L4 BMD in MHO and MUHO base on Karelis criteria.

METHODS

We enrolled 272 Iranian obese women and men (BMI ≥ 30). According to Karelis criteria, the participants were grouped base to MHO and MUHO. The body composition and BMD were assessed for all cases. Serum HDL-C, LDL-C, total cholesterol, triglyceride (TG), fasting blood glucose, homeostatic model assessment-insulin resistance (HOMA-IR), and hypersensitive C-reactive protein (hs-CRP) levels were quantified by ELISA method.

RESULTS

Our results demonstrate MUHO phenotype have high total BMD more than MHO (P = 0.01, CI = 0.12 to 0.21). Also, the results of logistic regression analysis showed MUHO have strongly associated with total BMD (β = -0.42, CI = - 0.31 to - 0.04, P = 0.009), but did not affected L2-L4 BMD (β = - 0.09, CI = - 0.14 to 0.08, P = 0.578); this represents that there was discordance in MUHO subjects. Our evidence implicated that HOMA-IR, high level serum TG, hs-CRP, and low level serum HDL had mediatory effect on relationship between obesity and high BMD at the hip region in MUHO subjects (P < 0.05).

CONCLUSION

Present evidence indicates that, could be a novel link between difference in MUH phenotype and MH phenotype with bone status. Also, component metabolic can effect on BMD in different sites.

Aging human body: changes in bone, muscle and body fat with consequent changes in nutrient intake

Aging affects almost all physiological processes, but changes in body composition and body phenotype are most observable. In this review, we focus on these changes, including loss of bone and muscle and increase in body fat or redistribution of the latter, possibly leading to osteosarcopenic obesity syndrome. We also address low-grade chronic inflammation, prevalent in aging adults and a cause of many disorders including those associated with body composition. Changes in dietary intake and nutritional requirements of older individuals, that all may lead to some disturbances on tissue and organ levels, are discussed as well. Finally, we discuss the hormonal changes in the aging body, considering each of the tissues, bone, muscle and fat as separate endocrine organs, but yet in the continuous interface and communication with each other. Although there are still many unanswered questions in this field, this review will enable the readers to better understand the aging human body and measures needing to be implemented toward reducing impaired health and disability in older individuals.

Adipokines may mediate the relationship between resting metabolic rates and bone mineral densities in obese women

The researchers sought to test the possible link between resting metabolic rate and bone mineral density through four adipokines. Participants with lower resting metabolic rate (RMR) per kilogram demonstrated higher total bone mineral density (BMD), total T-score, and total Z-score. Omentin-1 had a mediatory effect on the relationship between RMR/kg of body weight and bone parameters.

INTRODUCTION

The previous results of studies regarding the links between obesity and bone health are controversial. For this reason, the researchers sought to test the possible link between RMR and BMD through the following four adipokines: vaspin, retinol binding protein 4, angiopoietin-like 6 (ANGPL6), and omentin-1.

METHODS

We enrolled 312 obese Iranian women (30 ≤ body mass index <40) in this cross-sectional study. In order to examine the association of serum adipokine levels with RMR and BMD, the participants were grouped based on RMR per body weight. Body composition, dietary intake, bone mineral density, and resting metabolic rate were assessed in all participants. Serum adipokine levels were quantified by the enzyme-linked immunosorbent assay (ELISA) method.

RESULTS

Low levels of RMR/kg were strongly associated with higher weight, body mass index, fat mass, and visceral fat levels. In fact, participants with an RMR/kg of body weight <20 kcal/24 h/kg were more obese (p < 0.05). Another noteworthy finding was that participants with lower RMR/kg demonstrated higher total BMD, total T-score, and total Z-score. Our results showed that omentin-1 had a mediatory effect on the relationship between RMR per kilogram of body weight and bone parameters (p < 0.05). Nevertheless, other adipokines such as vaspin, retinol-binding protein 4 (RBP4), and ANGPL6 did not affect the relationship between RMR and BMD (p > 0.05).

CONCLUSIONS

The inhibitory effect of omentin-1 on TNF-alpha seems to be able to reduce the amount of circulating leptin as adipokine, affecting energy expenditure and improving bone loss induced by estrogen deficiency and controlled effect of RMR on BMD.

Vertebral bone marrow fat, bone mineral density and diabetes: The Osteoporotic Fractures in Men (MrOS) study

Elevated vertebral bone marrow fat (BMF) among individuals with osteoporosis has been established in histomorphometric studies. Several studies have found a negative correlation between BMF and bone mineral density (BMD) at the spine in men and women across different age groups. Animal studies have also observed bone loss with increased BMF in mice with induced diabetes. Our study objective was to test the hypothesis that the association between BMF and BMD varies by diabetic status. We performed a cross-sectional study of 156 men aged 74-96years from the Osteoporotic Fractures in Men study at the Pittsburgh clinical site. All men had spine BMF scans using proton magnetic resonance spectroscopy and spine and hip BMD scans by dual-energy X-ray absorptiometry. BMF was expressed as lipid to "lipid+water" ratio (%). Men were considered diabetic if they self-reported a physician diagnosis of diabetes, diabetes medication or had a fasting glucose ≥126mg/dl. Men with diabetes (n=38) had a significantly higher spine BMF (58.9 vs. 54.6%, p=0.0035), spine BMD (1.20 vs. 1.10g/cm², P=0.007) and total hip BMD (1.00 vs. 0.94g/cm², p=0.04) than those without, while no differences were observed for body weight, body mass index or waist circumference. Pearson correlation tests showed no significant correlation of spine BMF with age or BMD in non-diabetics. Significant inverse correlations were observed between BMF and BMD (-0.30 for femoral neck and -0.39 for total hip) among diabetic men. In conclusion, men with diabetes had a higher BMF compared to non-diabetic men. The correlation between BMF and BMD differed by diabetes status. Further investigation of the association of diabetes with BMF and BMD may provide a better understanding of the high fracture rates among individuals with diabetes despite their higher BMD.

