Wnt/β-catenin signaling in osteoblasts regulates global energy metabolism

Association between surrogate marker of insulin resistance and bone mineral density in US adults without diabetes

This study examines the relationship between TyG-BMI, an indicator of insulin resistance, and bone mineral density in US adults without diabetes, revealing a positive association. The findings suggest that higher TyG-BMI levels may be linked to a lower risk of osteoporosis, providing a basis for future research in this area.

OBJECTIVE

Patients with osteoporosis are often diagnosed with type 2 diabetes or prediabetes. Insulin resistance is a prediabetic state, and triglyceride glucose-body mass index (TyG-BMI) has been recognized as a potential predictor of it, valuable in assessing prediabetes, atherosclerosis, and other diseases. However, the validity of TyG-BMI in osteoporosis studies remains inadequate.

PURPOSE

The purpose of this study was to evaluate the relationship between TyG-BMI and BMD as well as the effect of TyG-BMI on the odds of developing osteoporosis in US adults without diabetes.

METHODS

National Health and Nutrition Examination Survey data were obtained. The relationship between TyG-BMI and BMD was evaluated via multivariate linear regression models. Smoothed curve fitting and threshold effect analysis explored potential non-linear relationships, and age, gender, and race subgroup analyses were performed. In addition, multivariate logistic regression models were employed to analyze its potential role in the development of osteoporosis.

RESULTS

In a study of 6501 participants, we observed a significant positive correlation between the TyG-BMI index and BMD, even after adjusting for covariates and categorizing TyG-BMI. The study identified specific TyG-BMI folding points-112.476 for the total femur BMD, 100.66 for the femoral neck BMD, 107.291 for the intertrochanter BMD, and 116.58 for the trochanter BMD-indicating shifts in the relationship's strength at these thresholds. While the association's strength slightly decreased after the folding points, it remained significant. Subgroup analyses further confirmed the positive TyG-BMI and BMD correlation. Multivariate linear regression analyses indicated a lower osteoporosis risk in participants with higher TyG-BMI levels, particularly in menopausal women over 40 and men over 60.

CONCLUSION

This study suggests a positive correlation between BMD and TyG-BMI in US adults without diabetes. Individuals with higher levels of TyG-BMI may have a lower risk of osteoporosis.

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.

Dietary intervention reprograms bone marrow cellular signaling in obese mice

Objectives

The current study aimed to investigate the pathogenesis of obesity-induced impaired bone mass accrual and the impact of dietary intervention on bone density in the mouse model of obesity.

Methods

Mice were fed with chow diet (CD) for 10 months, high-fat-diet (HFD) for 10 months, or HFD for 6 months then transferred to chow diet for 4 months (HFDt).

Results

Weight loss and decreased intrahepatic lipid accumulation were observed in mice following dietary intervention. Additionally, HFD feeding induced bone mass accrual, while diet intervention restrained trabecular bone density. These changes were further reflected by increased osteogenesis and decreased adipogenesis in HFDt mice compared to HFD mice. Furthermore, HFD feeding decreased the activity of the Wingless-related integration site (Wnt)-β-Catenin signaling pathway, while the Wnt signaling was augmented by diet intervention in the HFDt group.

Conclusions

Our findings suggest that a HFD inhibits bone formation and that dietary intervention reverses this inhibition. Furthermore, the dietary intervention was able to compensate for the suppressed increase in bone mass to a level comparable to that in the CD group. Our study suggests that targeting the Wnt signaling pathway may be a potential approach to treat obesity-induced impaired bone mass accrual.

Apelin-13, a regulator of autophagy, apoptosis and inflammation in multifaceted bone protection

Apelin/APJ is widely distributed in various tissues in the body and participates in the regulation of physiological and pathological mechanisms such as autophagy, apoptosis, inflammation, and oxidative stress. Apelin-13 is an adipokine family member with multiple biological roles and has been shown to be involved in the development and progression of bone diseases. In the process of osteoporosis and fracture healing, Apelin-13 plays an osteoprotective role by regulating the autophagy and apoptosis of BMSCs, and promotes the osteogenic differentiation of BMSCs. In addition, Apelin-13 also attenuates the progression of arthritis by regulating the inflammatory response of macrophages. In conclusion, Apelin-13 has an important connection with bone protection, which provides a new strategy for the clinical treatment of bone-related diseases.

The Metabolic Features of Osteoblasts: Implications for Multiple Myeloma (MM) Bone Disease

The study of osteoblast (OB) metabolism has recently received increased attention due to the considerable amount of energy used during the bone remodeling process. In addition to glucose, the main nutrient for the osteoblast lineages, recent data highlight the importance of amino acid and fatty acid metabolism in providing the fuel necessary for the proper functioning of OBs. Among the amino acids, it has been reported that OBs are largely dependent on glutamine (Gln) for their differentiation and activity. In this review, we describe the main metabolic pathways governing OBs' fate and functions, both in physiological and pathological malignant conditions. In particular, we focus on multiple myeloma (MM) bone disease, which is characterized by a severe imbalance in OB differentiation due to the presence of malignant plasma cells into the bone microenvironment. Here, we describe the most important metabolic alterations involved in the inhibition of OB formation and activity in MM patients.

