Bone has classically been viewed as an inert structure that is necessary for mobility, calcium homeostasis, and maintenance of the hematopoietic niche. Recent advances in bone biology using complex genetic manipulations in mice have highlighted the importance of bone not only as a structural scaffold to support the human body, but also as a regulator of a number of metabolic processes that are independent of mineral metabolism. These advances point to the skeleton as an endocrine organ that modulates glucose tolerance and testosterone production by secretion of a bone-specific protein, osteocalcin. This review will detail how bone has emerged as a bona-fide endocrine “gland”, and with that, the potential therapeutic implications that could be realized for this hormone-secreting tissue.
Vitamin D an ancient secosteroid is essential for mineral homeostasis, bone remodeling, immune modulation, and energy metabolism. Recently, debates have emerged about the daily vitamin D requirements for healthy and elderly adults, the safety and efficacy of long term supplementation and the role of vitamin D deficiency in several chronic disease states. Since this molecule acts as both a vitamin and a hormone, it should not be surprising that the effects of supplementation are multi-faceted and complex. Yet despite significant progress in the last decade, our understanding of vitamin D physiology and the clinical relevance of low circulating levels of this vitamin remains incomplete. The present review provides the reader with a comprehensive and up-to-date understanding of vitamin D requirements and safety. It also raises some provocative research questions.
Vitamin D; diagnosis; treatment
Caloric restriction is associated with a reduction in body weight and temperature but a reduction in trabecular bone volume and paradoxically an increase in adipocytes within the bone marrow. The nature of these adipocytes is uncertain, although there is emerging evidence of a direct relationship between bone remodeling and brown adipocytes. For example, in heterotropic ossification, brown adipocytes set up a hypoxic gradient that leads to vascular invasion, chondrocyte differentiation and subsequent bone formation. Additionally, deletion of retinoblastoma protein in an osteosarcoma model leads to increased hibernosarcoma (brown fat tumor). Interestingly, brown adipose tissue (BAT) senescences with age at a time when thermoregulation becomes altered,, bone loss becomes apparent and sympathetic activity increases. Interestingly, heart rate is an unexpected but good predictor of fracture risk in elderly individuals, pointing to a key role for the sympathetic nervous system in senile osteoporosis. Hence the possibility exists that BAT could play an indirect role in age-related bone loss. However, evidence of an indirect effect from thermogenic dysfunction on bone loss is currently limited. Here, we present current evidence for a relationship between brown adipose tissue and bone as well as provide novel insights into the effects of thermoregulation on bone mineral density.
Despite growing evidence for adipose tissue regulation of bone mass, the role of the adipokine leptin in bone remodeling remains controversial. The majority of in vitro studies suggest leptin enhances osteoblastic proliferation and differentiation while inhibiting adipogenic differentiation from marrow stromal cells. Alternatively, some evidence demonstrates either no effect or a pro-apoptotic action of leptin on stromal cells. Similarly, in vivo work has demonstrated both positive and negative effects of leptin on bone mass. Most of the literature supports the idea that leptin suppresses bone mass by acting in the brainstem to reduce serotonin-dependent sympathetic signaling from the ventromedial hypothalamus to bone. However, other studies have found partly or entirely contrasting actions of leptin. Recently one study found a significant effect of surgery alone with intracerebroventricular administration of leptin, a technique crucial for understanding centrally-mediated leptin regulation of bone. Thus, two mainstream hypotheses for the role of leptin on bone emerge: 1) direct regulation through increased osteoblast proliferation and differentiation and 2) indirect suppression of bone formation through a hypothalamic relay. At the present time, it remains unclear whether these effects are relevant in only extreme circumstances (i.e. models with complete deficiency) or play an important homeostatic role in the regulation of peak bone acquisition and skeletal remodeling. Ultimately, determining the actions of leptin on the skeleton will be critical for understanding how the obesity epidemic may be impacting the prevalence of osteoporosis.
Leptin; Bone remodeling; Adipocyte; Osteoblast; Sympathetic nervous system
Osteoporosis and obesity are chronic disorders that are increasing in prevalence. The pathophysiology of these diseases is multifactorial and includes genetic, environmental and hormonal determinants. Long considered as distinct disorders that rarely are found in the same individual, emerging evidence from basic and clinical studies support an important interaction between adipose tissue and the skeleton. Adiposity can influence bone remodeling through three possible mechanisms including secretion of cytokines that directly target bone, adipokines that influence the central nervous system thereby changing sympathetic impulses to bone, and paracrine influences on adjacent skeletal cells. This review will focus on our current understanding of bone-fat interactions and the clinical implications of recent studies linking obesity to osteoporosis.