Brief Report: Association of Adipokines With Bone Mineral Density in HIV-Infected and HIV-Uninfected Women

BACKGROUND

HIV infection is associated with low bone mineral density (BMD) and alterations in adipokines, which may mediate the relationship between fat and bone.

OBJECTIVE

To evaluate the relationship of adiponectin and leptin with BMD in HIV-infected and uninfected women.

METHODS

We measured BMD over 5 years at the lumbar spine, total hip (TH), and femoral neck (FN) using dual-energy X-ray absorptiometry in 318 HIV-infected and 122 HIV-uninfected participants of the multicenter Women's Interagency HIV Study (WIHS). Total adiponectin and leptin were assayed on stored sera. Multivariable linear mixed models assessed the effects of adipokines and HIV status on BMD.

RESULTS

HIV-infected women had higher adiponectin (median 6.2 vs. 5.6 μg/mL,) but lower leptin (11.7 vs. 19.8 ng/mL) levels at baseline (both P < 0.05) compared with HIV-uninfected women. HIV infection was associated with lower BMD at the lumbar spine (-0.074 g/cm), FN (-0.049 g/cm), and TH (-0.047 g/cm) (all P < 0.05) after adjusting for demographic, behavioral, and metabolic factors. HIV infection remained associated with lower BMD at each site, with little change in the effect sizes after additional adjustment for adiponectin or leptin. Among HIV-infected women, higher adiponectin was associated with lower TH BMD (-0.025 g/cm per 10-fold increase, P = 0.035), whereas higher leptin was associated with higher BMD at FN (+0.027 g/cm per 10-fold increase, P = 0.005) and TH (+0.019 g/cm, P = 0.028). After multivariable adjustment, the adipokines showed little association with BMD at any site (P > 0.8 for adiponectin; P > 0.2 for leptin).

CONCLUSIONS

Alterations in serum adiponectin and leptin do not explain low BMD in HIV-infected women.

Sequences of Regressions Distinguish Nonmechanical from Mechanical Associations between Metabolic Factors, Body Composition, and Bone in Healthy Postmenopausal Women

BACKGROUND

There is increasing recognition of complex interrelations between the endocrine functions of bone and fat tissues or organs.

OBJECTIVE

The objective was to describe nonmechanical and mechanical links between metabolic factors, body composition, and bone with the use of graphical Markov models.

METHODS

Seventy postmenopausal women with a mean ± SD age of 62.3 ± 3.7 y and body mass index (in kg/m2) of 24.9 ± 3.8 were recruited. Bone outcomes were peripheral quantitative computed tomography measures of the distal and diaphyseal tibia, cross-sectional area (CSA), volumetric bone mineral density (vBMD), and cortical CSA. Biomarkers of osteoblast and adipocyte function were plasma concentrations of leptin, adiponectin, osteocalcin, undercarboxylated osteocalcin (UCOC), and phylloquinone. Body composition measurements were lean and percent fat mass, which were derived with the use of a 4-compartment model. Sequences of Regressions, a subclass of graphical Markov models, were used to describe the direct (nonmechanical) and indirect (mechanical) interrelations between metabolic factors and bone by simultaneously modeling multiple bone outcomes and their relation with biomarker outcomes with lean mass, percent fat mass, and height as intermediate explanatory variables.

RESULTS

The graphical Markov models showed both direct and indirect associations linking plasma leptin and adiponectin concentrations with CSA and vBMD. At the distal tibia, lean mass, height, and adiponectin-UCOC interaction were directly explanatory of CSA (R2 = 0.45); at the diaphysis, lean mass, percent fat mass, leptin, osteocalcin, and age-adiponectin interaction were directly explanatory of CSA (R2 = 0.49). The regression models exploring direct associations for vBMD were much weaker, with R2 = 0.15 and 0.18 at the distal and diaphyseal sites, respectively. Lean mass and UCOC were associated, and the global Markov property of the graph indicated that this association was explained by osteocalcin.

CONCLUSIONS

This study, to our knowledge, offers a novel approach to the description of the complex physiological interrelations between adiponectin, leptin, and osteocalcin and the musculoskeletal system. There may be benefits to jointly targeting both systems to improve bone health.

Gender- and region-specific alterations in bone metabolism in Scarb1-null female mice

A positive correlation between plasma levels of HDL and bone mass has been reported by epidemiological studies. As scavenger receptor class B, type I (SR-BI), the gene product of Scarb1, is known to regulate HDL metabolism, we recently characterized bone metabolism in Scarb1-null mice. These mice display high femoral bone mass associated with enhanced bone formation. As gender differences have been reported in HDL metabolism and SR-BI function, we investigated gender-specific bone alterations in Scarb1-null mice by microtomography and histology. We found 16% greater relative bone volume and 39% higher bone formation rate in the vertebrae from 2-month-old Scarb1-null females. No such alteration was seen in males, indicating gender- and region-specific differences in skeletal phenotype. Total and HDL-associated cholesterol levels, as well as ACTH plasma levels, were increased in both Scarb1-null genders, the latter being concurrent to impaired corticosterone response to fasting. Plasma levels of estradiol did not differ between null and WT females, suggesting that the estrogen metabolism alteration is not relevant to the higher vertebral bone mass in female Scarb1-null mice. Constitutively, high plasma levels of leptin along with 2.5-fold increase in its expression in white adipose tissue were measured in female Scarb1-null mice only. In vitro exposure of bone marrow stromal cells to ACTH and leptin promoted osteoblast differentiation as evidenced by increased gene expression of osterix and collagen type I alpha. Our results suggest that hyperleptinemia may account for the gender-specific high bone mass seen in the vertebrae of female Scarb1-null mice.

