Leptin and bone: does the brain control bone biology?

Analysis of bone metabolic alterations linked with osteoporosis progression in type 2 diabetic db/db mice

Type 2 diabetes (T2D) is a common chronic disease, characterized by persistent hyperglycemia and insulin resistance. This disorder is associated with decreased bone quality and an elevated risk of bone fractures. However, evidence on the relationship between systemic metabolic change and the development of type 2 diabetic osteoporosis (T2DOP) remains elusive. Herein, we investigate the changes of bone metabolites with bone loss in db/db mice (an animal model of T2DOP exhibited bone loss with age progression), and explore the potential metabolic mechanism underlying type 2 diabetes and osteoporosis. C57BKS male mice were distributed in four groups, consisting six mice in each group: 8w m/m, 24w m/m, 8w db/db and 24w db/db. Bone morphometric and biomechanical parameters of db/db mice were analyzed by micro-CT and materials tester, it was found that 24w db/db mice showed severe bone loss and decreased bone tissue hardness compared with misty/misty littermates. The tibia of misty/misty mice (8 weeks, 24 weeks) and db/db mice (8 weeks, 24 weeks) were screened for differential metabolites by UPLC-Orbitrap MS. Ninety-eight metabolites were identified (35 and 63 metabolites are associated with early staged and late staged, respectively), consisting of amino acids, fatty acyls, and nucleotides. Notably, fatty acyls (such as 18-HEPE, 16(17)-EpDPE, arachidonic acid) and glycerophospholipids (such as phosphocholines (PC) (O-10:1(9E)/0:0), PC (O-16:1(9E)/0:0) [U] and phosphatidylethanolamines (PE) (P-16:0/0:0)) were significantly increased, and metabolites of amino acid pathway (such as l-glutamine, proline, phenylalanine) showed a downregulation trend. Dysregulation of lipid and glutathione pathways is the major contributor to progression of T2DOP in C57BKS mice.

Implications of the circadian clock in implant dentistry

Circadian rhythms are approximately 24-h cell-autonomous cycles driven by transcription and translation feedback loops of a set of core circadian clock genes, such as circadian locomoter output cycles kaput (Clock), brain and muscle arnt-like protein-1 (Bmal1), period (Per), and cryptochrome (Cry). The genetic clockwork of these genes produces circadian rhythms in cells throughout the body, including the craniofacial region. During development, dento-alveolar bone tissue formation could be regulated by site-specific circadian patterns. Studies using knockout mice and mesenchymal stem cells (MSCs) to evaluate clock genes revealed regulatory effects of clock function on bone remodeling, suggesting involvement of the circadian clockwork in osseointegration of titanium implants. Indeed, rough surface titanium modulates specific clock genes, Neuronal PAS domain protein-2 (Npas2) and Per, in MSCs to facilitate osseointegration. Further understanding of the bone clock machinery associated with biomaterial surface properties might improve preoperative diagnosis for dental implant treatments.

Neuronal PAS domain 2 (Npas2) facilitated osseointegration of titanium implant with rough surface through a neuroskeletal mechanism

Titanium (Ti) biomaterials have been applied to a wide range of implantable medical devices. When placed in bone marrow, Ti-biomaterials integrate to the surrounding bone tissue by mechanisms that are not fully understood. We have previously identified an unexpected upregulation of circadian clock molecule neuronal PAS domain 2 (Npas2) in successfully integrated implant with a rough surface. This study aimed to elucidate the molecular mechanism of osseointegration through determining the role of Npas2. Human bone marrow stromal cells (BMSC) that were cultured on a Ti disc with SLA surface exhibited increased NPAS2 expression compared to BMSC cultured on a machined surface. A mouse model was developed in which miniature Ti implants were surgically placed into femur bone marrow. The implant push-out test and bone-to-implant contact measurements demonstrated the establishment of osseointegration in 3 weeks. By contrast, in Npas2 functional knockout (KO) mice, the implant push-out value measured for SLA surface Ti implant was significantly decreased. Npas2 KO mice demonstrated normal femur bone structure surrounding the Ti implant; however, the recovered implants revealed abnormal remnant mineralized tissue, which lacked dense collagen architecture typically found on recovered implants from wild type mice. To explore the mechanisms leading to the induced Npas2 expression, an unbiased chemical genetics analysis was conducted using mouse BMSC carrying an Npas2-reporter gene for high throughput screening of Library of Pharmacologically Active Compounds. Npas2 modulating compounds were found clustered in regulatory networks of the α2-adrenergic receptor and its downstream cAMP/CREB signaling pathway. Mouse primary BMSC exposed to SLA Ti disc significantly increased the expression of α2-adrenergic receptors, but the expression of β2-adrenergic receptor was unaffected. Our data provides the first evidence that peripheral clock gene component Npas2 plays a role in facilitating the enhanced osseointegration through neuroskeletal regulatory pathways induced by BMSC in contact with rough surface Ti implant.

