Sclerostin influences body composition by regulating catabolic and anabolic metabolism in adipocytes

Bone and muscle crosstalk in ageing and disease

Interorgan communication between bone and skeletal muscle is central to human health. A dysregulation of bone-muscle crosstalk is implicated in several age-related diseases. Ageing-associated changes in endocrine, inflammatory, nutritional and biomechanical stimuli can influence the differentiation capacity, function and survival of mesenchymal stem cells and bone-forming and muscle-forming cells. Consequently, the secretome phenotype of bone and muscle cells is altered, leading to impaired crosstalk and, ultimately, catabolism of both tissues. Adipose tissue acts as a third player in the bone-muscle interaction by secreting factors that affect bone and muscle cells. Physical exercise remains the key biological stimulus for bone-muscle crosstalk, either directly via the release of cytokines from bone, muscle or adipocytes, or indirectly through extracellular vesicles. Overall, bone-muscle crosstalk is considered an inherent process necessary to maintain the structure and function of both tissues across the life cycle. This Review summarizes the latest biomedical advances in bone-muscle crosstalk as it pertains to human ageing and disease. We also outline future research priorities to accommodate the understanding of this rapidly emerging field.

Influence of diabetes mellitus on metabolic networks in lung cancer patients: an analysis using dynamic total-body PET/CT imaging

INTRODUCTION

The intricate interplay between organs can give rise to a multitude of physiological conditions. Disruptions such as inflammation or tissue damage can precipitate the development of chronic diseases such as tumors or diabetes mellitus (DM). While both lung cancer and DM are the consequences of disruptions in homeostasis, the relationship between them is intricate. This study sought to investigate the potential influence of DM on lung cancer by employing total-body dynamic PET imaging.

METHODS

The present study proposes a framework for metabolic network analysis using total-body dynamic PET imaging of 20 lung cancer patients with DM (DM group) and 20 lung cancer patients without DM (Non-DM group), with the residuals of a third-order polynomial fit serving as an indicator of Pearson correlation.

RESULTS

The framework successfully captured the deviation of the DM group from the Non-DM group at both the edge and organ levels. At the edge level, there was a significant difference in the lesion- left ventricle (LV) between the DM and Non-DM groups (P < 0.05). Furthermore, we discovered a positive correlation between the absolute value of Z-score (ZCC) of lesion - LV and the duration of DM (R = 0.680, P < 0.001). At the organ level, there was a significant difference in the kidney, brain, and abdominal fat between the DM and Non-DM groups (P < 0.05).

CONCLUSION

This study demonstrated the feasibility of constructing metabolic networks to uncover complex alterations in lung cancer patients with DM. The findings contribute to understanding the systemic effects of DM on lung cancer metabolism and highlight the importance of personalized metabolic network analysis to comprehend the implications of concurrent diseases.

Bone-derived factors mediate crosstalk between skeletal and extra-skeletal organs

Bone has long been acknowledged as a fundamental structural entity that provides support and protection to the body's organs. However, emerging research indicates that bone plays a crucial role in the regulation of systemic metabolism. This is achieved through the secretion of a variety of hormones, cytokines, metal ions, extracellular vesicles, and other proteins/peptides, collectively referred to as bone-derived factors (BDFs). BDFs act as a medium through which bones can exert targeted regulatory functions upon various organs, thereby underscoring the profound and concrete implications of bone in human physiology. Nevertheless, there remains a pressing need for further investigations to elucidate the underlying mechanisms that inform the effects of bone on other body systems. This review aims to summarize the current findings related to the roles of these significant modulators across different organs and metabolic contexts by regulating critical genes and signaling pathways in vivo. It also addresses their involvement in the pathogenesis of various diseases affecting the musculoskeletal system, circulatory system, glucose and lipid metabolism, central nervous system, urinary system, and reproductive system. The insights gained from this review may contribute to the development of innovative therapeutic strategies through a focused approach to bone secretomes. Continued research into BDFs is expected to enhance our understanding of bone as a multifunctional organ with diverse regulatory roles in human health.

Impact of 24-week concurrent training on bone parameters and plasma levels of osteoglycin and sclerostin in young, sedentary adults: secondary analyses from the ACTIBATE randomized controlled trial

OBJECTIVE

To examine the effects of 24-week moderate (MOD-EX) and vigorous-intensity concurrent training (VIG-EX) on bone parameters and plasma levels of osteoglycin and sclerostin and their interplay with body composition and cardiometabolic risk factors in young, sedentary men and women.

DESIGN

Secondary study from the ACTIBATE randomized controlled trial (ClinicalTrials.gov ID: NCT02365129).

METHODS

This study was performed at the Sport and Health University Research Institute and the Virgen de las Nieves University Hospital of the University of Granada. Bone parameters were measured by dual-energy X-ray absorptiometry, and osteoglycin and sclerostin levels, by enzyme-linked immunosorbent assay.

RESULTS

145 young sedentary adults were assigned to a control (CON, n = 54), a MOD-EX (n = 48), or a VIG-EX (n = 43). 106 participants were included in the per-protocol analyses (CON, n = 42; MOD-EX, n = 33; and VIG-EX, n = 31). After 24 weeks of concurrent training, we observed no differences in changes in bone parameters (all P time × group ≥ .300), osteoglycin (P time × group = .250), and sclerostin levels (P time × group = .489). Moreover, we found no correlations between osteoglycin and sclerostin levels with body composition (all P ≥ .639) and cardiometabolic risk factors (all P ≥ .119).

CONCLUSION

24 weeks of concurrent training did not alter bone parameters, and plasma levels of osteoglycin and sclerostin in young, sedentary adults. Moreover, osteoglycin and sclerostin are not related with bone parameters and cardiometabolic risk factors in this population. These findings suggest that longer concurrent training interventions may be needed to enhance bone parameters in young, sedentary adults.

Bone marrow adipocytes in cancer: Mechanisms, models, and therapeutic implications

Adipose tissue is the primary site of energy storage in the body and a key regulator of metabolism. However, different adipose depots exhibit distinct molecular and phenotypic characteristics that have yet to be fully unraveled. While initially considered inert, bone marrow adipocytes (BMAs) have been recognized as key regulators of bone homeostasis, and more recently bone pathologies, although many unknowns remain. In this review, we summarize the current knowledge on BMAs, focusing on their distinct characteristics, functional significance in bone physiology and metabolism, as well as their emerging role in cancer pathogenesis. We present and discuss the current methodologies for investigating BMA-cancer interactions, encompassing both in vitro 3D culture systems and in vivo models, and their limitations in accurately replicating the phenotypes and biological processes of the human species. We highlight the imperative for advancing towards humanized models to better mimic the complexities of human physiology and disease progression. Finally, therapeutic strategies targeting metabolism or BMA-secreted factors, such as anti-cholesterol drugs, hold considerable promise in cancer treatment. We present the synergistic avenue of combining conventional cancer therapies with agents targeting adipocyte signaling to amplify treatment efficacy. Developing preclinical models that more faithfully replicate human pathological and physiological processes will lead to more accurate mechanistic understanding of the role of BMAs in bone metastasis and lead to more relevant preclinical drug screening.