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.

Bone and mineral metabolism in patients undergoing Roux-en-Y gastric bypass

Despite effective weight reduction, the impact of bariatric surgery on bone is a major concern. Mechanisms include decreased mechanical loading, calcium and vitamin D malabsorption, deficiency in other nutrients, and alterations in fat- and gut-derived hormones. The evidence to support clinical care pathways to prevent bone loss and fractures is at this point weak.

INTRODUCTION

There is a growing concern regarding the potential deleterious impact of bariatric surgery on bone metabolism. This comprehensive review addresses this controversial topic.

METHODS

We reviewed and analyzed articles evaluating bone metabolism and mechanisms for the ensuing putative bone loss in adult patients exclusively undergoing Roux-en-Y gastric bypass (RYGB) surgery, for the period spanning 1942 till September 2012.

RESULTS

Mechanisms identified to contribute to alterations in bone metabolism after bypass surgery include: decreased mechanical loading, calcium and vitamin D malabsorption with secondary hyperparathyroidism, deficiency in other nutrients, in addition to alterations in adipokines, gonadal steroids, and gut-derived hormones favoring bone loss, with the exception of serotonin and glucagon-like peptide-1. The relative contribution of each of these hormones to changes in bone homeostasis after bypass surgery remains undefined. Bone loss reflected by a decline in bone mineral density (BMD) and an increase in bone turnover markers have been reported in many studies, limited for the most part by the exclusive use of dual energy X-ray absorptiometry. Well-designed long-term prospective trials with fractures as an outcome, and studies investigating the magnitude, reversibility, and impact of the observed metabolic changes on fracture outcomes are lacking.

CONCLUSION

Robust conclusions regarding bone loss and fracture outcome after RYGB surgery cannot be drawn at this time. Although not evidence based, baseline evaluation and sequential monitoring with measurement of BMD and calciotropic hormones seem appropriate, with adequate calcium and vitamin D replacement. Beneficial interventions remain unclear.

Ghrelin and bone

Ghrelin is a gut-derived peptide hormone, first isolated from the stomach. Ghrelin was initially characterized as a growth hormone (GH) secretagogue, but it plays a more important role as a potent orexigen and modulator of whole-body energy homeostasis. Ghrelin itself is closely regulated by metabolic status. Bone remodeling constantly renews the skeleton in a highly energy-dependent fashion. Accordingly, bone metabolism is tightly coupled to energy metabolism through the integration of peripheral and central mechanisms, involving the sympathetic nervous system and factors such as leptin. Ghrelin has been shown to modulate osteoblast differentiation and function, both directly and perhaps also through regulation of the GH-insulin-like growth factor axis. However, recently it has also been shown that ghrelin interacts with leptin in modulating bone structure, constituting a new mechanism that couples bone metabolism with energy homeostasis. In this review, we discuss the role that ghrelin plays modulating bone cell function, and its integrative role in coupling bone metabolism with energy metabolism.

Changes of leptin concentration in plasma in patients with spinal cord injury: a meta-analysis

OBJECTIVE

The aim of this study was to investigate changes of leptin concentration in plasma in patients with spinal cord injury to come to a single concept by using a Meta-analysis.

SETTING

Systematic Review.

METHODS

Searching relevant articles was performed in Ovid data base, Medline (PubMed) EMBASE, Google Scholar, Cochrane and Scopus up to February 2013. Five articles were selected using two independent reviewers. Analysis were performed using SPSS version 18 and Comparative Meta-analysis software version 2.0.

RESULTS

The combined analysis with confidence interval of 95% using comprehensive meta-analysis showed significant higher leptin levels in patients with spinal cord injury in comparison with able bodies (P<0.0001). The effect of spinal lesion level on plasma leptin concentration was also statistically significant (P<0.0001). Body mass index was positively related to plasma leptin concentration in both groups (P<0.0001).

CONCLUSION

This Meta analysis approves increased level of leptin in spinal cord injured patients which can be due to fat distribution changes and sympathetic dysfunction in these patients. Our results also showed that patients with higher spinal lesion level have higher plasma leptin concentration.

Association of weight-adjusted body fat and fat distribution with bone mineral density in middle-aged chinese adults: a cross-sectional study

Background

Although it is well established that a higher body weight is protective against osteoporosis, the effects of body fat and fat distribution on bone mineral density (BMD) after adjustment for body weight remains uncertain.

Objective

To examine the relationship between body fat and fat distribution and BMD beyond its weight-bearing effect in middle-aged Chinese adults.

Method

The study had a community-based cross-sectional design and involved 1,767 women and 698 men aged 50–75 years. The BMD of the lumbar spine, total hip, and whole body, and the fat mass (FM) and percentage fat mass (%FM) of the total body and segments of the body were measured by dual-energy X-ray absorptiometry. General information on the participants was collected using structured questionnaire interviews.

Result

After adjusting for potential confounders, an analysis of covariance showed the weight-adjusted (WA-) total FM (or %FM) to be negatively associated with BMD in all of the studied sites (P<0.05) in both women and men. The unfavorable effects of WA-total FM were generally more substantial in men than in women, and the whole body was the most sensitive site related to FM, followed by the total hip and the lumbar spine, in both genders. The mean BMD of the lumbar spine, total hip, and whole body was 3.93%, 3.01%, and 3.65% (in women) and 5.02%, 5.57%, 6.03% (in men) lower in the highest quartile (vs. lowest quartile) according to the WA-total FM (all p<0.05). Similar results were noted among the groups for WA-total FM%. In women, abdominal fat had the most unfavorable association with BMD, whereas in men it was limb fat.

Conclusion

FM (or %FM) is inversely associated with BMD beyond its weight-bearing effect. Abdominal fat in women and limb fat in men seems to have the greatest effect on BMD.

