Leptin is a potent stimulator of bone growth in ob/ob mice

Local leptin production in osteoarthritis subchondral osteoblasts may be responsible for their abnormal phenotypic expression

INTRODUCTION

Leptin is a peptide hormone with a role in bone metabolism and rheumatic diseases. The subchondral bone tissue plays a prominent role in the pathophysiology of osteoarthritis (OA), related to abnormal osteoblast (Ob) differentiation. Although leptin promotes the differentiation of Ob under normal conditions, a role for leptin in OA Ob has not been demonstrated. Here we determined if endogenous leptin produced by OA Ob could be responsible for the expression of the abnormal phenotypic biomarkers observed in OA Ob.

METHODS

We prepared primary normal and OA Ob from subchondral bone of tibial plateaus removed for knee surgery of OA patients or at autopsy. We determined the production of leptin and of the long, biologically active, leptin receptors (OB-Rb) using reverse transcriptase-polymerase chain reaction, ELISA and Western blot analysis. We determined the effect of leptin on cell proliferation by BrdU incorporation and 3-(4,5-Dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assays, and we determined by Western blot analysis phospho 42/44 MAPK (p42/44 Erk1/2) and phospho p38 levels. We then determined the effect of the addition of exogenous leptin, leptin receptor antagonists, inhibitors of leptin signaling or siRNA techniques on the phenotypic features of OA Ob. Phenotypic features of Ob were determined by measuring alkaline phosphatase activity (ALP), osteocalcin release (OC), collagen type 1 production (CICP) and of Transforming Growth Factor-beta1 (TGF-beta1).

RESULTS

Leptin expression was increased approximately five-fold and protein levels approximately two-fold in OA Ob compared to normal. Leptin stimulated its own expression and the expression of OB-Rb in OA Ob. Leptin dose-dependently stimulated cell proliferation of OA Ob and also increased phosphorylated p42/44 Erk1/2 and p38 levels. Inactivating antibodies against leptin reduced ALP, OC, CICP and TGF-beta1 levels in OA Ob. Tyrphostin (AG490) and piceatannol (Pce), inhibitors of leptin signaling, reproduced this effect. Inhibition of endogenous leptin levels using siRNA for leptin or inhibiting leptin signaling using siRNA for OB-Rb expression both reduced ALP and OC about 60%. Exogenous leptin addition stimulated ALP, yet this failed to further increase OC or CICP.

CONCLUSIONS

These results suggest that abnormal production of leptin by OA Ob could be responsible, in part, for the elevated levels of ALP, OC, collagen type 1 and TGF-beta1 observed in these cells compared to normal. Leptin also stimulated cell proliferation, and Erk 1/2 and p38 signaling. Taken together, these data suggest leptin could contribute to abnormal osteoblast function in OA.

Variation in within-bone stiffness measured by nanoindentation in mice bred for high levels of voluntary wheel running

The hierarchical structure of bone, involving micro-scale organization and interaction of material components, is a critical determinant of macro-scale mechanics. Changes in whole-bone morphology in response to the actions of individual genes, physiological loading during life, or evolutionary processes, may be accompanied by alterations in underlying mineralization or architecture. Here, we used nanoindentation to precisely measure compressive stiffness in the femoral mid-diaphysis of mice that had experienced 37 generations of selective breeding for high levels of voluntary wheel running (HR). Mice (n = 48 total), half from HR lines and half from non-selected control (C) lines, were divided into two experimental groups, one with 13-14 weeks of access to a running wheel and one housed without wheels (n = 12 in each group). At the end of the experiment, gross and micro-computed tomography (microCT)-based morphometric traits were measured, and reduced elastic modulus (E(r)) was estimated separately for four anatomical quadrants of the femoral cortex: anterior, posterior, lateral, and medial. Two-way, mixed-model analysis of covariance (ancova) showed that body mass was a highly significant predictor of all morphometric traits and that structural change is more apparent at the microCT level than in conventional morphometrics of whole bones. Both line type (HR vs. C) and presence of the mini-muscle phenotype (caused by a Mendelian recessive allele and characterized by a approximately 50% reduction in mass of the gastrocnemius muscle complex) were significant predictors of femoral cortical cross-sectional anatomy. Measurement of reduced modulus obtained by nanoindentation was repeatable within a single quadrant and sensitive enough to detect inter-individual differences. Although we found no significant effects of line type (HR vs. C) or physical activity (wheel vs. no wheel) on mean stiffness, anterior and posterior quadrants were significantly stiffer (P < 0.0001) than medial and lateral quadrants (32.67 and 33.09 GPa vs. 29.78 and 30.46 GPa, respectively). Our findings of no significant difference in compressive stiffness in the anterior and posterior quadrants agree with previous results for mice, but differ from those for large mammals. Integrating these results with others from ongoing research on these mice, we hypothesize that the skeletons of female HR mice may be less sensitive to the effects of chronic exercise, due to decreased circulating leptin levels and potentially altered endocannabinoid signaling.

Reduced size-independent mechanical properties of cortical bone in high-fat diet-induced obesity

Overweight and obesity are rapidly expanding health problems in children and adolescents. Obesity is associated with greater bone mineral content that might be expected to protect against fracture, which has been observed in adults. Paradoxically, however, the incidence of bone fractures has been found to increase in overweight and obese children and adolescents. Prior studies have shown some reduced mechanical properties as a result of high-fat diet (HFD) but do not fully address size-independent measures of mechanical properties, which are important to understand material behavior. To clarify the effects of HFD on the mechanical properties and microstructure of bone, femora from C57BL/6 mice fed either a HFD or standard laboratory chow (Chow) were evaluated for structural changes and tested for bending strength, bending stiffness and fracture toughness. Here, we find that in young, obese, high-fat fed mice, all geometric parameters of the femoral bone, except length, are increased, but strength, bending stiffness, and fracture toughness are all reduced. This increased bone size and reduced size-independent mechanical properties suggests that obesity leads to a general reduction in bone quality despite an increase in bone quantity; yield and maximum loads, however, remained unchanged, suggesting compensatory mechanisms. We conclude that diet-induced obesity increases bone size and reduces size-independent mechanical properties of cortical bone in mice. This study indicates that bone quantity and bone quality play important compensatory roles in determining fracture risk.