Research Progress on the Influence of Novel Targeted Drugs for Osteoporosis on Glucose Metabolism

Both diabetes and osteoporosis are serious chronic conditions. Evidence is mounting that several bone-derived hormones play a role in glucose metabolism in patients with diabetes. Notably, novel biotargeted anti-osteoporotic agents have been recently found to reduce the risk of diabetes. This review explores the correlation of osteokines, including the receptor activator of nuclear factor-κB ligand (RANKL), sclerostin, and Dickkopf-1 (DKK1) with glycemic indicators in patients with diabetes, as well as the effects of their respective monoclonal antibodies on glucose metabolism and their possible mechanisms. Denosumab, the monoclonal antibody against RANKL, has been shown to reduce glycated hemoglobin (HbA1c) and the risk of diabetes, possibly by enhancing pancreatic β-cell survival and glucagon-like peptide-1 secretion. Sclerostin was positively correlated with HbA1c and may induce insulin resistance via endoplasmic reticulum stress. The association of DKK1 with fasting plasma glucose and HbA1c is still unclear, though decreasing DKK1 levels may correlate with β-cell survival. However, few studies have investigated the effects of antibodies against sclerostin or DKK1 on glucose metabolism. Further research is required to elucidate the influence of novel anti-osteoporotic biotargeted agents on glucose homeostasis in patients with diabetes and their underlying mechanisms.

Lack of PTEN in Osteocytes Increases Lipocalin-2 Level and Confers Resistance to High-Fat Diet-Induced Obesity in Mice

Osteocytes have been shown to play critical roles in the regulation of a wide range of metabolic processes. However, their role in the regulation of glucose metabolism remains to be determined despite accumulating evidence of the integral role of osteoblasts in this regulation, in which osteoblast-derived lipocalin-2 (LCN2) has been shown to regulate glucose metabolism. Additionally, Lcn2 expression is induced by AKT activation. These results led us to hypothesize that AKT activation in osteocytes regulates glucose metabolism by modulating Lcn2 expression. Therefore, in this study, the Pten gene was deleted in osteocytes to activate AKT signaling by crossing Pten-flox mice with Dmp1-Cre mice (PtenOcy-/- mice). Deleting Pten expression in osteocytes resulted in osteocytic AKT activation, which was associated with decreased adipose tissue mass and enhanced insulin sensitivity. Expression of Pparg2 and lipogenesis-associated genes were decreased in the adipose tissue of PtenOcy-/- mice. Mechanistically, the lack of Phosphatase and Tensin Homolog Deleted from Chromosome 10 (PTEN) in osteocytes increased Lcn2 expression in the femur, which was associated with increased serum and urine LCN2 levels. The urinary LCN2 level was negatively associated with white adipose tissue mass. Additionally, the treatment of primary white adipocytes with recombinant LCN2 reduced the expression of Pparg2 and lipogenesis-related genes. These results suggest that the absence of PTEN in osteocytes increases the expression of Lcn2, which acts in the adipose tissue to suppress lipogenesis, resulting in enhanced insulin sensitivity in these mice. This study provides novel insights into the critical role of AKT activation in osteocytes in regulating glucose metabolism by increasing Lcn2 expression.

Wnt7b overexpression in osteoblasts stimulates bone formation and reduces obesity in mice on a high-fat diet

Previous studies have shown that Wnt7b potently stimulates bone formation by promoting osteoblast differentiation and activity. As high-fat feeding leads to obesity and systemic metabolic dysregulation, here we investigate the potential benefit of Wnt7b overexpression in osteoblasts on both bone and whole-body metabolism in mice fed with a high-fat diet (HFD). Wnt7b overexpression elicited massive overgrowth of trabecular and cortical bone but seemed to ameliorate body fat accumulation in mice with prolonged HFD feeding. In addition, Wnt7b overexpression modestly improved glucose tolerance in male mice on HFD. Collectively, the results indicate that targeted overexpression of Wnt7b in osteoblasts not only stimulates bone formation but also improves certain aspects of global metabolism in overnourished mice.

The role of bone in energy metabolism: A focus on osteocalcin

Understanding the mechanisms involved in whole body glucose regulation is key for the discovery of new treatments for type 2 diabetes (T2D). Historically, glucose regulation was largely focused on responses to insulin and glucagon. Impacts of incretin-based therapies, and importance of muscle mass, are also highly relevant. Recently, bone was recognized as an endocrine organ, with several bone proteins, known as osteokines, implicated in glucose metabolism through their effects on the liver, skeletal muscle, and adipose tissue. Research efforts mostly focused on osteocalcin (OC) as a leading example. This review will provide an overview on this role of bone by discussing bone turnover markers (BTMs), the receptor activator of nuclear factor kB ligand (RANKL), osteoprotegerin (OPG), sclerostin (SCL) and lipocalin 2 (LCN2), with a focus on OC. Since 2007, some, but not all, research using mostly OC genetically modified animal models suggested undercarboxylated (uc) OC acts as a hormone involved in energy metabolism. Most data generated from in vivo, ex vivo and in vitro models, indicate that exogenous ucOC administration improves whole-body and skeletal muscle glucose metabolism. Although data in humans are generally supportive, findings are often discordant likely due to methodological differences and observational nature of that research. Overall, evidence supports the concept that bone-derived factors are involved in energy metabolism, some having beneficial effects (ucOC, OPG) others negative (RANKL, SCL), with the role of some (LCN2, other BTMs) remaining unclear. Whether the effect of osteokines on glucose regulation is clinically significant and of therapeutic value for people with insulin resistance and T2D remains to be confirmed.

Crosstalk between Lipid Metabolism and Bone Homeostasis: Exploring Intricate Signaling Relationships

Bone is a dynamic tissue reshaped by constant bone formation and bone resorption to maintain its function. The skeletal system accounts for approximately 70% of the total volume of the body, and continuous bone remodeling requires quantities of energy and material consumption. Adipose tissue is the main energy storehouse of the body and has a strong adaptive capacity to participate in the regulation of various physiological processes. Considering that obesity and metabolic syndrome have become major public health challenges, while osteoporosis and osteoporotic fractures have become other major health problems in the aging population, it would be interesting to explore these 2 diseases together. Currently, an increasing number of researchers are focusing on the interactions between multiple tissue systems, i.e., multiple organs and tissues that are functionally coordinated together and pathologically pathologically interact with each other in the body. However, there is lack of detailed reviews summarizing the effects of lipid metabolism on bone homeostasis and the interactions between adipose tissue and bone tissue. This review provides a detailed summary of recent advances in understanding how lipid molecules and adipose-derived hormones affect bone homeostasis, how bone tissue, as a metabolic organ, affects lipid metabolism, and how lipid metabolism is regulated by bone-derived cytokines.

Bone controls browning of white adipose tissue and protects from diet-induced obesity through Schnurri-3-regulated SLIT2 secretion

The skeleton has been suggested to function as an endocrine organ controlling whole organism energy balance, however the mediators of this effect and their molecular links remain unclear. Here, utilizing Schnurri-3-/- (Shn3-/-) mice with augmented osteoblast activity, we show Shn3-/-mice display resistance against diet-induced obesity and enhanced white adipose tissue (WAT) browning. Conditional deletion of Shn3 in osteoblasts but not adipocytes recapitulates lean phenotype of Shn3-/-mice, indicating this phenotype is driven by skeleton. We further demonstrate osteoblasts lacking Shn3 can secrete cytokines to promote WAT browning. Among them, we identify a C-terminal fragment of SLIT2 (SLIT2-C), primarily secreted by osteoblasts, as a Shn3-regulated osteokine that mediates WAT browning. Lastly, AAV-mediated Shn3 silencing phenocopies the lean phenotype and augmented glucose metabolism. Altogether, our findings establish a novel bone-fat signaling axis via SHN3 regulated SLIT2-C production in osteoblasts, offering a potential therapeutic target to address both osteoporosis and metabolic syndrome.