We recently characterized Sprouty1 (Spry1), a growth factor signaling inhibitor as a regulator of marrow progenitor cells promoting osteoblast differentiation at the expense of adipocytes. Adipose tissue specific Spry1 expression in mice resulted in increased bone mass and reduced body fat while conditional knockout of Spry1 had the opposite effect with decreased bone and increased body fat. Because Spry1 suppresses normal fat development, we tested the hypothesis that Spry1 expression prevents high fat diet-induced obesity, bone loss, and associated lipid abnormalities and demonstrate that Spry1 has a long-term protective effect on mice fed a high caloric diet. We studied diet-induced obesity in mice with fatty acid binding promoter (aP2)-driven expression or conditional knockout of Spry1 in adipocytes. Phenotyping was performed by whole body dual-energy X-ray absorptiometry, microCT, histology and blood analysis. In conditional Spry1 null mice, high fat diet increased body fat by 40%, impaired glucose regulation, and led to liver steatosis. However, over-expression of Spry1 led to 35% lower body fat, reduced bone loss, and normal metabolic function compared to single transgenics. This protective phenotype was associated with decreased circulating insulin (70%) and leptin (54%) compared to controls on a high fat diet. Additionally, Spry1 expression decreased adipose tissue inflammation by 45%. We show that conditional Spry1 expression in adipose tissue protects against high fat diet-induced obesity and associated bone loss.
Sprouty; high fat diet; body fat; obesity; bone loss
Sclerostin, product of the SOST gene, is an important determinant of bone formation and resorption. Adolescents with anorexia nervosa (AN) have low bone density and decreased levels of bone turnover markers. However, sclerostin has not been examined in AN as a potential mediator of impaired bone metabolism. Our study objectives were to (i) assess associations of sclerostin with surrogate bone turnover markers in girls with AN and controls and (ii) examine effects of transdermal estradiol on sclerostin in AN. 69 girls (44 with AN and 25 normal-weight controls) 13–18 years old were studied at baseline. 22 AN girls were randomized to transdermal estradiol (plus cyclic medroxyprogesterone) or placebo in a double-blind study for 12 months. Sclerostin correlated positively with P1NP and CTX in controls (r = 0. 67 and 0. 53, p = 0. 0002 and 0. 005, respectively) but not in AN despite comparable levels at baseline. Changes in sclerostin over twelve months did not differ in girls randomized to estradiol or placebo. The relationship between sclerostin and bone turnover markers is disrupted in adolescent girls with AN. Despite an increase in BMD with estradiol administration in AN, estrogen does not impact sclerostin levels in this group.
anorexia nervosa; sclerostin; osteoporosis
Women with anorexia nervosa (AN) have elevated marrow fat mass despite low visceral and subcutaneous fat depots, which is inversely associated with bone mineral density (BMD). Whether marrow fat mass remains persistently elevated or decreases with recovery from AN is currently unknown. In this study, we investigated changes in marrow fat with recovery from AN and the relationship between preadipocyte factor (Pref)-1 -- a member of the EGF-like family of proteins and regulator of adipocyte and osteoblast differentiation -- and fat depots and BMD in women who have recovered from AN (AN-R) compared to women with AN and healthy controls (HC). We studied 29 women: 14 with active or recovered AN (30.7 + 2.2 years (mean ± SEM)) and 15 normal-weight controls (27.8 ± 1.2 years). We measured marrow adipose tissue (MAT) of the L4 vertebra and femur by 1H-magnetic resonance spectroscopy, BMD of the spine, hip and total body by DXA and serum Pref-1 and leptin levels. We found that MAT of the L4 vertebra was significantly lower in AN-R as compared to AN (p = 0.03) and was comparable to levels in HC. Pref-1 levels were also significantly lower in AN-R as compared to AN (p = 0.02) and comparable to levels in healthy controls. Although Pref-1 was positively associated with MAT of the L4 vertebra in AN (R = 0.94; p = 0.002), we found that it was inversely associated with MAT of the L4 vertebra in HC (R = −0.71; p = 0.004). Therefore, we have demonstrated that MAT and Pref-1 levels decrease with recovery from AN. Our data suggest that Pref-1 may have differential effects in states of nutritional deprivation as compared to nutritional sufficiency.