Pathogenesis of adolescent idiopathic scoliosis in girls - a double neuro-osseous theory involving disharmony between two nervous systems, somatic and autonomic expressed in the spine and trunk: possible dependency on sympathetic nervous system and hormones with implications for medical therapy

Anthropometric data from three groups of adolescent girls - preoperative adolescent idiopathic scoliosis (AIS), screened for scoliosis and normals were analysed by comparing skeletal data between higher and lower body mass index subsets. Unexpected findings for each of skeletal maturation, asymmetries and overgrowth are not explained by prevailing theories of AIS pathogenesis. A speculative pathogenetic theory for girls is formulated after surveying evidence including: (1) the thoracospinal concept for right thoracic AIS in girls; (2) the new neuroskeletal biology relating the sympathetic nervous system to bone formation/resorption and bone growth; (3) white adipose tissue storing triglycerides and the adiposity hormone leptin which functions as satiety hormone and sentinel of energy balance to the hypothalamus for long-term adiposity; and (4) central leptin resistance in obesity and possibly in healthy females. The new theory states that AIS in girls results from developmental disharmony expressed in spine and trunk between autonomic and somatic nervous systems. The autonomic component of this double neuro-osseous theory for AIS pathogenesis in girls involves selectively increased sensitivity of the hypothalamus to circulating leptin (genetically-determined up-regulation possibly involving inhibitory or sensitizing intracellular molecules, such as SOC3, PTP-1B and SH2B1 respectively), with asymmetry as an adverse response (hormesis); this asymmetry is routed bilaterally via the sympathetic nervous system to the growing axial skeleton where it may initiate the scoliosis deformity (leptin-hypothalamic-sympathetic nervous system concept = LHS concept). In some younger preoperative AIS girls, the hypothalamic up-regulation to circulating leptin also involves the somatotropic (growth hormone/IGF) axis which exaggerates the sympathetically-induced asymmetric skeletal effects and contributes to curve progression, a concept with therapeutic implications. In the somatic nervous system, dysfunction of a postural mechanism involving the CNS body schema fails to control, or may induce, the spinal deformity of AIS in girls (escalator concept). Biomechanical factors affecting ribs and/or vertebrae and spinal cord during growth may localize AIS to the thoracic spine and contribute to sagittal spinal shape alterations. The developmental disharmony in spine and trunk is compounded by any osteopenia, biomechanical spinal growth modulation, disc degeneration and platelet calmodulin dysfunction. Methods for testing the theory are outlined. Implications are discussed for neuroendocrine dysfunctions, osteopontin, sympathoactivation, medical therapy, Rett and Prader-Willi syndromes, infantile idiopathic scoliosis, and human evolution. AIS pathogenesis in girls is predicated on two putative normal mechanisms involved in trunk growth, each acquired in evolution and unique to humans.

Leptin plays a catabolic role on articular cartilage

Leptin has been shown to play a crucial role in the regulation of body weight. There is also evidence that this adipokine plays a key role in the process of osteoarthritis. However, the precise role of leptin on articular cartilage metabolism is not clear. We investigate the role of leptin on articular cartilage in vivo in this study. Recombinant rat leptin (100 μg) was injected into the knee joints of rats, 48 h later, messenger RNA (mRNA) expression and protein levels of basic fibroblast growth factor (bFGF), vascular endothelial growth factor (VEGF), matrix metalloproteinases 2 and 9 (MMP-2, MMP-9), cathepsin D, and collagen II from articular cartilage were analyzed by real-time quantitative polymerase chain reaction (PCR) and western blot. Two important aggrecanases ADAMTS-4 and -5 (a disintegrin and metalloproteinase with thrombospondin motifs 4 and 5) were also analyzed by real-time quantitative PCR. Besides, articular cartilage was also assessed for proteoglycan/GAG content by Safranin O staining. Leptin significantly increased both gene and protein levels of MMP-2, MMP-9, cathepsin D, and collagen II, while decreased bFGF markedly in cartilage. Moreover, the gene expression of ADAMTS-4 and -5 were markedly increased, and histologically assessed depletion of proteoglycan in articular cartilage was observed after treatment with leptin. These results strongly suggest that leptin plays a catabolic role on cartilage metabolism and may be a disadvantage factor involve in the pathological process of OA.

Consumption of green tea extract results in osteopenia in growing male mice

Consumption of green tea may reduce body weight gain. Although many disorders are related to obesity, bone mass is positively correlated with body mass. Therefore, our purpose in this study was to determine the effects of green tea extract (GTE) on bone mass and architecture in rapidly growing lean [C57BL/6 wild type (WT)] and genetically obese, leptin-deficient (ob/ob) male mice. Five-week-old lean and ob/ob mice were assigned to diets containing GTE at 0, 1, or 2% for 6 wk. Femoral and lumbar vertebral bone volume and architecture were evaluated by micro-computed tomography (muCT). Following muCT analysis, femora were ashed to determine bone mineral content and density. Compared with WT mice, ob/ob mice had shorter femora (P < 0.001), lower femoral bone volume (P < 0.001), and lower femoral bone mineral content (P < 0.001), but higher cancellous bone volume in lumbar vertebrae (P < 001). Neither genotype nor treatment affected femoral bone mineral density, indicating normal mineralization. GTE consumption resulted in lower femur length, volume, mineral content, cortical volume, and cortical thickness (P < 0.001), as well as lower cancellous bone volume/tissue volume (P < 0.008) and trabecular thickness (P < 0.004) in lumbar vertebrae. The results indicate that leptin is not essential for the reduced gains in body weight and bone mass due to GTE in growing mice and suggest that consumption of large quantities of green tea may reduce the rate of bone accumulation during growth.

Nutrition and bone growth in pediatrics

Children's growth is a hallmark of their normal development and the association between nutrition and linear growth in children is well accepted. Growth requires an adequate supply of many different nutritional factors, some form the "building materials," whereas others play regulatory roles. In this article we describe the growth of the growth plate and discuss the role of nutritional affected hormones on this process. In addition we describe the effect of local regulators and nutritional factors on the growth process and suggest the involvement of new regulatory factors in the translation of nutrition to growth.