Assessing the Association between Leptin and Bone Mineral Density in HIV-Infected Men

HIV-infected individuals are at risk for decreased bone mineral density (BMD). The known risk factors for bone loss do not fully explain the increased risk in this population. There is emerging evidence that leptin, a hormone secreted by adipocytes, plays an important role in bone metabolism. Several studies have assessed the relationship between leptin and bone density in healthy adults, but there are few such studies in HIV-infected individuals. Furthermore, HIV infected individuals on antiretroviral therapy are at increased risk for altered fat distribution, which may impact the relationship between leptin and BMD. In a cross-sectional analysis of data in 107 HIV-infected men, we determined whether serum leptin levels were associated with whole-body BMD and bone mineral content measured by dual-energy X-ray absorptiometry (DEXA), after adjusting for confounders including body fat distribution. We found an inverse association between leptin and bone density in those with peripheral lipoatrophy, defined objectively as <3 kg appendicular fat by DEXA, but no such relationship was seen in those with >3 kg appendicular fat. This result suggests that fat distribution may modify the relationship between leptin and bone density.

The effects of adiponectin and leptin on changes in bone mineral density

We tested the hypothesis that low leptin and high adiponectin levels are associated with higher rates of bone mineral density (BMD) loss among 3,075 men and women, aged 70-79, from the Health Aging and Body Composition Study. Results suggest that adiponectin, but not leptin, is a risk factor for bone loss in women.

INTRODUCTION

Adiponectin and leptin are hormones secreted by adipose cells that may impact BMD. Few studies have evaluated the longitudinal association of leptin and adiponectin levels with rates of BMD change.

METHODS

Hip and whole-body areal BMD (aBMD) were measured five times using dual-energy X-ray absorptiometry over 10 years (average follow-up time, 7.95 ± 1.92 years). Trabecular lumbar spine volumetric BMD (vBMD) was measured using quantitative computed topography at baseline and year 6 in the Pittsburgh cohort only. Random slope and intercept models were used to account for within person correlation as a result of repeated measures of hip and whole-body aBMD. Linear regression was used to model changes in spine trabecular vBMD.

RESULTS

Among women, the annualized rate of hip aBMD loss in the highest tertile of adiponectin was -0.67% (95% CI -0.77, -0.58) compared to [-0.43% (95% CI -0.51, -0.35)] in the lowest tertile (p trend = 0.019) after adjusting for age, race, BMI, diabetes, baseline hip aBMD, and weight change. In men, hip aBMD loss was greatest in the high adiponectin group (tertile 3), however this association was not significant (p trend = 0.148). After adjusting for weight change in women, the association between higher leptin and lower hip aBMD loss was attenuated and no longer significant (p trend = 0.134). Leptin and adiponectin levels were not associated with whole-body aBMD or trabecular lumbar spine vBMD loss.

CONCLUSIONS

Adiponectin was associated with increased hip aBMD loss in women only, supporting evidence that adiponectin may have an important role in bone health.

The relationship between body composition and bone mineral density in postmenopausal Turkish women

In a retrospective cross-sectional study among 202 postmenopausal women aged 46-75 years, we aimed to investigate the relationship between body composition and bone mineral density (BMD) to determine whether fat mass or lean mass is a better determinant of BMD in Turkish postmenopausal women. Lumbar spine (L1-L4) and proximal femur BMD were measured by dual energy X-ray absorbsiometry. Body composition analysis was performed by bioelectric impedance method and fat mass, lean mass, and percent fat were measured. Both fat mass and lean mass were positively correlated with BMD at the lumbar spine and proximal femur, weight and body mass index. Lean mass was also positively correlated with height and negatively correlated with age and years since menopause (P < 0.01). The correlations of fat mass and lean mass with BMD at the lumbar spine and proximal femur remained significant after adjustment for age, years since menopause and height. When the lean mass was adjusted together with age, years since menopause and height, the significant relationship between the fat mass and BMD continued, however the significant correlation between the lean mass and BMD disappeared at all sites after adjustment for fat mass. In multiple regression analyses, fat mass was the significant determinant of all BMD sites. Our data suggest that fat mass is the significant determinant of BMD at the lumbar spine and proximal femur, and lean mass does not have an impact on BMD when fat mass was taken into account in Turkish postmenopausal women.

Leptin is an independent determinant of bone mineral density in men with type 2 diabetes mellitus

To investigate the possible relationship of leptin to bone mineral density (BMD) in men with type 2 diabetes mellitus (T2DM), we screened 168 Belarusian men aged 45-65 years. Plasma total cholesterol (TC), high-density lipoprotein cholesterol, and triglyceride concentrations were assessed, and low-density lipoprotein cholesterol and very low-density lipoprotein cholesterol (LDL-C) were calculated. Hemoglobin A(1c), immune-reactive insulin (IRI), serum total testosterone, and sex hormone-binding globulin were also evaluated. BMD was evaluated using dual-energy X-ray absorptiometry. By univariate linear regression analysis, BMD was significantly correlated with body mass index (r = 0.23, P = 0.002) and leptin (r = 0.21, P = 0.006). By multivariate regression analysis adjusting for confounding factors, log leptin was independently correlated with BMD (β = 0.058, P = 0.001). Our study revealed that leptin is an independent determinant of BMD in patients with T2DM. Further research is necessary to confirm this association and to develop ways to correct abnormalities of bone metabolism in patients with T2DM.