High-fat and high-carbohydrate diets increase bone fragility through TGF-β-dependent control of osteocyte function

Obesity can increase the risk of bone fragility, even when bone mass is intact. This fragility stems from poor bone quality, potentially caused by deficiencies in bone matrix material properties. However, cellular and molecular mechanisms leading to obesity-related bone fragility are not fully understood. Using male mouse models of obesity, we discovered TGF-β signaling plays a critical role in mediating the effects of obesity on bone. High-carbohydrate and high-fat diets increase TGF-β signaling in osteocytes, which impairs their mitochondrial function, increases cellular senescence, and compromises perilacunar/canalicular remodeling and bone quality. By specifically inhibiting TGF-β signaling in mouse osteocytes, some of the negative effects of high-fat and high-carbohydrate diets on bones, including the lacunocanalicular network, perilacunar/canalicular remodeling, senescence, and mechanical properties such as yield stress, were mitigated. DMP1-Cre-mediated deletion of TGF-β receptor II also blunted adverse effects of high-fat and high-carbohydrate diets on energy balance and metabolism. These findings suggest osteocytes are key in controlling bone quality in response to high-fat and high-carbohydrate diets. Calibrating osteocyte function could mitigate bone fragility associated with metabolic diseases while reestablishing energy balance.

Activation of the osteoblastic HIF-1α pathway partially alleviates the symptoms of STZ-induced type 1 diabetes mellitus via RegIIIγ

The hypoxia-inducible factor-1α (HIF-1α) pathway coordinates skeletal bone homeostasis and endocrine functions. Activation of the HIF-1α pathway increases glucose uptake by osteoblasts, which reduces blood glucose levels. However, it is unclear whether activating the HIF-1α pathway in osteoblasts can help normalize glucose metabolism under diabetic conditions through its endocrine function. In addition to increasing bone mass and reducing blood glucose levels, activating the HIF-1α pathway by specifically knocking out Von Hippel‒Lindau (Vhl) in osteoblasts partially alleviated the symptoms of streptozotocin (STZ)-induced type 1 diabetes mellitus (T1DM), including increased glucose clearance in the diabetic state, protection of pancreatic β cell from STZ-induced apoptosis, promotion of pancreatic β cell proliferation, and stimulation of insulin secretion. Further screening of bone-derived factors revealed that islet regeneration-derived protein III gamma (RegIIIγ) is an osteoblast-derived hypoxia-sensing factor critical for protection against STZ-induced T1DM. In addition, we found that iminodiacetic acid deferoxamine (SF-DFO), a compound that mimics hypoxia and targets bone tissue, can alleviate symptoms of STZ-induced T1DM by activating the HIF-1α-RegIIIγ pathway in the skeleton. These data suggest that the osteoblastic HIF-1α-RegIIIγ pathway is a potential target for treating T1DM.

Regulatory Effect of Osteocytes on Extramedullary and Bone Marrow Adipose Tissue Development and Function

PURPOSE OF REVIEW

This review summarizes evidence on osteocyte support of extramedullary and bone marrow adipocyte development and discusses the role of endogenous osteocyte activities of nuclear receptors peroxisome proliferator-activated receptor gamma (PPARG) and alpha (PPARA) in this support.

RECENT FINDINGS

PPARG and PPARA proteins, key regulators of glucose and fatty acid metabolism, are highly expressed in osteocytes. They play significant roles in the regulation of osteocyte secretome and osteocyte bioenergetics; both activities contributing to the levels of systemic energy metabolism in part through an effect on metabolic function of extramedullary and bone marrow adipocytes. The PPARs-controlled osteocyte endocrine/paracrine activities, including sclerostin expression, directly regulate adipocyte function, while the PPARs-controlled osteocyte fuel utilization and oxidative phosphorylation contribute to the skeletal demands for glucose and fatty acids, whose availability is under the control of adipocytes. Bone is an inherent element of systemic energy metabolism with PPAR nuclear receptors regulating osteocyte-adipocyte metabolic axes.

Osteocytes/Osteoblasts Produce SAA3 to Regulate Hepatic Metabolism of Cholesterol

Hypercholesterolaemia is a systemic metabolic disease, but the role of organs other than liver in cholesterol metabolism is unappreciated. The phenotypic characterization of the Tsc1Dmp1 mice reveal that genetic depletion of tuberous sclerosis complex 1 (TSC1) in osteocytes/osteoblasts (Dmp1-Cre) triggers progressive increase in serum cholesterol level. The resulting cholesterol metabolic dysregulation is shown to be associated with upregulation and elevation of serum amyloid A3 (SAA3), a lipid metabolism related factor, in the bone and serum respectively. SAA3, elicited from the bone, bound to toll-like receptor 4 (TLR4) on hepatocytes to phosphorylate c-Jun, and caused impeded conversion of cholesterol to bile acids via suppression on cholesterol 7 α-hydroxylase (Cyp7a1) expression. Ablation of Saa3 in Tsc1Dmp1 mice prevented the CYP7A1 reduction in liver and cholesterol elevation in serum. These results expand the understanding of bone function and hepatic regulation of cholesterol metabolism and uncover a potential therapeutic use of pharmacological modulation of SAA3 in hypercholesterolaemia.

Evaluating the correlation of sclerostin levels with obesity and type 2 diabetes in a multiethnic population living in Kuwait

Obesity and Type 2 Diabetes Mellitus (T2DM) are intricate metabolic disorders with a multifactorial etiology, often leading to a spectrum of complications. Recent research has highlighted the impact of these conditions on bone health, with a particular focus on the role of sclerostin (SOST), a protein molecule integral to bone metabolism. Elevated circulating levels of SOST have been observed in patients with T2DM compared to healthy individuals. This study aims to examine the circulating levels of SOST in a multiethnic population living in Kuwait and to elucidate the relationship between SOST levels, obesity, T2DM, and ethnic background. The study is a cross-sectional analysis of a large cohort of 2083 individuals living in Kuwait. The plasma level of SOST was measured using a bone panel multiplex assay. The study found a significant increase in SOST levels in individuals with T2DM (1008.3 pg/mL, IQR-648) compared to non-diabetic individuals (710.6 pg/mL, IQR-479). There was a significant gender difference in median SOST levels, with males exhibiting higher levels than females across various covariates (diabetes, IR, age, weight, and ethnicity). Notably, SOST levels varied significantly with ethnicity: Arabs (677.4 pg/mL, IQR-481.7), South Asians (914.6 pg/mL, IQR-515), and Southeast Asians (695.2 pg/mL, IQR-436.8). Furthermore, SOST levels showed a significant positive correlation with gender, age, waist circumference, systolic and diastolic blood pressure, fasting blood glucose, HbA1c, insulin, total cholesterol, triglycerides, HDL, LDL, ALT, and AST (p-Value ≥0.05). South Asian participants, who exhibited the highest SOST levels, demonstrated the most pronounced associations, even after adjusting for age, gender, BMI, and diabetes status (p-Value ≥0.05). The observed correlations of SOST with various clinical parameters suggest its significant role in the diabetic milieu, particularly pronounced in the South Asian population compared to other ethnic groups.

Effects of sclerostin injection on soleus and extensor digitorum longus muscle tissue in male mice

Sclerostin, a potent inhibitor of the Wnt signaling pathway, plays a critical role in bone homeostasis. Evidence suggests that sclerostin may also be involved in crosstalk between other tissues, including muscle. This pilot study attempted to examine the effects of sclerostin on soleus and extensor digitorum longus (EDL) muscle tissue from male mice that were given continuous recombinant sclerostin injections for 4 weeks. A total of 48 10-week-old male C57BL/6J mice were assigned to be sedentary or perform 1 h treadmill running per day for 4 weeks and administered subcutaneous injections of either saline or recombinant sclerostin 5 days/week. Sclerostin injection led to a reduction in the soleus myosin heavy chain (MHC) I, MHC I/IIA, MHC IIA/X, and MHC IIB cross-sectional area (p < 0.05) with no exercise effects on these reductions. In contrast, there were no effects of sclerostin injections or exercise on the fast-twitch EDL muscle in terms of size, MHC protein, or markers of Wnt signaling. These findings provide preliminary evidence of sclerostin's endocrine role in muscle via decreases in myofiber cross-sectional area, which seems to be independent of fiber type but muscle type-specific. More studies, however, are needed to confirm these preliminary results.