Serum Levels of Leptin in Multiple Myeloma Patients and Its Relation to Angiogenic and Inflammatory Cytokines

Background

Leptin, apart from the regulation of food intake, has been implicated in hematopoiesis, the immune response and angiogenesis. Leptin has been found to be decreased in various hematological malignancies. In the present study leptin was measured in multiple myeloma (MM) patients before and after treatment and correlated with other angiogenic molecules and markers of disease activity.

Methods

Serum leptin, vascular endothelial growth factor (VEGF), basic fibroblast growth factor (b-FGF), interleukin-1 beta (IL-1β), beta 2 microglobulin (β2M) and C-reactive protein (CRP) were measured in 62 newly diagnosed MM patients, 22 of whom obtaining disease stabilization after treatment. The same parameters were measured in 20 healthy controls. Disease stage was defined according to the Durie-Salmon criteria.

Results

Leptin, VEGF, b-FGF, IL-1β, and β2M were significantly higher in newly diagnosed MM patients than in controls (p<0.05). VEGF, b-FGF, IL-1β, β2M, CRP but not leptin increased with advancing stage of disease (p<0.01). All parameters decreased significantly following treatment (p<0.001). Although IL-1β correlated positively with VEGF, β2M, b-FGF and CRP, leptin did not correlate with any of the measured parameters.

Conclusion

Leptin serum levels do not reflect disease severity in MM. However, there seems to be a decrease in leptin following treatment, which may be associated with an alteration in the metabolic state or the chemokine milieu.

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.

Plasma adipokines and body composition in response to modest dietary manipulations in the mouse

OBJECTIVE

The relationship between adipokine levels and body composition has not been carefully examined. Most studies in humans are cross-sectional, and the few studies in mice have been restricted to a comparison of control animals with markedly obese, insulin-resistant mice. Our objective was to study changes in adipokine levels and body composition in response to modest dietary intervention.

RESEARCH METHODS AND PROCEDURES

Plasma resistin, adiponectin, and leptin levels were examined in mice fed ad libitum, a 75% restricted diet, or a diet supplemented with 10% sucrose. Body composition was determined by whole-body DXA.

RESULTS

The percentage body fat was reduced in mice subjected to the restricted diet and increased in mice supplemented with 10% dextrose. Adipokine levels were not different in either of these groups compared with the control mice. A significant inverse correlation was observed between resistin levels and total body fat, whereas there was no significant correlation between body fat and adiponectin levels. Positive correlations were observed between leptin levels and percentage body fat, total body fat, and abdominal fat. Leptin levels correlated with plasma glucose, but multivariate analysis revealed that this correlation was the result of a strong positive correlation between leptin and insulin levels. There were no correlations between glycemia and resistin or glycemia and adiponectin levels, and no correlation was observed between any of the adipokine levels and bone mineral content or density.

DISCUSSION

These data suggest that in the mouse, modest dietary perturbations have little effect on resistin and adiponectin levels despite significant effects on glycemia, insulin levels, and bone parameters.