Leptin stimulates protein synthesis-activating translation machinery in human trophoblastic cells

Leptin was originally considered as an adipocyte-derived signaling molecule for the central control of metabolism. However, pleiotropic effects of leptin have been identified in reproduction and pregnancy, particularly in placenta, where it may work as an autocrine hormone, mediating angiogenesis, growth, and immunomodulation. Leptin receptor (LEPR, also known as Ob-R) shows sequence homology to members of the class I cytokine receptor (gp130) superfamily. In fact, leptin may function as a proinflammatory cytokine. We have previously found that leptin is a trophic and mitogenic factor for trophoblastic cells. In order to further investigate the mechanism by which leptin stimulates cell growth in JEG-3 cells and trophoblastic cells, we studied the phosphorylation state of different proteins of the initiation stage of translation and the total protein synthesis by [(3)H]leucine incorporation in JEG-3 cells. We have found that leptin dose-dependently stimulates the phosphorylation and activation of the translation initiation factor EIF4E as well as the phosphorylation of the EIF4E binding protein EIF4EBP1 (PHAS-I), which releases EIF4E to form active complexes. Moreover, leptin dose-dependently stimulates protein synthesis, and this effect can be partially prevented by blocking mitogen-activated protein kinase (MAPK) and phosphatidylinositol 3 kinase (PIK3) pathways. In conclusion, leptin stimulates protein synthesis, at least in part activating the translation machinery, via the activation of MAPK and PIK3 pathways.

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.

Skeletal changes associated with the onset of type 2 diabetes in the ZDF and ZDSD rodent models

The incidence and prevalence of type 2 diabetes (T2D) continue to escalate at an unprecedented rate in the United States, particularly among populations with high rates of obesity. The impact of T2D on bone mass, geometry, architecture, strength, and resistance to fracture has yet to be incontrovertibly characterized because of the complex and heterogeneous nature of this disease. This study utilized skeletally mature male diabetic rats of the commonly used Zucker diabetic fatty (ZDF) and Zucker diabetic Sprague-Dawley (ZDSD) strains as surrogate models to assess alterations in bone attributable to T2D-like states. After the animals were euthanized, bone data were collected using dual-energy X-ray absorptiometry, peripheral quantitative tomography, and micro-CT imaging modalities and via three-point bending or compression mechanical testing methods. ZDF and ZDSD diabetic rats exhibited lower bone mineral densities, which coincided with declines in structural strength and increased fragility at the femoral midshaft and the L4 vertebral body in response to monotonic loading. Vertebral trabecular morphology was compromised in both diabetic rodent strains, and ZDSD diabetic rats exhibited additional phenotypic impairments to bone material properties at the spine. Because the metabolic origin of the T2D-like state that develops in the ZDSD rat strain is highly relevant to adult-onset diabetes, it is a particularly attractive novel model for future preclinical research.

Leptin, ghrelin, and endocannabinoids: potential therapeutic targets in anorexia nervosa

Anorexia nervosa (AN) has the highest mortality rate between psychiatric disorders, and evidence for managing it is still very limited. So far, pharmacological treatment has focused on a narrow range of drugs and only a few controlled studies have been performed. Furthermore, the studies have been of short duration and included a limited number of subjects, often heterogenic with regard to stage and acute nutritive status. Thus, novel approaches are urgently needed. Body weight homeostasis is tightly regulated throughout life. With the discovery of orexigenic and anorectic signals, an array of new molecular targets to control eating behavior has emerged. This review focuses on recent advances in three important signal systems: leptin, ghrelin, and endocannabinoids toward the identification of potential therapeutical breakthroughs in AN. Our review of the current literature shows that leptin may have therapeutic potentials in promoting restoration of menstrual cycles in weight restored patients, reducing motor restlessness in severely hyperactive patients, and preventing osteoporosis in chronic patients. Ghrelin and endocannabinoids exert orexigenic effects which may facilitate nutritional restoration. Leptin and endocannabinoids may exert antidepressive and anxiolytic effects. Finally, monitoring serum concentration of leptin may be useful in order to prevent refeeding syndrome.

Body mass influences cortical bone mass independent of leptin signaling

Obesity in humans is associated with increased bone mass. Leptin, a hormone produced by fat cells, functions as a sentinel of energy balance, and may mediate the putative positive effects of body mass on bone. We performed studies in male C57Bl/6 wild type (WT) and leptin-deficient ob/ob mice to determine whether body mass gain induced by high fat intake increases bone mass and, if so, whether this requires central leptin signaling. The relationship between body mass and bone mass and architecture was evaluated in 9-week-old and 24-week-old WT mice fed a regular mouse diet. Femora and lumbar vertebrae were analyzed by micro computed tomography. In subsequent studies, slowly and rapidly growing ob/ob mice were injected in the hypothalamus with a recombinant adeno-associated virus containing the leptin gene (rAAV-lep) or a control vector, rAAV-GFP (green fluorescent protein). The mice were maintained on a regular control diet for 5 or 7 weeks and then subdivided into groups and either continued on the control diet or fed a high fat diet (45% of kcal from fat) for 8 weeks. In the WT mice, femoral and vertebral bone mass was positively correlated with body mass (Pearson's r=0.65-0.88 depending on endpoint). rAAV-lep therapy dramatically decreased body mass (-61%) but increased femur length. However, in the distal femur and lumbar vertebra, rAAV-lep therapy reduced cancellous bone volume/tissue volume, trabecular number and trabecular thickness, and increased trabecular spacing. The high fat diet increased body mass, irrespective of vector treatment. Total femur bone volume, length, cross-sectional volume, and cortical volume and thickness were increased in mice with increased body mass, independent of rAAV treatment. In the distal femur, increased body mass had no effect on cancellous architecture and there were no vector x body mass interactions. In WT mice, increased body mass resulted in increased (+33%) vertebral cancellous bone volume/tissue volume. Increased body mass had minimal independent effect on cancellous vertebral bone mass in ob/ob mice. Taken together, these findings suggest that increased body mass has a positive effect on femur cortical bone mass that is independent of leptin signaling.

Bone mineral density and bone turnover in relation to serum leptin, alpha-ketoglutarate and sex steroids in overweight and obese postmenopausal women

OBJECTIVE

Recent studies have shown that parallel changes in body weight and bone mass can be partially mediated via circulating leptin. Therefore, among the hormones involved in bone and mineral metabolism, such as oestrogens, testosterone and parathormone, leptin has recently become a subject of considerable interest. The aim of this study was to assess associations between leptin, E(2), testosterone, dehydroepiandrosterone sulphate (DHEA-S), SHBG, alpha-ketoglutaric acid (AKG) and bone mineral density (BMD) and bone turnover markers in overweight and obese postmenopausal women.