Anti-osteoporotic treatments in the era of non-alcoholic fatty liver disease: friend or foe

Over the last years non-alcoholic fatty liver disease (NAFLD) has grown into the most common chronic liver disease globally, affecting 17-38% of the general population and 50-75% of patients with obesity and/or type 2 diabetes mellitus (T2DM). NAFLD encompasses a spectrum of chronic liver diseases, ranging from simple steatosis (non-alcoholic fatty liver, NAFL) and non-alcoholic steatohepatitis (NASH; or metabolic dysfunction-associated steatohepatitis, MASH) to fibrosis and cirrhosis with liver failure or/and hepatocellular carcinoma. Due to its increasing prevalence and associated morbidity and mortality, the disease-related and broader socioeconomic burden of NAFLD is substantial. Of note, currently there is no globally approved pharmacotherapy for NAFLD. Similar to NAFLD, osteoporosis constitutes also a silent disease, until an osteoporotic fracture occurs, which poses a markedly significant disease and socioeconomic burden. Increasing emerging data have recently highlighted links between NAFLD and osteoporosis, linking the pathogenesis of NAFLD with the process of bone remodeling. However, clinical studies are still limited demonstrating this associative relationship, while more evidence is needed towards discovering potential causative links. Since these two chronic diseases frequently co-exist, there are data suggesting that anti-osteoporosis treatments may affect NAFLD progression by impacting on its pathogenetic mechanisms. In the present review, we present on overview of the current understanding of the liver-bone cross talk and summarize the experimental and clinical evidence correlating NAFLD and osteoporosis, focusing on the possible effects of anti-osteoporotic drugs on NAFLD.

Osteocyte-derived sclerostin impairs cognitive function during ageing and Alzheimer's disease progression

Ageing increases susceptibility to neurodegenerative disorders, such as Alzheimer's disease (AD). Serum levels of sclerostin, an osteocyte-derived Wnt-β-catenin signalling antagonist, increase with age and inhibit osteoblastogenesis. As Wnt-β-catenin signalling acts as a protective mechanism for memory, we hypothesize that osteocyte-derived sclerostin can impact cognitive function under pathological conditions. Here we show that osteocyte-derived sclerostin can cross the blood-brain barrier of old mice, where it can dysregulate Wnt-β-catenin signalling. Gain-of-function and loss-of-function experiments show that abnormally elevated osteocyte-derived sclerostin impairs synaptic plasticity and memory in old mice of both sexes. Mechanistically, sclerostin increases amyloid β (Aβ) production through β-catenin-β-secretase 1 (BACE1) signalling, indicating a functional role for sclerostin in AD. Accordingly, high sclerostin levels in patients with AD of both sexes are associated with severe cognitive impairment, which is in line with the acceleration of Αβ production in an AD mouse model with bone-specific overexpression of sclerostin. Thus, we demonstrate osteocyte-derived sclerostin-mediated bone-brain crosstalk, which could serve as a target for developing therapeutic interventions against AD.

El diálogo oculto entre el hueso y los tejidos a través del remodelado óseo

El hueso es mucho más que un reservorio de calcio y fósforo. Su disposición lacuno-canalicular ofrece una importante vía de intercambio con la circulación y actualmente, el esqueleto se considera un gran órgano endocrino, con acciones que van más allá del control del balance fosfocálcico mediado por el factor fibroblástico 23 (FGF23). Paralelamente al efecto modulador de las adipoquinas sobre el remodelado óseo, diversas proteínas óseas, como la osteocalcina y la esclerostina, ejercen cierta acción contra-reguladora sobre el metabolismo energético, posiblemente en un intento de asegurar los enormes requerimientos energéticos del remodelado. En esta interacción del hueso con otros tejidos, especialmente el adiposo, participa la señalización canónica Wnt/β-catenina y por ello la esclerostina, una proteína osteocítica que inhibe esta señalización, emerge como un potencial biomarcador. Es más, su participación en diversas patologías le posiciona como diana terapéutica, existiendo un anticuerpo anti-esclerostina, recientemente aprobado en nuestro país para el tratamiento de la osteoporosis. Esta revisión aborda el carácter endocrino del hueso, el papel de la osteocalcina y, especialmente, el papel regulador y modulador de la esclerostina sobre remodelado óseo y la homeóstasis energética a través de su interacción con la señalización canónica Wnt/β-catenina, así como su potencial utilidad como biomarcador.

The hidden cross talk between bone and tissues through bone turnover

Bone is more than a reservoir of calcium and phosphorus. Its lacuno-canalicular arrangement provides an important pathway for exchange with circulation and currently, the skeleton is considered a large endocrine organ with actions that go beyond the control of calcium-phosphorus balance mediated by fibroblastic growth factor 23 (FGF23). Parallel to the modulating effect of adipokines on bone turnover, certain bone proteins, such as osteocalcin and sclerostin, play a counter-regulatory role on energy metabolism, probably in an attempt to ensure its high energy requirement for bone turnover. In this crosstalk between bone and other tissues, especially with adipose tissue, canonical Wnt/β-catenin signaling is involved and therefore, sclerostin, an osteocyte derived protein that inhibits this signalling, emerges as a potential biomarker. Furthermore, its involvement in diverse pathologic conditions supports sclerostin as a therapeutic target, with an anti-sclerostin antibody recently approved in our country for the treatment of osteoporosis. This review addresses the endocrine nature of bone, the role of osteocalcin, and specially, the regulatory and modulatory role of sclerostin on bone turnover and energy homeostasis through its inhibitory effect on canonical Wnt/β-catenin signaling, as well as its potential utility as a biomarker.

Causal effects of osteoporosis on structural changes in specific brain regions: a Mendelian randomization study

Observational studies have reported that osteoporosis is associated with cortical changes in the brain. However, the inherent limitations of observational studies pose challenges in eliminating confounding factors and establishing causal relationships. And previous observational studies have not reported changes in specific brain regions. By employing Mendelian randomization, we have been able to infer a causal relationship between osteoporosis and a reduction in the surficial area (SA) of the brain cortical. This effect is partially mediated by vascular calcification. We found that osteoporosis significantly decreased the SA of global brain cortical (β = -1587.62 mm2, 95%CI: -2645.94 mm2 to -529.32 mm2, P = 0.003) as well as the paracentral gyrus without global weighted (β = - 19.42 mm2, 95%CI: -28.90 mm2 to -9.95 mm2, P = 5.85 × 10-5). Furthermore, we estimated that 42.25% and 47.21% of the aforementioned effects are mediated through vascular calcification, respectively. Osteoporosis leads to a reduction in the SA of the brain cortical, suggesting the presence of the bone-brain axis. Vascular calcification plays a role in mediating this process to a certain extent. These findings establish a theoretical foundation for further investigations into the intricate interplay between bone, blood vessels, and the brain.

Novel insights into osteocyte and inter-organ/tissue crosstalk

Osteocyte, a cell type living within the mineralized bone matrix and connected to each other by means of numerous dendrites, appears to play a major role in body homeostasis. Benefiting from the maturation of osteocyte extraction and culture technique, many cross-sectional studies have been conducted as a subject of intense research in recent years, illustrating the osteocyte-organ/tissue communication not only mechanically but also biochemically. The present review comprehensively evaluates the new research work on the possible crosstalk between osteocyte and closely situated or remote vital organs/tissues. We aim to bring together recent key advances and discuss the mutual effect of osteocyte and brain, kidney, vascular calcification, muscle, liver, adipose tissue, and tumor metastasis and elucidate the therapeutic potential of osteocyte.