Skeletal phenotype of the leptin receptor-deficient db/db mouse

Leptin, a major hormonal product of the adipocyte, regulates appetite and reproductive function through its hypothalamic receptors. The leptin receptor is present in osteoblasts and chondrocytes, and previously we have shown leptin to be an anabolic bone factor in vitro, stimulating osteoblast proliferation and inhibiting osteoclastogenesis. Leptin increases bone mass and reduces bone fragility when administered peripherally but also can indirectly reduce bone mass when administered into the central nervous system. However, data from animal models deficient in either leptin (ob/ob) or its receptor (db/db) remain contradictory. We compared the bone phenotype of leptin receptor-deficient (db/db) and wild-type mice using micro-computed tomographic (µCT) analysis of the proximal tibias and vertebrae. In the tibia, db/db mice had reduced percent trabecular bone volume (13.0 ± 1.62% in wild-type versus 6.01 ± 0.601% in db/db mice, p = .002) and cortical bone volume (411 ± 21.5 µm(3) versus 316 ± 3.53 µm(3), p = .0014), trabecular thickness (48.4 ± 001.07 µm versus 45.1 ± 0.929 µm, p = .041) and trabecular number (2.68 ± 0.319 mm(-1) versus 1.34 ± 0.148 mm(-1), p = .0034). In the fifth lumbar vertebral body, the trabecular thickness and cortical thickness were decreased in the db/db versus wild-type mice (0.053 ± 0.0011 mm versus 0.047 ± 0.0013 mm, p = .0002 and 0.062 ± 0.00054 mm versus 0.056 ± 0.0009 mm, p = .0001), respectively, whereas the trabecular and cortical percent bone volume and trabecular number did not reach significance. The total (endosteal and periosteal) cortical perimeter (12.2 ± 0.19 mm versus 13.2 ± 0.30 mm, p = .01) was increased. The serum osteocalcin levels were reduced in the db/db mice, suggesting that bone formation rates are decreased. The material properties of db/db femurs were determined by three-point bending and nanoindentation, showing decreased bone strength (13.3 ± 0.280 N versus 7.99 ± 0.984 N, p = .0074) and material stiffness (28.5 ± 0.280 GPa versus 25.8 ± 0.281 GPa, p < .0001). These results demonstrate that bone mass and strength are reduced in the absence of leptin signaling, indicating that leptin acts in vivo as an anabolic bone factor. This concurs with results of in vitro studies and of peripheral leptin administration in vivo and suggests that leptin's direct effects on bone cells are likely to override its actions via the central nervous system.

Adipokines and the risk of fracture in older adults

Adiponectin and leptin are adipokines that influence bone metabolism in vitro and in animal models. However, less is known about the longitudinal association of leptin and adiponectin with fracture. We tested the hypothesis that low leptin and high adiponectin levels are each individually associated with fracture risk in a prospective cohort study in Memphis and Pittsburgh among 3075 women and men aged 70 to 79 years from the Health Aging and Body Composition (Health ABC) study. There were 406 incident fractures (334 nonvertebral and 72 vertebral) over a mean of 6.5 ± 1.9 years. Cox regression was used to estimate the hazard ratios for fracture. Sex modified the association between adiponectin and fracture (p = .025 for interaction). Men with the highest adiponectin level (tertile 3) had a 94% higher risk of fracture [hazard ratio (HR) = 1.94; 95% confidence interval (CI) 1.20-3.16] compared with the lowest tertile (tertile 1; p = .007 for trend) after adjusting age, race, body mass index (BMI), education, diabetes, weight change, and hip bone mineral density (BMD). Among women, after adjusting for age and race, this association was no longer significant (p = .369 for trend). Leptin did not predict fracture risk in women (p = .544 for trend) or men (p = .118 for trend) in the multivariate models. Our results suggest that adiponectin, but not leptin, may be a novel risk factor for increased fracture risk independent of body composition and BMD and that these relationships may be influenced by sex. More research is needed to understand the physiologic basis underlying these sex differences.

Predictors of bone responsiveness to growth hormone (GH) replacement in adult GH-deficient patients

Growth hormone (GH) replacement in adulthood results in variable bone responses as a function of the gonadic hormonal milieu. We performed a retrospective analysis of a large cohort of adult males and females with confirmed GH deficiency (GHD) prior to treatment and during 3 years of replacement therapy. Potential confounders and effect modifiers were taken into account. Sixty-four adult patients with GHD (20 females and 44 males; mean age 34 years, range 18-64) were included in the analysis. GH replacement induced a different effect on bone in males compared to females. Bone mineral content increased in males and decreased in females at the lumbar spine, total femur, and femoral neck; bone mineral density showed a similar trend at the lumbar spine and femoral neck. There was no significant gender difference in bone area at any measured bone site. In both sexes we observed a similar trend for serum markers of bone remodeling. Sex predicted bone outcome on multivariate analysis, as did age, onset of GHD (childhood/adulthood), pretreatment bone mass, baseline body mass index (BMI), and BMI change during GH replacement. Serum IGF-I levels during treatment did not show any relationship with bone outcome at any measured site. This study confirms that bone responsiveness to GH replacement in adult GHD varies as a function of sex even after controlling for potential confounders and highlights the importance of other cofactors that may affect the interaction between GH replacement therapy and bone remodeling.

Serum leptin concentration positively correlates with body weight and total fat mass in postmenopausal Japanese women with osteoarthritis of the knee

The objective of the present study was to identify factors correlated with the serum leptin concentration in women with knee OA. Fifty postmenopausal Japanese women with knee OA (age: 50-88 years) were recruited in our outpatient clinic. Plain radiographs of the knee were taken, and urine and blood samples were collected. Dual-energy X-ray absorptiometry (DXA) scanning was performed for the whole body and lumbar spine, and factors correlated with the serum leptin concentration were identified. A simple linear regression analysis showed that body weight, body mass index, whole-body bone mineral density (BMD), total fat mass, and total fat percentage, but not age, height, lumbar spine BMD, lean body mass, serum and urinary bone turnover markers, or the radiographic grade of knee OA, were significantly correlated with the serum leptin concentration. A multiple regression analysis showed that among these factors, only body weight and total fat mass exhibited a significant positive correlation with the serum leptin concentration. These results suggest that the serum leptin concentration might be related to increases in body weight and total fat mass, but not to BMD or bone turnover markers, in postmenopausal women with OA.