Marrow Adiposity; Anorexia Nervosa; Pref-1; Bone mineral density; Adipose depots
Insulin-like growth factor (IGF)-I is important in the acquisition and maintenance of both soft and hard tissues. Skeletal remodeling requires energy and recent work has demonstrated that bone can influence insulin sensitivity and thereby regulate metabolic processes. New insights from mouse models into the role of IGF-binding proteins (IGFBPs) as more than mere depots for the IGFs has reignited investigations into the metabolic targets influenced by the IGF regulatory system and the pathways that link bone to adipose tissue. Although there remains continued uncertainty about the relative balance between the effects of circulating vs tissue IGF-I actions, the role of the IGFBPs has been redefined both as modulators of IGF-I action and as independent signaling factors. This review highlights several recent findings that shed new light on the physiologic role of the IGF regulatory system and its influence on skeletal and fat metabolism.
Osteoporosis, a syndrome characterized by thin bones and fractures, has become more prevalent in both women and men. Established therapies for this disorder consist primarily of drugs that prevent bone loss, such as the bisphosphonates and selective estrogen receptor modulators. Although these drugs have been shown to reduce fractures in randomized trials, there is an urgent need for treatments that could lower fracture risk further without additional adverse effects. The introduction of parathyroid hormone (teriparatide), which significantly increases bone mineral density, albeit for a relatively short duration, raised expectations that drugs which stimulate bone formation might cure osteoporosis. After outlining current approaches to treating osteoporosis, this review focuses on emerging therapeutic opportunities for osteoporosis that are based on recent insights into skeletal physiology. Such novel strategies offer promise for not only reducing age-related bone loss and the associated risk of fractures, but restoring bone mineral density to healthy levels.
Only three decades ago adipose tissue was considered inert with little relationship to insulin resistance. Similarly bone has long been thought purely in its structural context. In the last decade, emerging evidence has revealed important endocrine roles for both bone and adipose tissue. The interaction between these two tissues is remarkable. Bone marrow mesenchymal stem cells give rise to both osteoblasts and adipocytes. Leptin and adiponectin, two adipokines secreted by fat tissue, control energy homeostasis, but also have complex actions on the skeleton. In turn, the activities of bone cells are not limited to their bone remodeling activities, but also to modulation of adipose sensitivity and insulin secretion. This review will discuss these new insights linking bone remodeling to the control of fat metabolism and the association between diabetes mellitus and osteoporosis.
Osteoporosis; diabetes mellitus; obesity; leptin; osteocalcin
Bone Morphogenetic Protein 2 (BMP2) is a growth factor that initiates osteoblast differentiation. Recent studies show that BMP2 signaling regulates bone mineral density (BMD). BMP2 interacts with BMP receptor type Ia (BMPRIa) and type II receptor leading to the activation of the Smad signaling pathway. BMPRIa must shuttle between distinct plasma membrane domains, enriched of Caveolin-1 alpha and Caveolin-1 beta isoforms, and receptor activation occurs in these domains. Yet it remains unknown whether the molecular mechanism that regulates BMP2 signaling is driving mineralization and BMD. Therefore the B6.C3H-1-12 congenic mouse model with increased BMD and osteoblast mineralization was utilized in this study. Using the Family Image Correlation Spectroscopy, we determined if BMP2 led to a significant re-localization of BMPRIa to caveolae of the alpha/beta isoforms in bone marrow stromal cells (BMSCs) isolated from B6.C3H-1-12 mice compared to the C57BL/6J mice, which served as controls. The control, C57BL/6J mice, was selected due to only 4Mb of Chromosome 1 from the C3H/HeJ mouse was backcrossed to a C57BL/6J background. Using reporter gene assays, the B6.C3H-1-12 BMSCs responded to BMP2 with increased Smad activation. Furthermore disrupting caveolae reduced the BMP2-induced Smad signaling in BMSCs isolated from B6.C3H-1-12 and C57BL/6J. This study suggests for the first time a regulatory mechanism of BMPRIa signaling at the plasma membrane of BMSCs that 1) associated with genetic differences in the distal Chromosome 1 segment carried by the B6.C3H-1-12 congenic and 2) contributes to increase BMD of the B6.C3H-1-12 compared to the C57BL/6J control mice.