Crosstalk Between the Neuroendocrine System and Bone Homeostasis

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

Epithelial-like transport of mineral distinguishes bone formation from other connective tissues

We review unique properties of bone formation including current understanding of mechanisms of bone mineral transport. We focus on formation only; mechanism of bone degradation is a separate topic not considered. Bone matrix is compared to other connective tissues composed mainly of the same proteins, but without the specialized mechanism for continuous transport and deposition of mineral. Indeed other connective tissues add mechanisms to prevent mineral formation. We start with the epithelial-like surfaces that mediate transport of phosphate to be incorporated into hydroxyapatite in bone, or in its ancestral tissue, the tooth. These include several phosphate producing or phosphate transport-related proteins with special expression in large quantities in bone, particularly in the bone-surface osteoblasts. In all connective tissues including bone, the proteins that constitute the protein matrix are mainly type I collagen and γ-carboxylate-containing small proteins in similar molar quantities to collagen. Specialized proteins that regulate connective tissue structure and formation are surprisingly similar in mineralized and non-mineralized tissues. While serum calcium and phosphate are adequate to precipitate mineral, specialized mechanisms normally prevent mineral formation except in bone, where continuous transport and deposition of mineral occurs.

Muscle-bone crosstalk via endocrine signals and potential targets for osteosarcopenia-related fracture

BACKGROUND

Osteosarcopenia is a syndrome coexisting sarcopenia and osteopenia/osteoporosis, with a high fracture risk. Recently, skeletal muscle and bone have been recognized as endocrine organs capable of communication through secreting myokines and osteokines, respectively. With a deeper understanding of the muscle-bone crosstalk, these endocrine signals exhibit an important role in osteosarcopenia development and fracture healing.

METHODS

This review summarizes the role of myokines and osteokines in the development and treatment of osteosarcopenia and fracture, and discusses their potential for osteosarcopenia-related fracture treatment.

RESULTS

Several well-defined myokines (myostatin and irisin) and osteokines (RANKL and SOST) are found to not only regulate skeletal muscle and bone metabolism but also influence fracture healing processes. Systemic interventions targeting these biochemical signals has shown promising results in improving the mass and functions of skeletal muscle and bone, as well as accelerating fracture healing processes.

CONCLUSION

The regulation of muscle-bone crosstalk via biochemical signals presents a novel and promising strategy for treating osteosarcopenia and fracture by simultaneously enhancing bone and muscle anabolism. We propose that myostatin, irisin, RANKL, and SOST may serve as potential targets to treat fracture patients with osteosarcopenia.

THE TRANSLATIONAL POTENTIAL OF THIS ARTICLE

Osteosarcopenia is an emerging geriatric syndrome where sarcopenia and osteoporosis coexist, with high fracture risk, delayed fracture healing, and increased mortality. However, no pharmacological agent is available to treat fracture patients with osteosarcopenia. This review summarizes the role of several myokines and osteokines in the development and treatment of osteosacropenia and fracture, as well as discusses their potential as intervention targets for osteosarcopenia-related fracture, which provides a novel and promising strategy for future osteosarcopenia-related fracture treatment.

The role of sclerostin in lipid and glucose metabolism disorders

Lipid and glucose metabolism are critical for human activities, and their disorders can cause diabetes and obesity, two prevalent metabolic diseases. Studies suggest that the bone involved in lipid and glucose metabolism is emerging as an endocrine organ that regulates systemic metabolism through bone-derived molecules. Sclerostin, a protein mainly produced by osteocytes, has been therapeutically targeted by antibodies for treating osteoporosis owing to its ability to inhibit bone formation. Moreover, recent evidence indicates that sclerostin plays a role in lipid and glucose metabolism disorders. Although the effects of sclerostin on bone have been extensively examined and reviewed, its effects on systemic metabolism have not yet been well summarized. In this paper, we provide a systemic review of the effects of sclerostin on lipid and glucose metabolism based on in vitro and in vivo evidence, summarize the research progress on sclerostin, and prospect its potential manipulation for obesity and diabetes treatment.

Evidence in Humans for Bone as an Endocrine Organ Regulating Energy Metabolism

There is increasing evidence from animal models that bone, in addition to its traditional function of providing structural support for the organism, has a rich network of interactions with multiple other tissues. This perspective focuses on evidence from human studies demonstrating that bone is an endocrine organ regulating energy metabolism, with the specific examples being osteocalcin, lipocalin 2, RANKL, and sclerostin. Conversely, animal studies have also demonstrated that a key hormone regulating energy metabolism, leptin, regulates bone metabolism via the sympathetic nervous system. Studies in humans have established a role for the sympathetic nervous system in regulating bone turnover; indeed, the potential therapeutic benefit of targeting this pathway in humans to prevent postmenopausal bone loss is currently being evaluated.

Sclerostin: clinical insights in muscle-bone crosstalk

Sclerostin, a protein encoded by the sclerostin (SOST) gene, is mostly expressed in osteocytes. First described in the pathogenesis of three disorders, sclerosteosis, van Buchem's disease, and craniodiaphyseal dysplasia, sclerostin has been identified as an important regulator of bone homeostasis, controlling bone formation by osteoblasts through inhibition of the canonical Wnt signaling pathway. Recent studies have highlighted a hypothetical role of sclerostin in myogenesis, thus modulating the interaction between bone and muscle. This narrative review provides an overview of the clinical implications of sclerostin modulation on skeletal muscle mass and function, and bone metabolism. Improving knowledge about muscle-bone crosstalk may represent a turning point in the development of therapeutic strategies for musculoskeletal disorders, particularly osteosarcopenia.

Urinary Calcium Is Associated with Serum Sclerostin among Stone Formers

BACKGROUND

Sclerostin plays an important role in bone metabolism and adipose tissue. Animal studies suggest that sclerostin influences urinary calcium (UCa), but this relationship has not been evaluated in stone formers (SFs). We aimed to investigate the association of UCa with serum sclerostin, bone mineral density (BMD), and body composition among SFs.

METHODS

Clinical and laboratorial data were retrieved from medical records. Determinants of UCa were studied using linear regression.

RESULTS

A total of 107 SFs (35.8 ± 9.3 years, 54% male) with eGFR 99.8 ± 14.5 mL/min/1.73 were studied. Subjects were split by sex and grouped into tertiles of UCa levels. Men in the highest UCa tertile had higher body mass index (BMI) and serum sclerostin, lower lean mass, and a trend towards higher fat mass. Women in the highest tertile had higher BMI and a trend towards higher serum sclerostin. Hypertension and metabolic syndrome, but not lower BMD, were more prevalent in the highest UCa tertile for both sexes. Sclerostin was positively correlated with fat mass and inversely correlated with lean mass among men, but not among women. BMD corrected for BMI at lumbar spine was inversely associated with UCa in a univariate analysis, but only serum sclerostin, hypertension, and NaCl intake were independent determinants of UCa in the multivariate model.

CONCLUSION

The present findings disclose that in addition to hypertension and salt intake, serum sclerostin is associated with urinary calcium in stone formers, suggesting that in addition to the hormones traditionally thought to alter calcium reabsorption in the kidney, sclerostin may play a significant additional role, warranting further investigation.