DESIGN

Eighty healthy, postmenopausal Caucasian women were studied. BMD of the lumbar spine (L(2)-L(4)) and femoral neck regions were examined using the dual X-ray absorptiometry (DXA) method. Associations were evaluated in stepwise multiple regression analysis, including information on the possible confounders and effect modifiers, for example, age, years since menopause, height and weight.

RESULTS

Femoral neck BMD was positively correlated with weight (r = 0.52, P < 0.000001), body mass index (BMI) (r = 0.48, P < 0.000006), hipline (r = 0.48, P < 0.00006), waistline (r = 0.45, P < 0.00002) and DHEA-S (r = 0.36, P < 0.0008). Correlations of E(2), SHBG, testosterone and leptin, as well as biochemical markers of bone turnover with L(2)-L(4) and femoral neck BMD were not found. In the whole study group, significant predictors of L(2)-L(4 )BMD were BMI (beta = 0.35, P < 0.01) testosterone (beta = 0.27, P < 0.05) and osteocalcin (OC) (beta = 0.22, P < 0.05) (R(2) = 0.23), while predictors of femoral neck BMD were BMI (beta = 0.42, P < 0.001), testosterone (beta = 0.24, P < 0.05), E(2) (beta = 0.19, P < 0.05), as well as osteocalcin (beta = 0.20, P < 0.05) (R(2) = 0.41). In the subgroup with BMI 30-39.9, the significant predictors of both L(2)-L(4 )and femoral neck BMD were testosterone (beta = 0.32, P < 0.05, R(2) = 0.19; beta = 0.33, P < 0.05, R(2) = 0.29) and osteocalcin (beta = 0.34, P < 0.05, R(2) = 0.19; beta = 0.45, P < 0.01, R(2) = 0.29). In the subgroup with waist : hip ratio (WHR > or = 0.85, the predictor of L(2)-L(4 )BMD was E(2) (beta = 0.38, P < 0.05) (R(2) = 0.21), whereas the predictors of femoral neck BMD were BMI (beta = 0.29, P < 0.05) and testosterone (beta = 0.35, P < 0.01) (R(2) = 0.36).

CONCLUSION

The main endocrine variable predicting lumbar spine BMD in overweight and obese postmenopausal females was testosterone, while the main determinants of femoral neck BMD were both testosterone and E(2). No effect was found of serum leptin on examined indicators of bone status.

Identification and characterization of a leptin-responsive neuroblastoma cell line

The adipocyte-derived hormone leptin plays a critical role in a variety of physiological and pathological actions. As such the determination of leptin signal transduction pathways are important both for understanding the molecular mechanisms of leptin action and for identifying sites for possible therapeutic intervention. Since the hypothalamus is the primary site of leptin action, we sought to identify a neuronal-derived human cell line containing the long form of the leptin receptor (OBRb). To this end, we screened several neuroblastoma cell lines and isolated a sub-line of SH-SY5Y cells, which we designated as SH-OBRb, for further studies. We characterized the transduction pathways induced by leptin in SH-OBRb cells and demonstrated that OBRb mediates tyrosine phosphorylation of STAT3, phosphorylation of ERK1/2, but not SAPK/JNK and p38 MAPK, in a dose and time dependent fashion. In addition, Akt appears to be phosphorylated in the basal state and to be insensitive to further activation by leptin. In summary, we have isolated a unique cell line that can be utilized as a model for use in the study of leptin action and molecular mechanisms.

The Ay allele at the agouti locus reduces the size and alters the shape of the mandible in mice

To confirm my previous findings that the A(y) allele at the agouti locus reduced the mandible size and therefore altered the mandible shape in a KK mouse strain background, I further investigated the effects of the A(y) allele on mandible morphology on different strain backgrounds, DDD and B6. Principal component analysis revealed that the mandible was significantly smaller in A(y) mice (DDD-A(y) and B6-A(y)) than in corresponding non-A(y) mice (DDD and B6, respectively). Discriminant and canonical discriminant analyses revealed that most mice were classified correctly in their own strains, and misclassification was not observed between DDD (-A(y)) and B6 (-A(y)). The results confirmed that the A(y) allele reduced the mandible size and altered the mandible shape regardless of the strain background. However, the difference in mandible morphology between A(y) mice and the corresponding non-A(y) mice within a strain was not as large as that which intrinsically underlay the two strains. Possible mechanisms of the A(y) action are discussed.

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

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

Impaired energetic metabolism after central leptin signaling leads to massive appendicular bone loss in hindlimb-suspended rats

We previously showed in rats that the leptin effects on bone were dose dependent. Positive effects were observed when serum leptin concentration was in a physiological range. In contrast, important increases in serum leptin levels led to negative effects on bone formation similar to those reported after intracerebroventricular leptin administration in mice. To clarify whether leptin effects on bone depend on administration route and/or animal model, female rats were hindlimb unloaded or not and treated either with intracerebroventricular infusion of leptin or vehicle for 14 days. By increasing cerebrospinal fluid (CSF) leptin concentration, intracerebroventricular infusion of leptin significantly reduced food intake and consequently body weight, abdominal fat, and lean mass of the animals. Leptin infusion inhibited bone elongation over the 14 days and blunted cortical bone thickening at the femoral diaphysis site. Interestingly, leptin effects were site dependent in the cancellous bone envelopes, because tibia metaphysis BMD was lower and lumbar spine BMD was higher under intracerebroventricular leptin. Treated groups showed reduced bone remodeling independently of hindlimb unloading. Multiple downstream pathways were implicated in the mediation of these negative leptin effects on bone including not only stimulation of the sympathetic nervous system but also a decrease in somatotropic axis activity. Therefore, the intracerebroventricular leptin-induced bone loss could be largely related to the concurrent alteration of energetic and metabolic status. In summary, our study supports the hypothesis of a concentration-dependent balance between peripheral and central control of leptin on bone.