The relationship between blood leptin level and bone density is specific to ethnicity and menopausal status

Leptin, the obesity hormone, has been linked to bone mineralization and tumorigenesis. In addition, both bone mineral density (BMD) and postmenopausal breast cancer are associated with obesity, but the interrelationships between obesity, leptin, BMD, and breast cancer are not yet clear. In particular, there is little published research comparing white and black women in terms of these variables. We obtained blood specimens for leptin analysis from a group of 320 breast cancer patients and controls with an ethnic composition of 49% white women and 51% black women. Distal and proximal radial BMD (DBMD and PBMD) were measured by dual-energy X-ray absorptiometry, and age- and ethnicity-specific standardized scores (Z-scores) were calculated for bone density. Blood leptin levels were determined by radioimmunoassay. Blood leptin level was not linked to breast cancer risk. Leptin levels were significantly higher in black women than in white women and were also significantly higher in obese and overweight women than in normal-weight women. Black women weighed more and had a higher body mass index (BMI) than white women. After controlling for BMI, leptin was correlated with DBMD ( r = .17; P < .05) and PBMD ( r = .21; P < .05) in whites, but not in blacks. Leptin was also correlated with both distal and proximal Z-scores in postmenopausal women ( r = .14 and .13; P < .05). Thus leptin may be a predictor for BMD in a population that is prone to have a low BMD, and this relationship is independent of the effect of body weight on leptin levels. Our results suggest that ethnicity and menopausal status should be considered when comparing results from different studies.

Regulation of apoptosis in osteoclasts and osteoblastic cells

In postnatal life, the skeleton undergoes continuous remodeling in which osteoclasts resorb aged or damaged bone, leaving space for osteoblasts to make new bone. The balance of proliferation, differentiation, and apoptosis of bone cells determines the size of osteoclast or osteoblast populations at any given time. Bone cells constantly receive signals from adjacent cells, hormones, and bone matrix that regulate their proliferation, activity, and survival. Thus, the amount of bone and its microarchitecture before and after the menopause or following therapeutic intervention with drugs, such as sex hormones, glucocorticoids, parathyroid hormone, and bisphosphonates, is determined in part by effects of these on survival of osteoclasts, osteoblasts, and osteocytes. Understanding the mechanisms and regulation of bone cell apoptosis will enhance our knowledge of bone cell function and help us to develop better therapeutics for the management of osteoporosis and other bone diseases.

Bone growth stimulators. New tools for treating bone loss and mending fractures

In the new millennium, humans will be traveling to Mars and eventually beyond with skeletons that respond to microgravity by self-destructing. Meanwhile in Earth's aging populations growing numbers of men and many more women are suffering from crippling bone loss. During the first decade after menopause all women suffer an accelerating loss of bone, which in some of them is severe enough to result in "spontaneous" crushing of vertebrae and fracturing of hips by ordinary body movements. This is osteoporosis, which all too often requires prolonged and expensive care, the physical and mental stress of which may even kill the patient. Osteoporosis in postmenopausal women is caused by the loss of estrogen. The slower development of osteoporosis in aging men is also due at least in part to a loss of the estrogen made in ever smaller amounts in bone cells from the declining level of circulating testosterone and is needed for bone maintenance as it is in women. The loss of estrogen increases the generation, longevity, and activity of bone-resorbing osteoclasts. The destructive osteoclast surge can be blocked by estrogens and selective estrogen receptor modulators (SERMs) as well as antiosteoclast agents such as bisphosphonates and calcitonin. But these agents stimulate only a limited amount of bone growth as the unaffected osteoblasts fill in the holes that were dug by the now suppressed osteoclasts. They do not stimulate osteoblasts to make bone--they are antiresorptives not bone anabolic agents. (However, certain estrogen analogs and bisphosphates may stimulate bone growth to some extent by lengthening osteoblast working lives.) To grow new bone and restore bone strength lost in space and on Earth we must know what controls bone growth and destruction. Here we discuss the newest bone controllers and how they might operate. These include leptin from adipocytes and osteoblasts and the statins that are widely used to reduce blood cholesterol and cardiovascular damage. But the main focus of this article is necessarily the currently most promising of the anabolic agents, the potent parathyroid hormone (PTH) and certain of its 31- to 38-aminoacid fragments, which are either in or about to be in clinical trial or in the case of Lilly's Forteo [hPTH-(1-34)] tentatively approved by the Food and Drug Administration for treating osteoporosis and mending fractures.