Bone; Image Cross Correlation Spectroscopy; Membrane; Caveolae
Anorexia nervosa (AN) is characterized by subnormal estrogen and dehydroepiandrosterone (DHEA) levels. We sought to determine whether the combination of DHEA + estrogen/progestin is superior to placebo in preserving skeletal health over 18 months in AN. Females with AN, aged 13 to 27 years, were recruited for participation in this double-blind, placebo-controlled, randomized trial. Ninety-four subjects were randomized, of whom 80 completed baseline assessments and received either study drug (oral micronized DHEA 50 mg + 20 µg ethinyl estradiol/0.1 mg levonorgestrel combined oral contraceptive pill [COC] daily; n = 43) or placebo (n = 37). Serial measurements of areal bone mineral density (aBMD), bone turnover markers, and serum hormone concentrations were obtained. Sixty subjects completed the 18-month trial. Spinal and whole-body aBMD z scores were preserved in the DHEA + COC group, but decreased in the placebo group (comparing trends, P = .008 and P = .001, respectively). Bone turnover markers initially declined in subjects receiving DHEA + COC and then returned to baseline. No differences in body composition, adverse effects of therapy, or alterations in biochemical safety parameters were observed. Combined therapy with DHEA + COC appears to be safe and effective for preventing bone loss in young women with AN, whereas placebo led to decreases in aBMD. Dehydroepiandrosterone + COC may be safely used to preserve bone mass as efforts to reverse the nutritional, psychological, and other hormonal components of AN are implemented.
Aging is associated with profound changes in bone mass and body composition. Emerging evidence supports the hypothesis that alterations in mesenchymal stromal cell fate are a critical etiologic factor. In addition, timekeeping at the cellular level is affected as aging progresses, particularly in the adipocyte. In this Extra View we discuss the interactive role of three molecules, PPARγ, nocturnin and IGF-I, in regulating stem cell fate in the marrow and the potential implications of this network for understanding cellular aging.
aging; bone; PPARγ; nocturnin; IGF-I
Insulin-like growth factors; Skeletal growth; Bone homeostasis; Insulin-like growth factor binding protein
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 stromal cells into adipocytes or osteoblasts. Mature bone cells secrete factors that influence insulin sensitivity and fat cells synthesize cytokines that regulate osteoblast differentiation. 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 review, we discuss three pathways that could favor pharmacologic intervention with the ultimate goal of enhancing bone mass and reducing osteoporotic fracture risk. Not surprisingly, because of the complex interactions across homeostatic networks, other pathways will likely be activated by this targeting and these 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 these potential agents into any future clinical armamentarium.
Abdominal adiposity is associated with increased cardiovascular risk and decreased growth hormone (GH) secretion. The objective of our study was to determine the effects of GH in abdominally obese women on body composition and cardiovascular risk markers.
Materials and Methods
In this randomized, double-blind, placebo-controlled study, 79 obese premenopausal women received GH vs. placebo for six months. Primary endpoints were: 1) total abdominal (TAT) fat by CT (body composition) and 2) high-sensitivity C-reactive protein (hsCRP) (cardiovascular risk marker). Body composition was assessed by CT, DXA and proton MR spectroscopy. Serum cardiovascular risk markers, carotid intima-media thickness and endothelial function were measured.
Mean 6-month GH dose was 1.7±0.1 mg/day, resulting in a mean IGF-1 SDS increase from −1.7±0.08 to −0.1±0.3 in the GH group. GH administration decreased TAT and hsCRP compared with placebo. In addition, it increased thigh muscle mass and lean body mass, and decreased subcutaneous abdominal and trunk fat, tPA, apoB, and apoB/LDL compared with placebo. Visceral adipose tissue decreased and IMCL increased within the GH group. Six-month change in IGF-1 levels was negatively associated with 6-month decrease in TAT and VAT. One subject had a 2-hour glucose >200 mg/mL at 3 months; four subjects, three of whom were randomized to GH, had 2-hour glucose levels >200 mg/mL at study end.