Peroxisome proliferator activated receptor-γ in osteoblasts controls bone formation and fat mass by regulating sclerostin expression

The nuclear receptor peroxisome proliferator activated receptor-γ (PPARγ) is a key contributor to metabolic function via its adipogenic and insulin-sensitizing functions, but it has negative effects on skeletal homeostasis. Here, we questioned whether the skeletal and metabolic actions of PPARγ are linked. Ablating Pparg expression in osteoblasts and osteocytes produced a high bone mass phenotype, secondary to increased osteoblast activity, and a reduction in subcutaneous fat mass because of reduced fatty acid synthesis and increased fat oxidation. The skeletal and metabolic phenotypes in Pparg mutants proceed from the regulation of sclerostin production by PPARγ. Mutants exhibited reductions in skeletal Sost expression and serum sclerostin levels while increasing production normalized both phenotypes. Importantly, disrupting the production of sclerostin synergized with the insulin-sensitizing actions of a PPARγ agonist while preventing bone loss. These data suggest that modulating sclerostin action may prevent bone loss associated with anti-diabetic therapies and augment their metabolic actions.

Regulation of the Osteocyte Secretome with Aging and Disease

As the most numerous and long-lived of all bone cells, osteocytes have essential functions in regulating skeletal health. Through the lacunar-canalicular system, secreted proteins from osteocytes can reach cells throughout the bone. Furthermore, the intimate connectivity between the lacunar-canalicular system and the bone vasculature allows for the transport of osteocyte-secreted factors into the circulation to reach the entire body. Local and endocrine osteocyte signaling regulates physiological processes such as bone remodeling, bone mechanoadaptation, and mineral homeostasis. However, these processes are disrupted by impaired osteocyte function induced by aging and disease. Dysfunctional osteocyte signaling is now associated with the pathogenesis of many disorders, including chronic kidney disease, cancer, diabetes mellitus, and periodontitis. In this review, we focus on the targeting of bone and extraskeletal tissues by the osteocyte secretome. In particular, we highlight the secreted osteocyte proteins, which are known to be dysregulated during aging and disease, and their roles during disease progression. We also discuss how therapeutic or genetic targeting of osteocyte-secreted proteins can improve both skeletal and systemic health.

Metabolic Health and Disease: A Role of Osteokines?

Maintenance of skeletal health is tightly regulated by osteocytes, osteoblasts, and osteoclasts via coordinated secretion of bone-derived factors, termed osteokines. Disruption of this coordinated process due to aging and metabolic disease promotes loss of bone mass and increased risk of fracture. Indeed, growing evidence demonstrates that metabolic diseases, including type 2 diabetes, liver disease and cancer are accompanied by bone loss and altered osteokine levels. With the persistent prevalence of cancer and the growing epidemic of metabolic disorders, investigations into the role of inter-tissue communication during disease progression are on the rise. While osteokines are imperative for bone homeostasis, work from us and others have identified that osteokines possess endocrine functions, exerting effects on distant tissues including skeletal muscle and liver. In this review we first discuss the prevalence of bone loss and osteokine alterations in patients with type 2 diabetes, non-alcoholic fatty liver disease, non-alcoholic steatohepatitis, cirrhosis, and cancer. We then discuss the effects of osteokines in mediating skeletal muscle and liver homeostasis, including RANKL, sclerostin, osteocalcin, FGF23, PGE2, TGF-β, BMPs, IGF-1 and PTHrP. To better understand how inter-tissue communication contributes to disease progression, it is essential that we include the bone secretome and the systemic roles of osteokines.

A novel multifunctional radioprotective strategy using P7C3 as a countermeasure against ionizing radiation-induced bone loss

Radiotherapy is a critical component of cancer care but can cause osteoporosis and pathological insufficiency fractures in surrounding and otherwise healthy bone. Presently, no effective countermeasure exists, and ionizing radiation-induced bone damage continues to be a substantial source of pain and morbidity. The purpose of this study was to investigate a small molecule aminopropyl carbazole named P7C3 as a novel radioprotective strategy. Our studies revealed that P7C3 repressed ionizing radiation (IR)-induced osteoclastic activity, inhibited adipogenesis, and promoted osteoblastogenesis and mineral deposition in vitro. We also demonstrated that rodents exposed to clinically equivalent hypofractionated levels of IR in vivo develop weakened, osteoporotic bone. However, the administration of P7C3 significantly inhibited osteoclastic activity, lipid formation and bone marrow adiposity and mitigated tissue loss such that bone maintained its area, architecture, and mechanical strength. Our findings revealed significant enhancement of cellular macromolecule metabolic processes, myeloid cell differentiation, and the proteins LRP-4, TAGLN, ILK, and Tollip, with downregulation of GDF-3, SH2B1, and CD200. These proteins are key in favoring osteoblast over adipogenic progenitor differentiation, cell matrix interactions, and shape and motility, facilitating inflammatory resolution, and suppressing osteoclastogenesis, potentially via Wnt/β-catenin signaling. A concern was whether P7C3 afforded similar protection to cancer cells. Preliminarily, and remarkably, at the same protective P7C3 dose, a significant reduction in triple-negative breast cancer and osteosarcoma cell metabolic activity was found in vitro. Together, these results indicate that P7C3 is a previously undiscovered key regulator of adipo-osteogenic progenitor lineage commitment and may serve as a novel multifunctional therapeutic strategy, leaving IR an effective clinical tool while diminishing the risk of adverse post-IR complications. Our data uncover a new approach for the prevention of radiation-induced bone damage, and further work is needed to investigate its ability to selectively drive cancer cell death.

Role of the Osteocyte in Musculoskeletal Disease

PURPOSE OF THE REVIEW

The purpose of this review is to summarize the role of the osteocyte in muscle atrophy in cancer patients, sarcopenia, spinal cord injury, Duchenne's muscular dystrophy, and other conditions associated with muscle deterioration.

RECENT FINDINGS

One type of bone cell, the osteocyte, appears to play a major role in muscle and bone crosstalk, whether physiological or pathological. Osteocytes are cells living within the bone-mineralized matrix. These cells are connected to each other by means of dendrites to create an intricately connected network. The osteocyte network has been shown to respond to different types of stimuli such as mechanical unloading, immobilization, aging, and cancer by producing osteocytes-derived factors. It is now becoming clear that some of these factors including sclerostin, RANKL, TGF-β, and TNF-α have detrimental effects on skeletal muscle. Bone and muscle not only communicate mechanically but also biochemically. Osteocyte-derived factors appear to contribute to the pathogenesis of muscle disease and could be used as a cellular target for new therapeutic approaches.

The roles of hepatokine and osteokine in liver-bone crosstalk: Advance in basic and clinical aspects

Both the liver and bone are important secretory organs in the endocrine system. By secreting organ factors (hepatokines), the liver regulates the activity of other organs. Similarly, bone-derived factors, osteokines, are created during bone metabolism and act in an endocrine manner. Generally, the dysregulation of hepatokines is frequently accompanied by changes in bone mass, and osteokines can also disrupt liver metabolism. The crosstalk between the liver and bone, particularly the function and mechanism of hepatokines and osteokines, has increasingly gained notoriety as a topic of interest in recent years. Here, based on preclinical and clinical evidence, we summarize the potential roles of hepatokines and osteokines in liver-bone interaction, discuss the current shortcomings and contradictions, and make recommendations for future research.