Peiminine regulates bone-fat balance by canonical Wnt/β-catenin pathway in an ovariectomized rat model

Peiminine is a major biologically active component of Fritillaria thunbergii Miq that exhibits good anticancer, antiinflammatory, and anti-osteoclast effects. However, its effects on osteoporosis (OP) remain unknown. This study aimed to explore whether Peiminine was able to regulate osteogenesis and adipogenesis in ovariectomized (OVX) rat. The effects on the differentiation of bone marrow stem cells (BMSCs), function of Wnt/β-catenin pathway, ALP activity, calcium nodule deposition, as well as adipocyte formation in vitro by Peiminine at different concentrations, were detected. The curative effects of Peiminine on the ovariectomy-induced osteoporosis model by micro-CT and bone histomorphology assays were analyzed. The promotion of osteogenic differentiation and inhibition of adipogenic differentiation by Peiminine (5-40 μg/mL) was detected and the optimum concentration was 20 μg/mL. Mechanistically, Peiminine regulated the fate of BMSCs in vitro, and activated Wnt/β-catenin signaling pathway by restraining phosphorylation of β-catenin and promoting the nuclear translocation of β-catenin. Moreover, Peiminine prevented ovariectomy-induced osteoporosis by alleviating trabecular bone loss and inhibiting adipose formation. Our data suggested that Peiminine could attenuate ovariectomy-induced osteoporosis by alleviating trabecular bone loss and inhibiting adipose formation. These encouraging discoveries could lay the foundation for Peiminine to be a promising preventive treatment strategy for skeletal diseases, such as osteoporosis.

Sclerostin Influences Exercise-Induced Adaptations in Body Composition and White Adipose Tissue Morphology in Male Mice

Sclerostin is an inhibitor of the osteogenic Wnt/β-catenin signaling pathway that also has an endocrine role in regulating adipocyte differentiation and metabolism. Additionally, subcutaneous white adipose tissue (scWAT) sclerostin content decreases following exercise training (EXT). Therefore, we hypothesized that EXT-induced reductions in adipose tissue sclerostin may play a role in regulating adaptations in body composition and whole-body metabolism. To test this hypothesis, 10-week-old male C57BL/6J mice were either sedentary (SED) or performing 1 hour of treadmill running at ~65% to 70% maximum oxygen consumption (VO_2max ) 5 day/week (EXT) for 4 weeks and had subcutaneous injections of either saline (C) or recombinant sclerostin (S) (0.1 mg/kg body mass) 5 day/week; thus, making four groups (SED-C, EXT-C, SED-S, and EXT-S; n = 12/group). No differences in body mass were observed between experimental groups, whereas food intake was higher in EXT (p = 0.03) and S (p = 0.08) groups. There was a higher resting energy expenditure in all groups compared to SED-C. EXT-C had increased lean mass and decreased fat mass percentage compared to SED-C and SED-S. No differences in body composition were observed in either the SED-S or EXT-S groups. Lower scWAT (inguinal), epididymal white adipose tissue (eWAT) (visceral epididymal) mass, and scWAT adipocyte cell size and increased percentage of multilocular cells in scWAT were observed in the EXT-C group compared to SED-C, whereas lower eWAT was only observed in the EXT-S group. EXT mice had increased scWAT low-density lipoprotein receptor-related protein 4 (Lrp4) and mitochondrial content and sclerostin treatment only inhibited increased Lrp4 content with EXT. Together, these results provide evidence that reductions in resting sclerostin with exercise training may influence associated alterations in energy metabolism and body composition, particularly in scWAT. © 2023 The Authors. Journal of Bone and Mineral Research published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research (ASBMR).

Signaling pathways in obesity: mechanisms and therapeutic interventions

Obesity is a complex, chronic disease and global public health challenge. Characterized by excessive fat accumulation in the body, obesity sharply increases the risk of several diseases, such as type 2 diabetes, cardiovascular disease, and nonalcoholic fatty liver disease, and is linked to lower life expectancy. Although lifestyle intervention (diet and exercise) has remarkable effects on weight management, achieving long-term success at weight loss is extremely challenging, and the prevalence of obesity continues to rise worldwide. Over the past decades, the pathophysiology of obesity has been extensively investigated, and an increasing number of signal transduction pathways have been implicated in obesity, making it possible to fight obesity in a more effective and precise way. In this review, we summarize recent advances in the pathogenesis of obesity from both experimental and clinical studies, focusing on signaling pathways and their roles in the regulation of food intake, glucose homeostasis, adipogenesis, thermogenesis, and chronic inflammation. We also discuss the current anti-obesity drugs, as well as weight loss compounds in clinical trials, that target these signals. The evolving knowledge of signaling transduction may shed light on the future direction of obesity research, as we move into a new era of precision medicine.