GH administration in abdominally obese premenopausal women exerts beneficial effects on body composition and cardiovascular risk markers, but is associated with a decrease in glucose tolerance in a minority of women.
growth hormone; visceral obesity; body composition; cardiovascular risk
Bone Morphogenetic Proteins (BMPs) are growth factors that initiate differentiation of bone marrow stromal cells to osteoblasts and adipocytes, yet the mechanism that decides which lineage the cell will follow is unknown. BMP2 is linked to the development of osteoporosis and variants of BMP2 gene have been reported to increase the development of osteoporosis. Intracellular signaling is transduced by BMP Receptors (BMPRs) of type I and type II that are serine/threonine kinase receptors. The BMP type I a receptor (BMPRIa) is linked to osteogenesis and bone mineral density (BMD). BMPRs are localized to caveloae enriched with Caveolin1 alpha/beta and Caveolin beta isoforms to facilitate signaling. BMP2 binding to caveolae was recently found to be crucial for the initiation of the Smad signaling pathway. Here we determined the role of BMP receptor localization within caveolae isoforms and aggregation of caveolae as well as BMPRIa in bone marrow stromal cells (BMSCs) on bone mineral density using the B6.C3H-6T as a model system. The B6.C3H-6T is a congenic mouse with decreased bone mineral density (BMD) with increased marrow adipocytes and decreased osteoprogenitor proliferation. C57BL/6J mice served as controls since only a segment of Chr6 from the C3H/HeJ mouse was backcrossed to a C57BL/6J background. Family of Image Correlation Spectroscopy was used to analyze receptor cluster density and co-localization of BMPRIa and caveolae. It was previously shown that BMP2 stimulation results in an aggregation of caveolae and BMPRIa. Additionally, BMSCs isolated from the B6.C3H-6T mice showed a dispersion of caveolae domains compared to C57BL/6J. The aggregation of BMPRIa that is necessary for signaling to occur was inhibited in BMSCs isolated from B6.C3H-6T. Additionally, we analyzed the co-localization of BMPRIa with caveolin-1 isoforms. There was increased percentage of BMPRIa co-localization with caveolae compared to C57BL/6J. BMP2 stimulation had no effect on the colocalization of BMPRIa with caveolin-1. Disrupting caveolae initiated Smad signaling in the isolated BMSCs from B6.C3H-6T. These data suggest that in congenic 6T mice BMP receptors aggregation is inhibited causing an inhibition of signaling and reduced bone mass.
Image Cross Correlation Spectroscopy; Caveolae; Caveolin; Osteoblast; Mineralization; Family Image Correlation Spectroscopy
Insulin-like growth factor I (IGF-I) is a mitogen for vascular smooth muscle cells (VSMC) and has been implicated in the development and progression of atherosclerosis. IGF binding proteins (IGFBPs) modify IGF-I actions independently of IGF binding, but a receptor-based mechanism by which they function has not been elucidated. We investigated the role of IGFBP-2 and receptor protein tyrosine phosphatase β (RPTPβ) in regulating IGF-I signaling and cellular proliferation. IGFBP-2 bound RPTPβ, which led to its dimerization and inactivation. This enhanced PTEN tyrosine phosphorylation and inhibited PTEN activity. Utilization of substrate trapping and phosphatase-dead mutants showed that RPTPβ bound specifically to PTEN and dephosphorylated it. IGFBP-2 knockdown led to decreased PTEN tyrosine phosphorylation and decreased AKT Ser473 activation. IGFBP-2 enhanced IGF-I-stimulated VSMC migration and proliferation. Analysis of aortas obtained from IGFBP-2−/− mice showed that RPTPβ was activated, and this was associated with inhibition of IGF-I stimulated AKT Ser473 phosphorylation and VSMC proliferation. These changes were rescued following administration of IGFBP-2. These findings present a novel mechanism for coordinate regulation of IGFBP-2 and IGF-I signaling functions that lead to stimulation of VSMC proliferation. The results have important implications for understanding how IGFBPs modulate the cellular response to IGF-I.
Recent studies have demonstrated an important physiologic link between bone and fat. Bone and fat cells arise from the same mesenchymal precursor cell within bone marrow, capable of differentiation into adipocytes or osteoblasts. Increased BMI appears to protect against osteoporosis. However, recent studies have suggested detrimental effects of visceral fat on bone health. Increased visceral fat may also be associated with decreased growth hormone (GH) and insulin-like growth factor 1 (IGF-1) levels which are important for maintenance of bone homeostasis. The purpose of our study was to assess the relationship between vertebral bone marrow fat and trabecular bone mineral density (BMD), abdominal fat depots, GH and IGF-1 in premenopausal women with obesity. We studied 47 premenopausal women of various BMI (range: 18–41 kg/m2, mean 30 ± 7 kg/m2) who underwent vertebral bone marrow fat measurement with proton magnetic resonance spectroscopy (1H-MRS), body composition, and trabecular BMD measurement with computed tomography (CT), and GH and IGF-1 levels. Women with high visceral fat had higher bone marrow fat than women with low visceral fat. There was a positive correlation between bone marrow fat and visceral fat, independent of BMD. There was an inverse association between vertebral bone marrow fat and trabecular BMD. Vertebral bone marrow fat was also inversely associated with IGF-1, independent of visceral fat. Our study showed that vertebral bone marrow fat is positively associated with visceral fat and inversely associated with IGF-1 and BMD. This suggests that the detrimental effect of visceral fat on bone health may be mediated in part by IGF-1 as an important regulator of the fat and bone lineage.