Identification of novel gene expression in healing fracture callus tissue by DNA microarray

Fracture healing requires controlled expression of thousands of genes. Only a small fraction of these genes have been isolated and fewer yet have been shown to play a direct role in fracture healing. The purpose of this study was threefold: (1) to develop a reproducible open femur model of fracture healing that produces consistent fracture calluses for subsequent RNA extraction, (2) to use this model to determine temporal expression patterns of known and unknown genes using DNA microarray expression profiling, and (3) to identify and validate novel gene expression in fracture healing. In the initial arm of the study, a total of 56 wild-type C57BL/6 mice were used. An open, stabilized diaphyseal femur fracture was created. Animals were killed at 1, 5, 7, 10, 14, 21, and 35 days after surgery and the femurs were harvested for analysis. At each time point, fractures were radiographed and sectioned for histologic analyses. Tissue from fracture callus at all stages following fracture yielded reproducibly large amounts of mRNA. Expression profiling revealed that genes cluster by function in a manner similar to the histologic stages of fracture healing. Based on the expression profiling of fracture tissue, temporal expression patterns of several genes known to be involved in fracture healing were verified. Novel expression of multiple genes in fracture callous tissue was also revealed including leptin and leptin receptor. In order to test whether leptin signaling is required for fracture repair, mice deficient in leptin or its receptor were fractured using the same model. Fracture calluses of mice deficient in both leptin or leptin receptor are larger than wild-type mice fractures, likely due to a delay in mineralization, revealing a previously unrecognized role of leptin signaling in fracture healing. This novel model of murine fracture repair is useful in examining both global changes in gene expression as well as individual signaling pathways, which can be used to identify specific molecular mechanisms of fracture healing.

Beta2-adrenergic receptors expressed on murine chondrocytes stimulate cellular growth and inhibit the expression of Indian hedgehog and collagen type X

The sympathetic nervous system has been demonstrated to have a role in regulating bone remodeling through beta-adrenergic receptors (beta-AR) expressed on osteoblasts. Studies using beta(2)-adrenergic receptor agonists in vivo have also suggested an effect on endochondral bone development; however, it was not clear if this effect was mediated through osteoblasts or chondrocytes. To more thoroughly examine the role of beta-AR in chondrocytes we characterized the expression and signal transduction systems activated by beta-AR in growth plate chondrocytes prepared from ribs of embryonic E18.5 mice. Using RT-PCR and immunohistochemistry we found that the chondrocytes expressed only beta(2)-AR. The receptors were coupled to stimulation of adenylyl cyclase, phosphorylation of the cyclic AMP response element binding protein (CREB) and extracellular signal-regulated kinase (ERK1/2). Stimulation of ERK1/2 was transient and limited by the concomitant stimulation of the mitogen-activated protein kinase phosphatase (MKP-1). Isoproterenol stimulated the growth of chondrocytes as assessed by increased incorporation of [(3)H]-thymidine into the cells. The cellular expression of two markers of chondrocyte differentiation, Indian hedgehog, expressed in pre-hypertrophic cells and collagen type X, expressed in hypertrophic chondrocytes, were both significantly inhibited after incubation with isoproterenol. Collectively, these findings demonstrate regulation of chondrocytes through beta(2)-AR expressed on the cells that stimulate their growth and inhibit their differentiation, indicating that the sympathetic nervous system may be an important regulator of embryonic cartilage development.

The bone-adipose axis in obesity and weight loss

Body fat and lean mass are correlated with bone mineral density, with obesity apparently exerting protection against osteoporosis. The pathophysiological relevance of adipose tissue in bone integrity resides in the participation of adipokines in bone remodeling through effects on deposition and resorption. On the other hand, the skeleton has recently emerged as an endocrine organ with effects on body weight control and glucose homeostasis through the actions of bone-derived factors such as osteocalcin and osteopontin. The cross-talk between adipose tissue and the skeleton constitutes a homeostatic feedback system with adipokines and molecules secreted by osteoblasts and osteoclasts representing the links of an active bone-adipose axis. Given the impact of bariatric surgery on absorption and the adipokine secretory pattern, to focus on the changes taking place following surgical-induced weight loss on this dynamic system merits detailed consideration.

Central control of bone remodelling

The discovery that the brain controls bone remodelling has provided a new paradigm for our understanding of bone biology. This review summarises the genetic, molecular and physiological bases for the central control of bone remodelling and discusses the future directions of this new research field of neuroskeletal biology.

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.

Leptin and regulation of linear growth

PURPOSE OF REVIEW

Leptin, first identified as the product of the ob gene in leptin-deficient obese (ob/ob) mice, was originally described as a circulating hormone involved in feeding behavior and energy homeostasis. It was later found to be a pleiotropic hormone involved in the regulation of a variety of physiological processes. This review summarizes our recent understanding of the role of leptin as a linear growth-stimulating factor.

RECENT FINDINGS

Leptin was found to have mitogenic effects on numerous cell types in vivo and in vitro, including several cancer cells, cells of the immune system, as well as chondrocytes of the epiphyseal growth plate.

SUMMARY

Leptin stimulates linear growth by regulating the energy balance of the organism and by stimulating the production and secretion of growth hormone from the hypothalamus; at the same time, it is involved with bone remodeling and has a direct effect on the chondrocytes of the growth plate.

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.

Genetic effect of zirconium oxide coating on osteoblast-like cells

Zirconium is widely used as material for prosthetic devices because its good mechanical and chemical properties. When exposed to oxygen, zirconium becomes zirconium oxide (ZrO(2)), which is biocompatible. ZrO(2) can be also prepared as a colloidal suspension and then used to coat surfaces. Zirconium oxide coating (ZrO(2)C) can potentially have specific biologic effects, and among them is bone formation related to implant osseointegration. How this biomaterial alters osteoblast activity to promote bone formation is poorly understood. We therefore attempted to address this question by using microarray techniques to identify genes that are differently regulated in osteoblasts exposed to ZrO(2)C. By using DNA microarrays containing 20,000 genes, we identified in osteoblast-like cell lines (MG-63) cultured with ZrO(2)C several genes whose expression was significantly upregulated or downregulated. The differentially expressed genes cover a broad range of functional activities: (a) cell cycle regulation, (b) signal transduction, (c) immunity, and (d) cytoskeleton component. The data reported are, to our knowledge, the first genetic portrait of ZrO(2)C effects. They can be relevant to better understand the molecular mechanism of bone regeneration and as a model for comparing other materials with similar clinical effects.

cAMP/PKA regulates osteogenesis, adipogenesis and ratio of RANKL/OPG mRNA expression in mesenchymal stem cells by suppressing leptin

Background

Mesenchymal stem cells (MSCs) are a pluripotent cell type that can differentiate into adipocytes, osteoblasts and other cells. The reciprocal relationship between adipogenesis and osteogenesis was previously demonstrated; however, the mechanisms remain largely unknown.