Associations between serum leptin level and bone turnover in kidney transplant recipients

BACKGROUND AND OBJECTIVES

Obesity is associated with increased parathyroid hormone (PTH) in the general population and in patients with chronic kidney disease (CKD). A direct effect of adipose tissue on bone turnover through leptin production has been suggested, but such an association has not been explored in kidney transplant recipients.

DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS

This study examined associations of serum leptin with PTH and with biomarkers of bone turnover (serum beta crosslaps [CTX, a marker of bone resorption] and osteocalcin [OC, a marker of bone formation]) in 978 kidney transplant recipients. Associations were examined in multivariable regression models. Path analyses were used to determine if the association of leptin with bone turnover is independent of PTH.

RESULTS

Higher leptin levels were associated with higher PTH and lower vitamin D levels, and adjustment for vitamin D attenuated the association between leptin and PTH. However, higher leptin was also significantly associated with lower levels of the bone turnover markers: 1 SD higher leptin was associated with 0.13 lower log-OC (-0.17, -0.08, P < 0.001) and 0.030 lower log-CTX (-0.045, -0.016, P < 0.001) after multivariable adjustments. Path analysis indicated that the association of leptin with PTH was mostly mediated through vitamin D, and that the association between leptin and bone turnover was independent of PTH and vitamin D.

CONCLUSIONS

Elevated leptin level is associated with lower bone turnover independent of its effects on serum PTH in kidney transplant recipients.

Concentration of adipogenic and proinflammatory cytokines in the bone marrow supernatant fluid of osteoporotic women

Osteoporosis is characterized by low bone mass, microarchitectural deterioration of bone tissue leading to increased bone fragility, and a resulting susceptibility to fractures. Distinctive environmental bone marrow conditions appear to support the development and maintenance of the unbalance between bone resorption and bone formation; these complex bone marrow circumstances would be reflected in the fluid surrounding bone marrow cells. The content of regulatory molecules in the extracellular fluid from the human bone marrow is practically unknown. Since the content of cytokines such as adiponectin, leptin, osteoprogeterin (OPG), soluble receptor activator of nuclear factor kappaB ligand (s-RANKL), tumor necrosis factor alpha, and interleukin 6 (IL-6) may elicit conditions promoting or sustaining osteoporosis, in this work we compared the concentrations of the above-mentioned cytokines and also the level of the soluble receptors for both IL-6 and leptin in the extracellular fluid from the bone marrow of nonosteoporotic and osteoporotic human donors. A supernatant fluid (bone marrow supernatant fluid [BMSF]) was obtained after spinning the aspirated bone marrow samples; donors were classified as nonosteoporotic or osteoporotic after dual-energy X-ray absorptiometry (DXA) measuring. Specific commercially available kits were used for all measurements. The cytokines' concentration in BMSF showed differently among nonosteoporotic and osteoporotic women; this last group was characterized by higher content of proinflammatory and adipogenic cytokines. Also, osteoporotic BMSF differentiated by decreased leptin bioavailability, suggesting that insufficient leptin action may distinguish the osteoporotic bone marrow.

The relationship between adipokines, body composition, and bone density in men with chronic obstructive pulmonary disease

Osteoporosis is common in patients with chronic obstructive pulmonary disease (COPD). Data regarding the relationship between adipokines and bone mineral density (BMD) in this population is lacking. The purpose of this pilot study was to determine associations between the adipokines tumor necrosis factor-alpha (TNF-α), leptin, adiponectin and resistin, body composition, and BMD in men with severe COPD. This was a cross-sectional study of men with severe COPD who visited the University of Colorado Hospital COPD Center. Bone density and parameters of body composition were measured by dual-energy X-ray absorptiometry. Twenty-three men were included (mean age = 66 years, mean percent predicted forced expiratory volume in one second = 32%). On bivariate analysis, there was no association between TNF-α and BMD. Parameters of body composition and serum concentrations of leptin and adiponectin were significantly associated with total hip and spine bone density. However, with partial correlation analysis, total body mass was the only independent predictor of total hip BMD, explaining approximately 50% of the variability. Overall, 18 out of 23 men enrolled (78%) had low bone density by T-score, and nine (39%) were classified as having osteoporosis. The men with osteoporosis had lower parameters of body composition, lower mean serum leptin concentrations, and a greater impairment in measures of lung function compared to the men without osteoporosis. We conclude that the effect of adipokines on BMD does not appear to be independent of body mass. However, larger studies are needed to further evaluate the relationship between adipokines, body weight, and BMD in patients with COPD.

Bone mineral density in men with and without the metabolic syndrome

The objective of this study was to measure bone mineral density (BMD) in middle-aged men with and without the metabolic syndrome according to the International diabetes federation (IDF) definition from 2005. We studied 80 men (mean age: 51.9 +/- 9.0 y, mean body mass index (BMI): 32.0 +/- 1.7 kg/m2) with and 92 men without the metabolic syndrome (mean age: 52.6 +/- 15.1 y, mean BMI: 24.9 +/- 2.8 kg/m2). Height (cm), weight (kg), waist circumference (cm) and blood pressure were measured. Fasting plasma glucose (FPG) and blood lipids were determined. BMD at the lumbar spine and total hip was measured by dual X-ray absorptiometry on a Hologic QDR 4500 bone densitometer. In men around 59.3% had a waist circumference > 94 cm (abdominal obesity). Among them 58.7% showed abnormal BP values. Around 30.7% had FPG > or = 5.6 mmol/L and 22.7% had low high density lipoprotein (HDL)-cholesterol and 36.6% had hypertriglyceridemia. In men with the metabolic syndrome, mean lumbar spine BMD was 0.986 +/- 0.210 g/cm2 and total hip BMD - 1.012 +/- 0.209 g/cm2. The corresponding values in men without this syndrome were 0.934 +/- 0.179 g/cm2 and 0.894 +/- 0.189 g/cm2, respectively. The inter-group BMD difference reached statistical significance only at the hip (p = 0.039). Respectively, the prevalence of osteoporosis at the central sites was significantly higher in men without the metabolic syndrome (MS) (13.2 versus 20.8%, p = 0.03). Our data confirmed the trend for higher BMD in the studied men with the metabolic syndrome.