Antibodies to sclerostin or G-CSF receptor partially eliminate bone or marrow adipocyte loss, respectively, following vertical sleeve gastrectomy

Vertical sleeve gastrectomy (VSG), the most utilized bariatric procedure in clinical practice, greatly reduces body weight and improves a variety of metabolic disorders. However, one of its long-term complications is bone loss and increased risk of fracture. Elevated circulating sclerostin (SOST) and granulocyte-colony stimulating factor (G-CSF) concentrations have been considered as potential contributors to VSG-associated bone loss. To test these possibilities, we administrated antibodies to SOST or G-CSF receptor and investigated alterations to bone and marrow niche following VSG. Neutralizing either SOST or G-CSF receptor did not alter beneficial effects of VSG on adiposity and hepatic steatosis, and anti-SOST treatment provided a further improvement to glucose tolerance. SOST antibodies partially reduced trabecular and cortical bone loss following VSG by increasing bone formation, whereas G-CSF receptor antibodies had no effects on bone mass. The expansion in myeloid cellularity and reductions in bone marrow adiposity seen with VSG were partially eliminated by treatment with Anti-G-CSF receptor. Taken together, these experiments demonstrate that antibodies to SOST or G-CSF receptor may act through independent mechanisms to partially block effects of VSG on bone loss or marrow niche cells, respectively.

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

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

Relationships between Circulating Sclerostin, Bone Marrow Adiposity, Other Adipose Deposits and Lean Mass in Post-Menopausal Women

Sclerostin is a Wnt signaling pathway inhibitor that negatively regulates bone formation. Bone-marrow-derived stromal cell (BMSC) differentiation is influenced by the Wnt pathway, leading to the hypothesis that higher levels of sclerostin might be associated with an increase in bone marrow adiposity (BMA). The main purpose of this study was to determine whether a relationship exists between circulating sclerostin and BMA in post-menopausal women with and without fragility fractures. The relationships between circulating sclerostin and body composition parameters were then examined. The outcomes measures included vertebral and hip proton density fat fraction (PDFF) using the water fat imaging (WFI) MRI method; DXA scans; and laboratory measurements, including serum sclerostin. In 199 participants, no significant correlations were found between serum sclerostin and PDFF. In both groups, serum sclerostin was correlated positively with bone mineral density (R = 0.27 to 0.56) and negatively with renal function (R = -0.22 to -0.29). Serum sclerostin correlated negatively with visceral adiposity in both groups (R = -0.24 to -0.32). Serum sclerostin correlated negatively with total body fat (R = -0.47) and appendicular lean mass (R = -0.26) in the fracture group, but not in the controls. No evidence of a relationship between serum sclerostin and BMA was found. However, serum sclerostin was negatively correlated with body composition components, such as visceral adiposity, total body fat and appendicular lean mass.

Myokine, a key cytokine for physical exercise to alleviate sarcopenic obesity

Skeletal muscle has a robust endocrine function as a powerful organ and can secrete and release cytokines or polypeptides known as myokines. These myokines have significant regulatory effects on signal transduction in skeletal muscle and the metabolism of peripheral tissues and organs and exert biological effects via autocrine, paracrine, or endocrine forms. Obesity and aging cause myokine secretion dysregulation, and hastening sarcopenic obesity (SO) development. Exercise is currently an excellent intervention and prevention method for SO. Meanwhile, exercise impacts many organs and tissues. These organs and tissues will produce various myokines in response to movement and metabolism throughout the body to govern muscle differentiation, growth, and remodeling. According to accumulating data, exercise can increase the release of myokines from diverse tissues into the blood and postpone the SO onset and progression by influencing protein metabolism, inflammation, mitochondrial quality control, and other mechanisms.

Energy Balance and Bone Health: a Nutrient Availability Perspective

PURPOSE OF REVIEW

Obesity is highly prevalent and is associated with bone fragility and fracture. The changing nutrient availability to bone in obesity is an important facet of bone health. The goal of this article is to summarize current knowledge on the effects of carbohydrate and dietary fat availability on bone, particularly in the context of other tissues.

RECENT FINDINGS

The skeleton is a primary site for fatty acid and glucose uptake. The trafficking of carbohydrates and fats into tissues changes with weight loss and periods of weight gain. Exercise acutely influences nutrient uptake into bone and may affect nutrient partitioning to bone. Bone cells secrete hormones that signal to the brain and other tissues information about its energetic state, which may alter whole-body nutrient trafficking. There is a critical need for studies to address the changes that metabolic perturbations have on nutrient availability in bone.

Endocrine Functions of Sclerostin

Sclerostin, the product of the SOST gene has primarily been studied for its profound impact on bone mass. By interacting with LRP5 and LRP6, the glycoprotein suppresses the propagation of Wnt signals to β-catenin and thereby suppresses new bone formation. In this review, we discuss emerging data which suggest that sclerostin also acts outside the skeleton to influence metabolism. In humans, serum sclerostin levels are associated with body mass index and indices of metabolic function. Likewise, genetic mouse models of Sost gene deficiency indicate sclerostin influences adipocyte development and insulin signaling. These data raise the possibility that sclerostin neutralization may be effective at treating two epidemic conditions: osteoporosis and obesity.

Characterization of sclerostin's response within white adipose tissue to an obesogenic diet at rest and in response to acute exercise in male mice

Introduction: It is well established that sclerostin antagonizes the anabolic Wnt signalling pathway in bone, however, its physiological role in other tissues remains less clear. This study examined the effect of a high-fat diet (HFD) on sclerostin content and downstream markers of the Wnt signaling pathway (GSK3β and β-catenin) within subcutaneous inguinal white adipose tissue (iWAT), and visceral epididymal WAT (eWAT) depots at rest and in response to acute aerobic exercise. Methods: Male C57BL/6 mice (n = 40, 18 weeks of age) underwent 10 weeks of either a low-fat diet (LFD) or HFD. Within each diet group, mice were assigned to either remain sedentary (SED) or perform 2 h of endurance treadmill exercise at 15 m min^-1 with 5° incline (EX), creating four groups: LFD + SED (N = 10), LFD + EX (N = 10), HFD + SED (N = 10), and HFD + EX (N = 10). Serum and WAT depots were collected 2 h post-exercise. Results: Serum sclerostin showed a diet-by-exercise interaction, reflecting HFD + EX mice having higher concentration than HFD + SED (+31%, p = 0.03), and LFD mice being unresponsive to exercise. iWAT sclerostin content decreased post-exercise in both 28 kDa (-31%, p = 0.04) and 30 kDa bands (-36%, main effect for exercise, p = 0.02). iWAT β-catenin (+44%, p = 0.03) and GSK3β content were higher in HFD mice compared to LFD (+128%, main effect for diet, p = 0.005). Monomeric sclerostin content was abolished in eWAT of HFD mice (-96%, main effect for diet, p < 0.0001), was only detectable as a 30 kDa band in LFD mice and was unresponsive to exercise. β-catenin and GSK3β were both unresponsive to diet and exercise within eWAT. Conclusion: These results characterized sclerostin's content to WAT depots in response to acute exercise, which appears to be specific to a reduction in iWAT and identified a differential regulation of sclerostin's form/post-translational modifications depending on diet and WAT depot.