Methods and Findings

We report that activation of PKA by 3-isobutyl-1 methyl xanthine (IBMX) and forskolin enhances adipogenesis, the gene expression of PPARγ2 and LPL, and downregulates the gene expression of Runx2 and osteopontin, markers of osteogenesis. PKA activation also decreases the ratio of Receptor Activator of the NF-κB Ligand to Osteoprotegerin (RANKL/OPG) gene expression – the key factors of osteoclastogenesis. All these effects are mediated by the cAMP/PKA/CREB pathway by suppressing leptin, and may contribute to PKA stimulators-induced in vivo bone loss in developing zebrafish.

Conclusions

Using MSCs, the center of a newly proposed bone metabolic unit, we identified cAMP/PKA signaling, one of the many signaling pathways that regulate bone homeostasis via controlling cyto-differentiation of MSCs and altering RANKL/OPG gene expression.

Correlation of obesity and osteoporosis: effect of fat mass on the determination of osteoporosis

It was previously believed that obesity and osteoporosis were two unrelated diseases, but recent studies have shown that both diseases share several common genetic and environmental factors. Body fat mass, a component of body weight, is one of the most important indices of obesity, and a substantial body of evidence indicates that fat mass may have beneficial effects on bone. Contrasting studies, however, suggest that excessive fat mass may not protect against osteoporosis or osteoporotic fracture. Differences in experimental design, sample structure, and even the selection of covariates may account for some of these inconsistent or contradictory results. Despite the lack of a clear consensus regarding the impact of effects of fat on bone, a number of mechanistic explanations have been proposed to support the observed epidemiologic and physiologic associations between fat and bone. The common precursor stem cell that leads to the differentiation of both adipocytes and osteoblasts, as well the secretion of adipocyte-derived hormones that affect bone development, may partially explain these associations. Based on our current state of knowledge, it is unclear whether fat has beneficial effects on bone. We anticipate that this will be an active and fruitful focus of research in the coming years.

Anorexia, bulimia, and the athletic triad: evaluation and management

Caloric restriction caused by undernutrition or over-exercise is increasingly common and has significant health consequences such as hypothalamic amenorrhea, infertility, attainment of low peak bone mass, and bone loss leading to fracture. In these patients, the pathophysiology of amenorrhea and bone loss is multifactorial, involving hormones that integrate the nutritional state with the hypothalamic-pituitary-ovarian axis, including leptin and possibly ghrelin. The pathophysiology of bone loss includes nutritional deficiencies, possibly estrogen deficiency, and direct and indirect effects of leptin on bone. Identifying patients at risk for low bone mineral density and fracture is important, as is screening with dual energy radiograph absorptiometry. Treatment has focused on oral contraceptive use, yet improved bone mineral density is marked by nutritional recovery and anovulation reversal. Therefore, resolving the nutrition deficiency should be the cornerstone of treatment. Cognitive-behavioral therapy aims for weight recovery, which can lead to reversal of amenorrhea and improvement in other associated metabolic abnormalities. During treatment, estradiol levels can be followed to assess hypothalamic-pituitary-ovarian recovery because estradiol secretion may increase well before ovulation occurs. In patients failing the above interventions, hormone replacement should be considered, but bone mineral density should be followed because patients may continue to lose bone despite treatment with oral contraceptives if nutrition is not improved.

Epigenetic regulation of leptin affects MMP-13 expression in osteoarthritic chondrocytes: possible molecular target for osteoarthritis therapeutic intervention

OBJECTIVE

To investigate whether epigenetic mechanisms can regulate leptin's expression and affect its downstream targets as metalloproteinases 3,9,13 in osteoarthritic chondrocytes.

METHODS

DNA methylation in leptin promoter was measured by DNA bisulfite sequencing, and mRNA expression levels were measured by real-time quantitative PCR in osteoarthritic as well as in normal cartilage. Osteoarthritic articular cartilage samples were obtained from two distinct locations of the knee (n = 15); from the main defective area of maximum load (advanced osteoarthritis (OA)) and from adjacent macroscopically intact regions (minimal OA). Using small interference RNA, we tested if leptin downregulation would affect matrix metalloproteinase (MMP)-13 activity. We also evaluated the effect of the demethylating agent, 5'-Aza-2-deoxycytidine (AZA) and of the histone deacetylase inhibitor trichostatin A (TSA) on leptin expression in chondrocyte cultures. Furthermore, we performed chromatin immunoprecipitation in leptin's promoter area.

RESULTS

We found a correlation between leptin expression and DNA methylation and also that leptin controls MMP-13 activity in chondrocytes. Leptin's downregulation with small interference RNA inhibited MMP-13 expression dramatically. After 5-AZA application in normal chondrocytes, leptin's methylation was decreased, while its expression was upregulated, and MMP-13 was activated. Furthermore, TSA application in normal chondrocyte cultures increased leptin's expression. Also, chromatin immunoprecipitation in leptin's promoter after TSA treatment revealed that histone H3 lysines 9 and 14 were acetylated.

CONCLUSION

We found that epigenetic mechanisms regulate leptin's expression in chondrocytes affecting its downstream target MMP-13. Small interference RNA against leptin deactivated directly MMP-13, which was upregulated after leptin's epigenetic reactivation, raising the issue of leptin's therapeutic potential for osteoarthritis.

Leptin stimulates parathyroid hormone related peptide expression in the endochondral growth plate

We have previously shown that growth plate chondrocytes expressed the long form of leptin receptor, and that within the growth plate, leptin stimulated cell proliferation and differentiation and epiphyseal growth in a balanced manner. These three cell processes are known to be regulated by the interactions of parathyroid hormone-related peptide (PTHrP) and Indian hedgehog (Ihh) protein. The aim of the present study was to examine the effect of leptin on the PTHrP/Ihh feedback loop. The effect of leptin was investigated in vivo in pair-fed experiments in ICR mice, and ex vivo in mandibular condyle explants incubated with leptin. Immunohistochemistry and in situ hybridization showed that in the pair-fed in vivo system as well as in the organ culture, leptin increased the level of PTHrP and reduced that of Ihh. Leptin may affect chondrocyte proliferation and differentiation by activating the PTHrP/Ihh growth-restraining feedback loop in the postnatal growth plate.