Cathelicidin LL-37 improves bone metabolic balance in rats with ovariectomy-induced osteoporosis via the Wnt/beta-catenin pathway

Osteoporosis is a bone disease characterized by low bone mineral density (BMD) and impaired bone microarchitecture due to the abnormal activity of osteoclasts. Cathelicidins are antimicrobial peptides present in the lysosomes of macrophages and polymorphonuclear leukocytes. LL-37, a cathelicidin, induces various biological effects, including modulation of the immune system, angiogenesis, wound healing, cancer growth, as well as inflammation, and bone loss. A previous study reported direct involvement of LL-37 suppressing osteoclastogenesis in humans. Here, we examined the role of LL-37 in the treatment of osteoporosis using an ovariectomy (OVX) rat model. Our results showed that LL-37 significantly reduced bone loss and pathological injury in OVX rats with osteoporosis. Furthermore, we found that LL-37 significantly increased the activity of the Wnt/beta-catenin pathway in OVX rats with osteoporosis, including the increased expression of beta-catenin, Osterix (Osx), and Runt-related transcription factor 2 (Runx2), whereas XAV-939, an inhibitor of the Wnt/beta-catenin pathway, significantly blocked the effects of LL-37 on bone loss and abnormal bone metabolism. Altogether, our findings suggested that LL-37 exerted a protective role in regulating bone loss and abnormal bone metabolism in rats with osteoporosis by activating the Wnt/beta-catenin pathway.

The role of the Wnt signalling pathway in the energy metabolism of bone remodelling

OBJECTIVES

Bone remodelling is necessary to repair old and impaired bone caused by aging and its effects. Injury in the process of bone remodelling generally leads to the development of various bone diseases. Energy metabolism plays crucial roles in bone cell formation and function, the disorder of which will disrupt the balance between bone formation and bone resorption.

MATERIALS AND METHODS

Here, we review the intrinsic interactions between bone remodelling and energy metabolism and the role of the Wnt signalling pathway.

RESULTS

We found a close interplay between metabolic pathways and bone homeostasis, demonstrating that bone plays an important role in the regulation of energy balance. We also discovered that Wnt signalling is associated with multiple biological processes regulating energy metabolism in bone cells.

CONCLUSIONS

Thus, targeted regulation of Wnt signalling and the recovery of the energy metabolism function of bone cells are key means for the treatment of metabolic bone diseases.

Effects of Low-Concentration Graphene Oxide Quantum Dots on Improving the Proliferation and Differentiation Ability of Bone Marrow Mesenchymal Stem Cells through the Wnt/β-Catenin Signaling Pathway

Graphene oxide quantum dots (GOQDs) are considered to be a new method for regulating the proliferation and differentiation of bone marrow mesenchymal stem cells (BMSCs). However, there are few reports on such regulation with different concentrations of GOQDs, and the molecular mechanism has not been fully elucidated. The purposes of this study were, first, to explore the effects of GOQDs on the proliferation and differentiation of BMSCs in vitro and in vivo, and, second, to provide a theoretical basis for the repair of bone defects. Live/Dead staining, EdU staining, immunofluorescence staining, alkaline phosphatase (ALP), western blotting, and qT-PCR were used for detecting the proliferation and differentiation of BMSCs after coculture with GOQDs of different concentrations. Hematoxylin and eosin (HE) staining and Van Gieson (VG) staining were used to detect new bone regeneration in vivo. The results showed that low-concentration GOQDs (0.1 and 1 μg/mL) promoted the proliferation and differentiation of BMSCs. Compared with the 1 μg/mL GOQD group, the 0.1 μg/mL GOQD group had better ability to promote the proliferation and differentiation of BMSCs. HE and VG staining results showed the greatest proportion of new bone area on sandblasted, large-grit, and acid-etched (SLA)/GOQD scaffolds. Furthermore, the ratio of active β-catenin and the phosphorylation level of GSK-3β (p-GSK-3β) increased after BMSCs treatment with 0.1 μg/mL GOQDs. Low concentrations of GOQDs improved the osteogenic differentiation ability of BMSCs by activating the Wnt/β-catenin signaling pathway.

Osteoblast MR deficiency protects against adverse ventricular remodeling after myocardial infarction

RATIONALE

Mineralocorticoid receptor (MR) antagonists have been clinically used to treat heart failure. However, the underlying cellular and molecular mechanisms remain incompletely understood.