Human osteoclasts express oxytocin receptor

Increasing evidences demonstrated many new targets for the hypothalamic hormone oxytocin, as the regulation of food balance and in some cases of leptin secretion. Considering that leptin is a potent inhibitor of bone formation and that oxytocin receptors (OTR) were detected in normal human osteoblasts, we investigated if OTR was expressed by human osteoclasts (hOCs) and the effect of the hormone on these cells. Here, we demonstrate by immunofluorescence and by Western blot analysis the expression of OTR by fully differentiated hOCs and by their precursors (pOCs). We also show that the receptor is functional, as OT treatment induces an increase of [Ca(2+)](i), and that the hormone may affect osteoclastogenesis, since it increases the number of pre-osteoclasts.

Leptin stimulates human osteoblastic cell proliferation, de novo collagen synthesis, and mineralization: Impact on differentiation markers, apoptosis, and osteoclastic signaling

Anabolic hormones, mechanical loading, and the obese protein leptin play separate roles in maintaining bone mass. We have previously shown that leptin, as well as its receptor, are expressed by normal human osteoblasts. Consequently, we have investigated how leptin affects proliferation, differentiation, and apoptosis of human osteoblasts. Iliac crest osteoblasts, incubated with either leptin (100 ng/ml), calcitriol (1,25(OH)(2)D(3); 10(-9) M) or 1-84 human parathyroid hormone (PTH; 10(-8) M), were cultured for 35 consecutive days and assayed for expression of various differentiation-related marker genes (as estimated by RT-PCR), de novo collagen synthesis, proliferation, in vitro mineralization, and osteoclast signaling. The effects of leptin on protection against retinoic acid (RA; 10(-7) M) induced apoptosis, as well as transition into preosteocytes, were also tested. Leptin exposure enhanced cell proliferation and collagen synthesis over both control condition and PTH exposure. Leptin inhibited in vitro calcified nodule production after 1-2 weeks in culture, however, subsequent to 4-5 weeks, leptin significantly stimulated mineralization. The mineralization profile throughout the entire incubation period was almost undistinguishable from the one induced by PTH. In comparison, 1,25(OH)(2)D(3) generally reduced proliferation and collagen production rates, whereas mineralization was markedly enhanced. Leptin exposure (at 2 and 5 weeks) significantly enhanced the expression of TGFbeta, IGF-I, collagen-Ialpha, ALP, and osteocalcin mRNA. Leptin also protected against RA-induced apoptosis, as estimated by soluble DNA fractions and DNA laddering patterns subsequent to 10 days of culture. The expression profiles of Bax-alpha and Bcl-2 mRNAs indicated that leptin per se significantly protected against apoptosis throughout the entire incubation period. Furthermore, the osteoblast marker OSF-2 was diminished, whereas the CD44 osteocyte marker gene expression was stimulated, indicating a transition into preosteocytes. In terms of osteoclastic signaling, leptin significantly augmented the mRNA levels of both interleukin-6 (IL-6) and osteoprotegerin (OPG). In summary, continuous leptin exposure of iliac crest osteoblasts, promotes collagen synthesis, cell differentiation and in vitro mineralization, as well as cell survival and transition into preosteocytes. Leptin may also facilitate osteoblastic signaling to the osteoclast.

Relationship of serum leptin concentration with bone mineral density in the United States population

Overweight is associated with both higher bone mineral density (BMD) and higher serum leptin concentrations. In humans, little is known about the relationship of leptin concentration and bone density. We studied this relationship in a large, national population-based sample. Participants included 5815 adults in the Third U.S. National Health and Nutrition Examination Survey (NHANES III; 1988-1994) who underwent DXA of the proximal femur and measurement of fasting serum leptin. Mean +/- SE BMD (gm/cm2) of the total hip was 1.01 +/- 0.005 in men, 0.94 +/- 0.004 in premenopausal women, and 0.78 +/- 0.007 in postmenopausal women. Bone density increased with increasing leptin concentration in men (p = 0.003), premenopausal women (p < 0.001), and postmenopausal women (p < 0.001). However, after adjusting for body mass index (BMI) and other bone density-related factors, an inverse association emerged in men (p < 0.001), being most evident among men < 60 years old. There was no association of leptin and BMD in premenopausal women (p = 0.66) or postmenopausal women (p = 0.69) in multivariate analysis. Controlling for leptin had no effect on the strong positive association of BMI and BMD in either men or women. Serum leptin concentration did not appear to affect directly BMD. If present, the association appeared to be limited to younger men who are at lower risk of osteoporosis.