Phosphatidylinositol 3-kinase, MEK-1 and p38 mediate leptin/interferon-gamma synergistic NOS type II induction in chondrocytes

In a previous study, we established that leptin acts synergistically with interferon-gamma in inducing nitric oxide synthase type II in cultured chondrocytes via Janus kinase-2 activation. However, the exact molecular mechanism that accounts for this synergism is not completely understood. The aim of the present study was to further delineate the signalling pathway used by leptin/interferon-gamma in the nitric oxide synthase type II induction in chondrocytes. Consequently, the roles of PI-3 kinase, MEK1 and p38 kinase were investigated using specific pharmacological inhibitors (Wortmannin, LY 294002, PD 098,059 and SB 203580). For this purpose, the amount of stable nitrite, the end product of NO generation by activated chondrocytes, has been evaluated by Griess colorimetric reaction in culture medium of human primary chondrocytes and in the murine ATDC5 cell line stimulated with leptin (400 nM) and interferon-gamma (1 ng/ml), alone or in combination. Specific inhibitors for PI-3K, MEK1 and p38 were added 1 h before stimulation. Nitric oxide synthase type II mRNA was investigated by real-time RT-PCR and NOS type II protein expression has been evaluated by western blot analysis. Our results showed that, as expected, leptin synergizes with IFN-gamma in inducing NO accumulation in the supernatant of co-stimulated cells. Pre-treatment with Wortmannin, LY 294002, PD 098,059 and SB 203580 caused a significant decrease in nitrite production, NOS type II protein expression and NOS type II mRNA expression induced by leptin and interferon-gamma co-stimulation. These findings were confirmed in 15 and 21-day differentiated ATDC5 cells, and in normal human primary chondrocytes. This is the first report showing that NOS type II induction triggered by co-stimulation with leptin and interferon-gamma is mediated by a signaling pathway involving PI-3K, MEK1 and p38.

Differential expression of leptin and leptin's receptor isoform (Ob-Rb) mRNA between advanced and minimally affected osteoarthritic cartilage; effect on cartilage metabolism

OBJECTIVE

To investigate leptin's effect on cartilage metabolism and the pathophysiology of osteoarthritis (OA).

METHODS

Messenger RNA (mRNA) expression and protein levels of leptin and leptin's receptor isoforms were measured by real-time reverse transcription-PCR and Western blot in osteoarthritic and normal cartilage. Osteoarthritic cartilage samples were obtained from two locations of the knee (n=11) and hip (n=6); from the main defective area (advanced OA) and from adjacent macroscopically and histological intact regions (minimal OA). Paired serum and synovial fluid (SF) leptin levels were measured. The effect of leptin was evaluated on chondrocyte proliferation, IL-1beta (interleukin-1beta), NO and metalloproteinases 9 and 13 (MMP-9, MMP-13) protein expression.

RESULTS

Leptin's and leptin's receptor (Ob-Rb) expression levels were significantly increased in advanced OA cartilage compared to minimal. Leptin was significantly increased in SF than serum samples. Also, leptin had a detrimental effect on chondrocyte proliferation and induced IL-1beta production and MMP-9 and MMP-13 protein expression. Furthermore, leptin's mRNA expression in advanced OA cartilage was significantly correlated with BMI of the patients.

CONCLUSION

The increased leptin levels in SF point toward a local effect of leptin in articular cartilage, while the observed intrajoint differences of leptin and Ob-Rb mRNA expression may be related to the grade of cartilage destruction. The observed production of IL-1beta, MMP-9 and MMP-13 by chondrocytes after leptin treatment indicates a pro-inflammatory and catabolic role of leptin on cartilage metabolism. Furthermore, the observed correlation of leptin's mRNA expression with BMI suggests that leptin may be a metabolic link between obesity and OA.

Opposite effects of leptin on bone metabolism: a dose-dependent balance related to energy intake and insulin-like growth factor-I pathway

Published data describing leptin effects on bone are at variance with both positive and negative consequences reported. These findings are consistent with a bimodal threshold response to serum leptin levels. To test this theory, two groups of female rats (tail-suspended and unsuspended) were treated with ip leptin at two different doses or vehicle for 14 d. In tail-suspended rats, low-dose leptin compensated the decrease in serum leptin levels observed with suspension and was able to prevent the induced bone loss at both the trabecular and cortical level (assessed by three-dimensional microtomography). In contrast, high-dose leptin inhibited femoral bone growth and reduced bone mass by decreasing bone formation rate and increasing bone resorption in both tail-suspended and unsuspended groups. High- and low-dose leptin administration resulted in a reduced medullar adipocytic volume in all groups. High-dose leptin (but not low) induced a decrease in body-weight abdominal fat mass and serum IGF-I levels. Thus, the observed bone changes at high-dose leptin are at least partly mediated by a leptin-induced energy imbalance. In conclusion, a balance between negative and positive leptin effects on bone is dependent on a bimodal threshold that is triggered by leptin serum concentration. Also, the negative effects of high leptin levels are likely induced by reduced energy intake and related hormonal changes. The respective part of each pathway will be unraveled by additional studies.

Loss of sex-specific difference in femoral bone parameters in male leptin knockout mice

Sex-dependent differences were identified in the femoral bone parameters of male and female ob/ob (leptin knockout) mice compared with their C57BL/6 wild-type background strain. Total fat, lean weight and body weight were not different between adult male and female leptin knockout mice. However, leptin knockout males exhibited lower lean weights than C57BL/6 males. Peripheral quantitative computerized tomographic measurements at the femoral midshaft revealed that the normal differences in the periosteal circumference, endosteal circumference, total bone mineral content, and polar moment of inertia normally observed between adult male and female wild-type mice were lost between adult male and female ob/ob mice. Significant reductions in these bone parameters were seen in male ob/ob mice compared to male wild-type mice but not in female ob/ob mice compared to female wild-type mice. In prepubertal mice, there were no differences in phenotype and femoral bone parameters between males and females within any strain, suggesting sex hormone functions. Serum free testosterone levels were 5.6-fold higher in adult male ob/ob mice than in adult male C57BL/6 wild-type mice, and serum estradiol levels were 1.8- and 1.3-fold greater in adult male and female ob/ob mice, respectively, than in their wild-type counterparts. Androgen receptor gene expression was not different in femur-derived bone cells of male ob/ob mice compared with wild-type mice. The loss of sex-related differences in these bone parameters in adult male ob/ob mice might result from deficient signaling in the androgen signaling pathway and the fact that leptin functions are permissive for androgen effects on bone development.