Multiple doses of SHR-1222, a sclerostin monoclonal antibody, in postmenopausal women with osteoporosis: A randomized, double-blind, placebo-controlled, dose-escalation phase 1 trial

SHR-1222, a novel humanized monoclonal antibody targeting sclerostin, has been shown to induce bone formation and decrease bone resorption at a single dose ranging 50-400 mg in our previous phase 1 trial. This study was a randomized, double-blind, placebo-controlled, dose-escalation phase 1 trial, which further investigated the safety, tolerability, pharmacokinetics (PK), pharmacodynamics (PD), and immunogenicity of multiple ascending doses of SHR-1222 in women with postmenopausal osteoporosis (POP). A total of 105 women with POP were enrolled and randomly assigned. Twenty-one received placebo and eighty-four received SHR-1222 sequentially (100 mg QM, n=4; 200 or 300 mg QM, n=20; and 400 or 600 mg Q2M, n=20). The most common adverse events included increased blood parathyroid hormone, increased low-density lipoprotein, increased blood alkaline phosphatase, increased blood cholesterol, back pain, and arthralgia, the majority of which were mild in severity without noticeable safety concerns. Serum SHR-1222 exposure (C_max,ss and AUC_0-tau,ss) increased in a greater than dose-proportional manner. Following multiple doses of SHR-1222, the bone formation markers (terminal propeptide of type I procollagen, bone-specific alkaline phosphatase, and osteocalcin) increased in a dose-dependent manner, whereas the bone resorption marker (β-C-telopeptide) was downregulated. Accordingly, BMD gains in the lumbar spine, total hip, and femoral neck were observed. The maximum BMD increase from baseline at the lumbar spine was detected in the 300 mg QM cohort (14.6% vs. 0.6% in the placebo group on day 169). Six (6/83; 7.2%) subjects developed anti-SHR-1222 antibodies with no discernible effects on PKs, PDs, and safety. Thus, multiple doses of SHR-1222 showed an acceptable safety profile and dose-dependent plasma exposure in women with POP, and could improve their BMD rapidly and prominently by promoting bone formation and inhibiting bone resorption. These findings further support SHR-1222 as a potential alternative agent for the treatment of POP.

Relationships between Sclerostin, Leptin and Metabolic Parameters in Non-Dialysis Chronic Kidney Disease Males

Sclerostin is an inhibitor of the Wnt-beta-catenin pathway. The relationship between sclerostin and adipose tissue or between sclerostin and nutritional status has been the subject of research interest in the last decade. Sclerostin concentrations are elevated in patients with chronic kidney disease (CKD). Leptin is an adipocytokine which inhibits food intake by stimulating the satiety center in the hypothalamus. Leptin concentrations rise with the reduction of eGFR (glomerular filtration rate). The aim of this study was to investigate the possible association between sclerostin and leptin, between sclerostin and selected poor prognostic factors of CKD progression, and between sclerostin and nutritional parameters in non-dialysis CKD male patients. 101 men with non-dialysis CKD stage 3-5 were included in the study. Bioimpedance spectroscopy (BIS) was used to measure body composition. Blood samples were drawn to measure the serum concentrations of sclerostin, leptin, creatinine, hemoglobin (Hgb), parathormone (PTH), inflammatory markers, and markers of nutritional status. We also measured homeostatic model assessment of insulin resistance (HOMA-IR) as well as blood pressure. We observed a significant, positive relationship between sclerostin and age, leptin, and glycated hemoglobin (HgbA1c) concentrations. A significant, negative association was observed between sclerostin and eGFR. Sclerostin is associated with leptin in non-dialysis CKD male patients. Sclerostin is also related to metabolic disturbances such as hyperglycemia in this population.

Osteoblast/osteocyte-derived interleukin-11 regulates osteogenesis and systemic adipogenesis

Exercise results in mechanical loading of the bone and stimulates energy expenditure in the adipose tissue. It is therefore likely that the bone secretes factors to communicate with adipose tissue in response to mechanical loading. Interleukin (IL)-11 is known to be expressed in the bone, it is upregulated by mechanical loading, enhances osteogenesis and suppresses adipogenesis. Here, we show that systemic IL-11 deletion (IL-11^-/-) results in reduced bone mass, suppressed bone formation response to mechanical loading, enhanced expression of Wnt inhibitors, and suppressed Wnt signaling. At the same time, the enhancement of bone resorption by mechanical unloading was unaffected. Unexpectedly, IL-11^-/- mice have increased systemic adiposity and glucose intolerance. Osteoblast/osteocyte-specific IL-11 deletion in osteocalcin-Cre;IL-11^fl/fl mice have reduced serum IL-11 levels, blunted bone formation under mechanical loading, and increased systemic adiposity similar to IL-11^-/- mice. Adipocyte-specific IL-11 deletion in adiponectin-Cre;IL-11^fl/fl did not exhibit any abnormalities. We demonstrate that osteoblast/osteocyte-derived IL-11 controls both osteogenesis and systemic adiposity in response to mechanical loading, an important insight for our understanding of osteoporosis and metabolic syndromes.

Low Bone Turnover Due to Hypothyroidism or Anti-Resorptive Treatment Does Not Affect Whole-Body Glucose Homeostasis in Male Mice

Bone is a large and dynamic tissue and its maintenance requires high amounts of energy as old or damaged bone structures need to be replaced during the process of bone remodeling. Glucose homeostasis is an essential prerequisite for a healthy bone and vice versa, the skeleton can act as an endocrine organ on energy metabolism. We recently showed that hypothyroidism in mice leads to an almost complete arrest of bone remodeling. Here, we aimed to investigate whether the profound suppression of bone remodeling affects whole-body glucose homeostasis. To that end, male C57BL/6JRj mice were rendered hypothyroid over 4 weeks using methimazole and sodium perchlorate in the drinking water. We confirmed trabecular bone gain due to decreased bone turnover in hypothyroid mice with decreased cortical but increased vertebral bone strength. Further, we found impaired glucose handling but not insulin resistance with hypothyroidism. In hypothyroid bone, glucose uptake and expression of glucose transporter Glut4 were reduced by 44.3% and 13.9%, respectively, suggesting lower energy demands. Nevertheless, hypothyroidism led to distinct changes in glucose uptake in muscle, liver, and epididymal white adipose tissue (eWAT). Reduced glucose uptake (−30.6%) and Glut1/Glut4 transcript levels (−31.9%/−67.5%) were detected in muscle tissue. In contrast, in liver and eWAT we observed increased glucose uptake by 25.6% and 68.6%, respectively, and upregulated expression of glucose transporters with hypothyroidism. To more specifically target bone metabolism and discriminate between the skeletal and systemic effects of hypothyroidism on energy metabolism, male mice were treated with zoledronate (ZOL), a bisphosphonate, that led to decreased bone turnover, trabecular bone gain, and reduced local glucose uptake into bone (−40.4%). However, ZOL-treated mice did not display alterations of systemic glucose handling nor insulin tolerance. Despite the close mutual crosstalk of bone and glucose metabolism, in this study, we show that suppressing bone remodeling does not influence whole-body glucose homeostasis in male mice.

The Gestational Effects of Maternal Bone Marker Molecules on Fetal Growth, Metabolism and Long-Term Metabolic Health: A Systematic Review

Fetal exposure in adverse environmental factors during intrauterine life can lead to various biological adjustments, affecting not only in utero development of the conceptus, but also its later metabolic and endocrine wellbeing. During human gestation, maternal bone turnover increases, as reflected by molecules involved in bone metabolism, such as vitamin D, osteocalcin, sclerostin, sRANKL, and osteoprotegerin; however, recent studies support their emerging role in endocrine functions and glucose homeostasis regulation. Herein, we sought to systematically review current knowledge on the effects of aforementioned maternal bone biomarkers during pregnancy on fetal intrauterine growth and metabolism, neonatal anthropometric measures at birth, as well as on future endocrine and metabolic wellbeing of the offspring. A growing body of literature converges on the view that maternal bone turnover is likely implicated in fetal growth, and at least to some extent, in neonatal and childhood body composition and metabolic wellbeing. Maternal sclerostin and sRANKL are positively linked with fetal abdominal circumference and subcutaneous fat deposition, contributing to greater birthweights. Vitamin D deficiency correlates with lower birthweights, while research is still needed on intrauterine fetal metabolism, as well as on vitamin D dosing supplementation during pregnancy, to diminish the risks of low birthweight or SGA neonates in high-risk populations.