Leptin in anorexia nervosa and bulimia nervosa: importance of assay technique and method of interpretation

Studies of the role of leptin in patients with anorexia nervosa and bulimia nervosa have conflicted in their data and interpretation. Such differences may be a result of the assay methods used or the way results are compared with those from normal controls. To investigate these possibilities, we analyzed serum leptin levels in anorexic, bulimic, obese, and control individuals, thereby spanning the full range of human body weights, using three frequently employed commercial kits. Kits from Linco (St Louis, MO) and DSL (Webster, TX) employ a radioimmunoassay method, and the R&D Systems kit (Minneapolis, MN) uses an enzyme-linked immunosorbent assay. We found that the three kits provide results that are highly linearly correlated with each other and remarkably linearly related to percent ideal body weight (%IBW) over more than three orders of magnitude (Linco, r = 0.90; R&D, r = 0.87; DSL, r = 0.86). For very low leptin levels, the more sensitive kits from R&D and Linco appeared to give more reliable results. Measurement method does not appear to explain the literature conflicts. We found that patients with anorexia nervosa have serum leptin values that lie above the line extrapolated from the %IBW/leptin curve generated from analysis of all non-anorexic patients. Therefore, in anorexia nervosa, inappropriately high leptin levels for %IBW may contribute to a blunted physiologic response to underweight and consequent resistance to dietary treatment. By contrast, most bulimic patients have leptin levels significantly below those predicted from the same %IBW/leptin curve. The relative leptin deficiency in bulimic subjects may contribute to food-craving behavior. We propose that using the %IBW/ leptin curve can facilitate identification of true pathophysiologic abnormalities in eating-disordered individuals and provide a basis for the design of therapeutic interventions or monitoring of response to treatment.

Neuroprotective effects of leptin in vivo and in vitro

The present study explored the efficacy of leptin in protecting against in vivo induction of excitotoxic lesions by the glutamatergic analogue ibotenate injected into the developing mouse brain and against in vitro NMDA-induced cell death in primary neuronal cultures. Ibotenate injected intracerebrally (i.c.) to mice on postnatal day 5 produced transcortical necrosis and white matter cysts. Co-treatment with leptin administered i.c. or i.p. reduced ibotenate-induced cortical lesions and white matter cysts by 50%. in vitro, leptin afforded significant neuroprotection of mouse cortical neurons against NMDA cytotoxicity. The neuroprotective effect of leptin was antagonized both in vivo and in vitro by the Jak2 inhibitor AG490, indicating that it was mediated via the leptin receptor and Jak2 activation. These findings are the first evidence for a role of leptin in neuroprotection.

Leptin regulation of the immune response and the immunodeficiency of malnutrition

Leptin is a 16 kDa protein mainly produced by adipose tissue in proportion to adipose tissue mass. Originally thought to be a satiety factor, leptin is a pleiotropic molecule. In addition to playing a role in energy regulation, leptin also regulates endocrine and immune functions. Both the structure of leptin and that of its receptor suggest that leptin might be classified as a cytokine. The secondary structure of leptin has similarities to the long-chain helical cytokines family, which includes interleukin 6 (IL-6), IL-11, CNTF, and LIF, and the leptin receptor is homologous to the gp-130 signal-transducing subunit of the IL-6-type cytokine receptors. Leptin plays a role in innate and acquired immunity. Leptin levels increase acutely during infection and inflammation, and may represent a protective component of the host response to inflammation. More important, leptin deficiency increases susceptibility to infectious and inflammatory stimuli and is associated with dysregulation of cytokine production. Leptin deficiency also causes a defect in hematopoiesis. Leptin regulates T cells responses, polarizing Th cells toward a Th1 phenotype. Low leptin levels occurring during starvation mediate the neuroendocrine and immune dysfunction of starvation.