Central leptin gene therapy corrects skeletal abnormalities in leptin-deficient ob/ob mice

Skeletal growth is tightly coupled to energy balance via complex and incompletely understood mechanisms. Leptin-deficient ob/ob mice are obese and develop multiple pathologies associated with the metabolic syndrome. Additionally, ob/ob mice have skeletal abnormalities. The objective of this study was to evaluate the effects of leptin deficiency and long duration selective central leptin repletion via recombinant adeno-associated virus-leptin (rAAV-lep) gene therapy on bone in growing ob/ob mice. The ob/ob mice were injected in the hypothalamus with either rAAV-lep or rAAV-GFP (control vector). Treated ob/ob and untreated wild-type (WT) mice were then maintained on a normal diet for 15 weeks. In a second experiment, similarly treated mice along with a group of pair-fed mice were maintained for 30 weeks. Leptin was not detected in blood of either rAAV-lep- or rAAV-GFP-treated mice although rAAV-lep-treated mice displayed leptin transgene expression in the hypothalamus. As expected, rAAV-lep normalized body weight and food intake. Compared to WT mice, rAAV-GFP-treated ob/ob mice had decreased femoral length (by 1.6 mm or 10%, P<0.001), decreased total femur bone volume (by 3.3 mm(3) or 19%, P<0.001), but increased cancellous bone volume in the distal femur (by 0.04 mm(3) or 60%, P<0.09) and lumbar vertebrae (by 0.26 mm(3) or 118%, P<0.001). Treatment with rAAV-lep rescued the ob/ob skeletal phenotype by increasing femoral length and total bone volume, and decreasing femoral and vertebral cancellous bone volume, so that at 15 weeks post-rAAV-lep injection the ob/ob mice no longer differed from WT mice. No further skeletal changes in either the femur or lumbar vertebra were observed at 30 weeks post-rAAV-lep administration. The results suggest that hypothalamic leptin functions as an essential permissive factor for normal bone growth.

Serum leptin and adiponectin are positively associated with bone mineral density at the distal radius in patients with type 2 diabetes mellitus

There have been several reports about associations of serum leptin or adiponectin with bone mineral density and biochemical markers of bone turnover. However, the precise roles of adipocytokines in bone metabolism have not been fully elucidated. We investigated the associations of serum level of leptin or adiponectin with bone mineral density, serum osteocalcin, and urinary N-terminal telopeptide of type I collagen (NTX) in 40 Japanese patients with type 2 diabetes mellitus. Bone mineral density was measured by using dual-energy x-ray absorptiometry at different sites (distal radius, femoral neck, and lumbar spine) and was expressed as z score. Multiple regression analysis revealed that there were significant positive correlations between serum leptin or adiponectin level and z score at the distal radius, but not at the femoral neck or the lumbar spine. Although no correlation was observed between serum leptin and serum osteocalcin, there was a significant negative correlation between serum leptin and urinary NTX, a marker of bone resorption. No correlation was observed between serum adiponectin and serum osteocalcin or urinary NTX. These results indicate that leptin and adiponectin may have a protective effect on bone metabolism in patients with type 2 diabetes mellitus.

The new field of neuroskeletal biology

The fields of neuroscience and bone biology have recently converged following the discovery that bone remodeling is directly regulated by the brain. This work has defined bone remodeling as one of the cardinal physiological functions of the body, subject to homeostatic regulation and integrated with the other major physiological functions by the hypothalamus. Central to this discovery was the definition of the adipocyte-derived hormone leptin as a regulator of both arms of bone remodeling, formation and resorption, through its action on the ventromedial hypothalamus and subsequently via the sympathetic nervous system to osteoblasts. The characterization of the sympathetic nervous system as a regulator of bone remodeling has led to several large clinical studies demonstrating a substantial protective effect of beta-blockers, particularly beta1-blockers, on fracture risk. Studies in model organisms have reinforced the role of the central nervous system in the regulation of bone remodeling in vivo by the identification of several additional genes, namely cocaine and amphetamine regulated transcript (Cart), melanocortin 4 receptor (Mc4R), neuropeptide Y (NPY), Y2 receptor, cannabinoid receptor CB1 (Cnbr1), and the genes of the circadian clock. These genes have several common features, including high levels of expression in the hypothalamus and the ability to regulate other major physiological functions in addition to bone remodeling including energy homeostasis, body weight, and reproduction. We review the major pathways that define the new field of neuroskeletal biology and identify further avenues of inquiry.

Leptin attenuates gene expression for renal 25-hydroxyvitamin D3-1alpha-hydroxylase in mice via the long form of the leptin receptor

Leptin, the ob gene product secreted by adipocytes, controls overall energy balance. We previously showed that leptin administration to leptin-deficient obese (ob/ob) mice suppressed mRNA expression and activity of renal 25-hydroxyvitamin D(3)-1alpha-hydroxylase (CYP27B1). In leptin receptor-deficient (db/db) mice, we presently examined whether leptin affects 1alpha-hydroxylase expression in renal tubules through the active form of the leptin receptor (ObRb). Elevated serum concentrations of calcium and 1,25-dihydroxyvitamin D(3) [1,25-(OH)(2)D(3)] in untreated ob/ob mice showed sharp reduction with leptin administration (4 mg/kg, i.p. every 12h for 2 days); no such reduction of elevation occurred in db/db mice. ObRb mRNA was expressed in kidney, brain, fat, lung, and bone in wild-type and ob/ob mice, but not db/db mice. The ob/ob and db/db mice showed large increases in renal 1alpha-hydroxylase mRNA expression and activity. Leptin administration (4 mg/kg) completely abrogated these increases in ob/ob but not db/db mice. Renal 25-hydroxyvitamin D(3)-24-hydroxylase (CYP24) mRNA synthesis also was greatly elevated in ob/ob and db/db mice; excesses decreased significantly with leptin administration in ob/ob mice, but increased in db/db mice. Renal tubular cells in primary culture expressed mRNAs including proximal tubules markers (1alpha-hydroxylase and megalin), parathyroid hormone receptor, and vitamin D receptor. Calcitonin receptor mRNA, synthesized mainly in distal tubules, was scant, indicating that most cultured cells were from proximal tubules. Cells did not express ObRb mRNA. Forskolin exposure at 10(-6)M for 3 or 6h significantly increased 1alpha-hydroxylase mRNA. Leptin at 10(-6)M did not change mRNA expression in either presence or absence of forskolin. Accordingly, leptin attenuates renal 1alpha-hydroxylase gene expression through ObRb. Furthermore, leptin appears to act indirectly on renal proximal tubules to regulate 1alpha-hydroxylase gene expression.