Bone marrow adipogenesis is a normal physiologic process in all mammals. However, its function is unknown. The mesenchymal stem cell is the marrow precursor for adipocytes as well as osteoblasts, and PPARγ is an essential differentiation factor for entrance into the fat lineage. Mouse models have provided significant insight into the molecular cues that define stromal cell fate. In humans, accelerated marrow adipogenesis has been associated with aging and several chronic conditions including diabetes mellitus and osteoporosis. Newer imaging techniques have been used to determine the developmental time course of fat generation in bone marrow. However, more studies are needed to understand the interrelationship among hematopoietic, osteoblastic, and adipogenic cells within the marrow niche.
adipogenesis; marrow stromal cells; osteoblastogenesis; bone mass; aging
The continuous release of bone-stored growth factors following bone resorption promotes the colonization of circulating cancer cells. However, the precise role of each of the various growth factors remains unclear. In this study, we investigated the role of bone-derived insulin-like growth factor (IGF) in the development of bone metastases in an animal model of breast cancer. We found that local stimulation of calvarial bone resorption prior to cell inoculation stimulated subsequent bone metastases to that site in vivo, while inhibition of bone resorption inhibited bone metastases. Anchorage-independent growth of cancer cells was stimulated by the culture supernatants from resorbed bones, which contained elevated levels of IGF type I (IGF-1). This stimulation was blocked by IGF-1 receptor (IGF1R) neutralizing antibody, but not antibody targeting other bone-stored growth factors including TGFβ, fibroblast growth factors, and platelet derived growth factors. While recombinant human IGF-I caused IGFIR tyrosine autophosphorylation, followed by activation of Akt and NF-κB in cancer cells, dominant-negative inhibition of IGFIR, Akt, or NF-κB significantly reduced bone metastases with increased apoptosis and decreased mitosis in metastatic cells. Together, our findings suggest that bone-derived IGF-I bridges the crosstalk between bone and metastasized cancer cells via activation of the IGFIR/Akt/NF-κB pathway. Disruption of this pathway therefore may represent a promising therapeutic intervention for bone metastasis.
IGF receptor signaling; Akt; NF-κB; apoptosis; mitosis
Second generation antipsychotics (SGAs) have been linked to metabolic and bone disorders in clinical studies, but the mechanisms of these side effects remain unclear. Additionally, no studies have examined whether SGAs cause bone loss in mice. Using in vivo and in vitro modeling we examined the effects of risperidone, the most commonly prescribed SGA, on bone in C57BL6/J (B6) mice. Mice were treated with risperidone orally by food supplementation at a dose of 1.25 mg/kg daily for 5 and 8 weeks, starting at 3.5 weeks of age. Risperidone reduced trabecular BV/TV, trabecular number and percent cortical area. Trabecular histomorphometry demonstrated increased resorption parameters, with no change in osteoblast number or function. Risperidone also altered adipose tissue distribution such that white adipose tissue mass was reduced and liver had significantly higher lipid infiltration. Next, in order to tightly control risperidone exposure, we administered risperidone by chronic subcutaneous infusion with osmotic minipumps (0.5 mg/kg daily for 4 weeks) in 7 week old female B6 mice. Similar trabecular and cortical bone differences were observed compared to the orally treated groups (reduced trabecular BV/TV, and connectivity density, and reduced percent cortical area) with no change in body mass, percent body fat, glucose tolerance or insulin sensitivity. Unlike in orally treated mice, risperidone infusion reduced bone formation parameters (serum P1NP, MAR and BFR/BV). Resorption parameters were elevated, but this increase did not reach statistical significance. To determine if risperidone could directly affect bone cells, primary bone marrow cells were cultured with osteoclast or osteoblast differentiation media. Risperidone was added to culture medium in clinically relevant doses of 0, 2.5 or 25 ng/ml. The number of osteoclasts was significantly increased by addition in vitro of risperidone while osteoblast differentiation was not altered. These studies indicate that risperidone treatment can have negative skeletal consequences by direct activation of osteoclast activity and by indirect non-cell autonomous mechanisms. Our findings further support the tenet that the negative side effects of SGAs on bone mass should be considered when weighing potential risks and benefits, especially in children and adolescents who have not yet reached peak bone mass.