Fat targets for skeletal health

Emerging evidence points to a critical role for the skeleton in several homeostatic processes, including energy balance. The connection between fuel utilization and skeletal remodeling begins in the bone marrow with lineage allocation of mesenchymal stem cells to adipocytes or osteoblasts. Mature bone cells secrete factors that influence insulin sensitivity, and fat cells synthesize cytokines that regulate osteoblast differentiation; thus, these two pathways are closely linked. The emerging importance of the bone-fat interaction suggests that novel molecules could be used as targets to enhance bone formation and possibly prevent fractures. In this article, we discuss three pathways that could be pharmacologically targeted for the ultimate goal of enhancing bone mass and reducing osteoporotic fracture risk: the leptin, peroxisome proliferator-activated receptor gamma and osteocalcin pathways. Not surprisingly, because of the complex interactions across homeostatic networks, other pathways will probably be activated by this targeting, which could prove to be beneficial or detrimental for the organism. Hence, a more complete picture of energy utilization and skeletal remodeling will be required to bring any potential agents into the future clinical armamentarium.

Relationship between age-related serum concentrations of TGF-beta1 and TGF-beta2 and those of osteoprotegerin and leptin in native Chinese women

BACKGROUND

Transforming growth factor-beta 1 (TGF-beta1), TGF-beta2, osteoprotegerin (OPG), and leptin are important cytokines in the regulation of bone remodeling. We investigated the relationship of TGF-beta1 and TGF-beta2 concentrations with those of OPG and leptin in Chinese females.

METHODS

The serum concentrations of TGF-beta1, TGF-beta2, OPG, and leptin were measured by ELISA in 459 healthy Chinese females aged 25-80 y.

RESULTS

The mean values (+/-SD) of the serum concentrations of TGF-beta1, TGF-beta2, OPG, and leptin in Chinese females were 29.7+/-1.69 microg/l, 13.7+/-3.86 microg/l, 3.81+/-1.96 pmol/l, and 10.5+/-2.01 microg/l, respectively. Further, the serum TGF-beta1 concentrations of postmenopausal women were significantly lower than those of perimenopausal and premenopausal women (24.3+/-1.59 vs 33.4+/-1.69 and 37.6+/-1.64, respectively), while the TGF-beta2 concentrations of postmenopausal women were significantly higher than those of perimenopausal and premenopausal women (14.6+/-3.91 vs 13.5+/-3.93 and 11.7+/-2.68, respectively). The serum TGF-beta1 concentration was found to be significantly negatively correlated with age (r=-0.335, P=0.000) and the TGF-beta2 concentration, to be significantly positively correlated with age (r=0.230, P=0.000). The TGF-beta1 concentration was found to be significantly negatively correlated with both TGF-beta2 (r=-0.261, P=0.000) and OPG (r=-0.313, P=0.000) concentrations; a significantly positive correlation was found between the TGF-beta1 and leptin concentrations (r=0.164, P=0.000) and between TGF-beta2 and OPG concentrations (r=0.432, P=0.000).

CONCLUSION

These results provide age-related reference values of TGF-beta1 and TGF-beta2 in Chinese adult women, and reveal the relationships between these cytokines.

Circulating leptin and bone mineral density in rheumatoid arthritis

OBJECTIVE

To evaluate the association between circulating leptin and bone mineral density (BMD) in patients with rheumatoid arthritis (RA).

METHODS

One-hundred thirty postmenopausal women with RA were assessed for body mass index (BMI), disease characteristics, history of drug use, rheumatoid factor, and erythrocyte sedimentation rate (ESR). BMD (g/cm(2)) was determined in the hip and spine by DEXA. Serum leptin concentrations were measured by ELISA. Spearman's correlation coefficients (rho) were determined between BMD and leptin and other variables. A multiple regression analysis was used to adjust for confounders.

RESULTS

Patients' serum leptin levels varied widely (range 2-128 ng/ml). Thirty-three patients (25%) had osteoporosis. Higher levels of leptin correlated significantly with BMD in the lumbar spine (rho = 0.17, p = 0.04) and total hip (rho = 0.21, p = 0.01). The variables that were negatively correlated with BMD were age, duration of menopause, and ESR. After adjustment for confounders, leptin was no longer associated with BMD. In the multivariate model, factors that remained associated with BMD in the total hip were age (p = 0.021) and BMI (p = 0.003); and the factors that remained associated with BMD in the lumbar spine were BMI (p = 0.03) and ESR (p = 0.01).

CONCLUSION

No relevant association was found between circulating leptin levels and BMD in patients with RA in this cross-sectional study. Followup studies are needed to evaluate whether abnormal leptin levels confer a risk for fractures due to osteoporosis.

Atorvastatin and BMD in coronary syndrome. Role of Lys656Asn polymorphism of leptin receptor gene

OBJECTIVES

To evaluate the effect of atorvastatin on bone mass and markers of bone remodeling in patients with acute coronary syndrome according to the Lys656Asn leptin receptor gene polymorphism.