Global deletion of the Igfbp2 gene results in the suppression of bone turnover. To investigate the role of IGFBP-2 in regulating osteoclast differentiation we cultured Igfbp2−/− bone marrow cells and found a reduction in the number of osteoclasts and impaired resorption. Addition of full length IGFBP-2 restored osteoclast differentiation, fusion and resorption. To determine the molecular domains of IGFBP-2 that were required for this effect to be manifest, Igfbp2−/− bone marrow cells mice were transfected with constructs in which the heparin binding (HBD) or the IGF- binding domains of IGFBP-2 were mutated. We found that both domains were necessary for osteoclastogenesis since expression of the mutated forms of either domain failed to support the formation of functionally mature osteoclasts. To discern the mechanism by which IGFBP-2 regulates osteoclast formation, PTEN abundance and phosphorylation status as well as AKT responsiveness to IGF-I were analyzed. Igfbp2−/− cells had elevated levels of PTEN and phospho-PTEN compared with controls. Expression of wild-type IGFBP-2 reduced the level of PTEN to that of wild-type cells. Cells expressing the IGF binding mutant showed suppression of PTEN and phospho-PTEN equivalent to the wild type protein, whereas those expressing the IGFBP-2 HBD mutant showed no PTEN suppression. When the ability of IGF-I to stimulate AKT activation, measured by Thr308 and Ser473 phosphorylation, was analyzed, stimulation of Ser473 in response to IGF-I in pre-osteoclasts required the presence of intact IGFBP-2. This effect was duplicated by the addition of a CK2 inhibitor that prevents the phosphorylation of PTEN. In contrast, in fully differentiated osteoclasts stimulation of Thr308 phosphorylation required the presence of intact IGFBP-2. We conclude that IGFBP-2 is an important regulator of osteoclastogenesis and that both the heparin and the IGF binding domains of IGFBP-2 are essential for the formation of fully differentiated and functional osteoclasts.
During endochondral ossification, the cartilage is surrounded by a layer of cells that constitute the perichondrium. Communication between osteoblasts in the perichondrium via N-cadherin adherens junctions is essential for endochondral bone growth. We observed that adherens junction molecule N-cadherin and its interacting partners p120, β-catenin and PTEN are expressed by cells present in the perichondrium. To study if N-cadherin mediated adherens junctions play a role in mediating signal transduction events during bone development, we utilized MC3T3E1 preosteoblasts plated at sub confluent (low) and confluent (high) densities to mimic adherens junction formation. When MC3T3E1 cells were plated at high density we observed an increase in phosphorylation of AKTSer473 and its downstream target GSK3Ser9, which coincided with an increase in Osterix, Osteomodulin and Osteoglycin gene expression. Using immunofluorescence, we identified N-cadherin, p120 and β-catenin localized at the membrane of MC3T3E1 cells. Treatment of confluent MC3T3E1cells with an N-cadherin junction inhibitor-EGTA and a PI3K inhibitor LY294002 resulted in reduction of phosphorylation levels of AKT and GSK3 and expression of Osterix, Osteomodulin and Osteoglycin. Furthermore, utilizing an N-cadherin blocking antibody resulted in reduced AKT signaling and Osterix gene expression, suggesting that osteoblast junction formation is linked to activation of PI3K signaling, which leads to osteoblast differentiation. To further explore the strength of this linkage, we utilized a conditional knockout approach using Dermo1cre to delete β-catenin and PTEN, two important proteins known to be essential for adherens junctions and PI3K signaling, respectively. In the absence of β-catenin, we observed a decrease in adherens junctions and AKT signaling in the perichondrium. PTEN deletion, on the other hand, increased the number of cells expressing N-cadherin in the perichondrium. These observations show that N-cadherin mediated junctions between osteoblasts are needed for osteoblast gene transcription.
N-cadherin; Osteoblasts; Osterix; Adherens junctions