METHODS

Sixty-two patients with acute coronary syndrome were included. Patients were allocated to low and high doses of atorvastatin according to baseline levels of cholesterol and triglycerides and the index of vascular risk and were studied at hospital admission and at 12 months. Cholesterol, triglycerides, total calcium, phosphorus, magnesium, osteocalcin and urinary deoxypyridinoline were determined in all patients at baseline and at 12 months of follow up. Densitometric studies were conducted in the lumbar spine and hip. Patients with a T-score<-2.5 were considered osteoporotic. The Lys656Asn leptin receptor gene polymorphism was determined by PCR.

RESULTS

Forty-two patients were Lys/Lys homozygotic and 20 Lys/Asn heterozygotic. The prevalence of osteoporosis was 31% for the Lys/Lys genotype and 27% for the Lys/Asn genotype with no significant differences between groups. There was a significant increase in bone mineral density in the lumbar spine (1.117 +/- 0.24 versus 1.135 +/- 0.24, P = 0.008) in patients with the Lys/Lys genotype.

CONCLUSION

Atorvastatin increases lumbar spine bone mineral density only in patients with the Lys/Lys genotype of the Lys656Asn polymorphism.

Is leptin a significant predictor of bone mineral density in postmenopausal Turkish women?

The objective of the present study was to investigate the relationship between leptin and bone mineral density in postmenopausal Turkish women. A total of 122 healthy postmenopausal women were enrolled in this cross-sectional study. Blood samples were obtained for analysis of serum leptin. Bone mineral density (BMD) was measured by dual-energy X-ray absorptiometry at the lumbar spine, femoral neck and trochanter on the same day. Leptin levels was significantly correlated with BMD of L(1-4) (P = 0.04), but not of femoral neck (P = 0.13), and trochanter (P = 0.39). However, Z scores of L(1-4) (P = 0.009), femur neck (P = 0.009), and femur trochanter (P = 0.025) were positively correlated with leptin levels. In multiple linear regression analysis, leptin was not found to be a statistically significant independent predictor for BMD. Leptin was associated with BMD and Z scores at various body sites; however, it was not an independent predictor of BMD.

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.

The relationship of ghrelin and adiponectin with bone mineral density and bone turnover markers in elderly men

Body weight is commonly considered a significant predictor of bone mineral density (BMD). Adiponectin, an adipocyte-derived hormone, could modulate BMD. Moreover, recent studies have reported that ghrelin is able to stimulate bone formation. In this study, we investigated any associations of adiponectin and ghrelin serum levels with bone turnover markers and BMD in elderly men. In 137 men aged 55 years and older (mean age 67.4 +/- 5.4 years, mean body mass index [BMI] 26.6 +/- 3.4 kg/m2), we evaluated serum adiponectin, serum ghrelin, body composition (fat mass and lean mass), BMD, bone alkaline phosphatase (ALP), and the carboxy-terminal telopeptide of type I collagen (betaCTX). Ghrelin showed significant correlations with BMD at the femoral neck (r = 0.25, P < 0.01), total femur (r = 0.22, P < 0.05), and whole body (r = 0.18, P < 0.05). However, after adjusting for age, BMI, and calcium intake, the correlation remained significant only for femoral neck BMD. Ghrelin showed a significant correlation with lean mass but not with fat mass and bone turnover markers. Adiponectin showed a positive association with both bone ALP and betaCTX; the correlation between adiponectin and bone ALP (r = 0.25, P < 0.01) remained significant after adjusting for confounding variables. No significant correlations between adiponectin and BMD at all skeletal sites were observed. In conclusion, our study suggests that in elderly Italian men serum ghrelin was significantly associated with femoral neck BMD and that adiponectin was positively associated with bone ALP. Further studies are needed to elucidate the role of adipocytokines in bone metabolism.

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.

Relationships between fat and bone

Body weight impacts both bone turnover and bone density, making it, therefore, an important risk factor for vertebral and hip fractures and ranking it alongside age in importance. The effect of body weight is probably contributed to by both fat mass and lean mass, though in postmenopausal women, fat mass has been more consistently demonstrated to be important. A number of mechanisms for the fat-bone relationship exist and include the effect of soft tissue mass on skeletal loading, the association of fat mass with the secretion of bone active hormones from the pancreatic beta cell (including insulin, amylin, and preptin), and the secretion of bone active hormones (e.g., estrogens and leptin) from the adipocyte. These factors alone probably do not fully explain the observed clinical associations, and study of the actions on bone of novel hormones related to nutrition is an important area of further research. An understanding of this aspect of bone biology may open the way for new treatments of osteoporosis. More immediately, the role of weight maintenance in the prevention of osteoporosis is an important public health message that needs to be more widely appreciated.

Prognostic indicators of changes in bone density measures in adolescent girls with anorexia nervosa-II

INTRODUCTION

Adolescents with anorexia nervosa (AN) have low bone mineral density (BMD). Baseline predictors of temporal BMD changes (DeltaBMD) in AN, including 1) gastrointestinal peptides regulating food intake and appetite that have been related to bone metabolism and 2) bone turnover markers, have not been well characterized. We hypothesized that baseline levels of nutritionally regulated hormones and of bone turnover markers would predict DeltaBMD overall.

METHODS

In a prospective observational study, lumbar and whole-body BMD was measured at 0, 6, and 12 months in 34 AN girls aged 12-18 yr and 33 controls. Baseline body mass index, lean mass, nutritionally regulated hormones [IGF-I, cortisol, ghrelin, leptin, and peptide YY (PYY)], bone formation, and resorption markers were examined to determine nutritional and hormonal predictors of bone density changes.

RESULTS

In a regression model, baseline ghrelin and PYY predicted changes in spine bone measures; and baseline ghrelin, cortisol, and PYY predicted changes in whole-body bone measures independent of baseline nutritional status.

CONCLUSIONS

Neuroendocrine gastrointestinal-derived peptides regulating food intake are independent predictors of changes in bone mass in AN.