METHODS AND RESULTS

Using osteoblast MR knockout (MR^obko) mouse in combination with myocardial infarction (MI) model, we demonstrated that MR deficiency in osteoblasts significantly improved cardiac function, promoted myocardial healing, as well as attenuated cardiac hypertrophy, fibrosis and inflammatory response after MI. Gene expression profiling using RNA sequencing revealed suppressed expression of osteocalcin (OCN) in calvaria from MR^obko mice compared to littermate control (MR^fl/fl) mice with or without MI. Plasma levels of undercarboxylated OCN (ucOCN) were also markedly decreased in MR^obko mice compared to MR^fl/fl mice. Administration of ucOCN abolished the protective effects of osteoblast MR deficiency on infarcted hearts. Mechanistically, ucOCN treatment promoted proliferation and inflammatory cytokine secretion in macrophages. Spironolactone, an MR antagonist, significantly inhibited the expression and secretion of OCN in post-MI mice. More importantly, spironolactone decreased plasma levels of ucOCN and inflammatory cytokines in heart failure patients.

CONCLUSIONS

MR deficiency in osteoblasts alleviates pathological ventricular remodeling after MI, likely through its regulation on OCN. Spironolactone may work through osteoblast MR/OCN axis to exert its therapeutic effects on pathological ventricular remodeling and heart failure in mice and human patients.

Postnatal deletion of β-catenin in preosteoblasts regulates global energy metabolism through increasing bone resorption and adipose tissue fibrosis

Many studies revealed bone can regulate global energy metabolism and our previous study also showed that Wnt/β-catenin pathway is involved in this process. To better understand the participation of canonical Wnt pathway in energy metabolism, we examined the β-catenin knock-out (β-cat KO) mice by crossing the osterix-cre transgenic mice with β-catenin^flox/flox mice. First, we identified that postnatal deletion of β-catenin in preosteoblasts led to decreased fat mass and increased energy expenditure in mice. Osteoprotegerin administration largely rescued the decreased fat mass and partly normalized the energy expenditure accompanied by the inhibition of bone resorption. Anti-resorption with alendronate or RANKL-antibody could also partly rescued the decreased bone mass, decreased fat mass and increased energy expenditure in β-cat KO mice. We further found that the adipose cells in the inguinal fat tissue were smaller and the extracellular matrix components around adipocytes accumulated more in β-cat KO mice than their controls by histomorphology. Gene analysis by RT-PCR showed that the expression of collagen VI is 4.8 folds in adipose tissue of the β-cat KO mice compared with the control mice. We further detected the expression of cytokines which were related to fibrosis and the data showed that the level of TGF-beta1 was elevated in both of bone marrow serum and adipose tissue derived from the β-cat KO mice. After administration of TGF-beta1 neutralizing antibody, the impaired energy metabolism was partly rescued in β-cat KO mice. Besides, anti-resorption treatment and TGF-beta1 antibody could partly suppress the increased expression of genes related to fat tissue fibrosis. These results indicate that the abnormal global energy metabolism in β-cat KO mice may be attributed to increasing bone resorption and adipose tissue fibrosis.

Osteoblastic glucocorticoid signaling exacerbates high-fat-diet- induced bone loss and obesity

Chronic high-fat diet (HFD) consumption not only promotes obesity and insulin resistance, but also causes bone loss through mechanisms that are not well understood. Here, we fed wild-type CD-1 mice either chow or a HFD (43% of energy from fat) for 18 weeks; HFD-fed mice exhibited decreased trabecular volume (-28%) and cortical thickness (-14%) compared to chow-fed mice. In HFD-fed mice, bone loss was due to reduced bone formation and mineral apposition, without obvious effects on bone resorption. HFD feeding also increased skeletal expression of sclerostin and caused deterioration of the osteocyte lacunocanalicular network (LCN). In mice fed HFD, skeletal glucocorticoid signaling was activated relative to chow-fed mice, independent of serum corticosterone concentrations. We therefore examined whether skeletal glucocorticoid signaling was necessary for HFD-induced bone loss, using transgenic mice lacking glucocorticoid signaling in osteoblasts and osteocytes (HSD2^OB/OCY-tg mice). In HSD2^OB/OCY-tg mice, bone formation and mineral apposition rates were not suppressed by HFD, and bone loss was significantly attenuated. Interestingly, in HSD2^OB/OCY-tg mice fed HFD, both Wnt signaling (less sclerostin induction, increased β-catenin expression) and glucose uptake were significantly increased, relative to diet- and genotype-matched controls. The osteocyte LCN remained intact in HFD-fed HSD2^OB/OCY-tg mice. When fed a HFD, HSD2^OB/OCY-tg mice also increased their energy expenditure and were protected against obesity, insulin resistance, and dyslipidemia. Therefore, glucocorticoid signaling in osteoblasts and osteocytes contributes to the suppression of bone formation in HFD-fed mice. Skeletal glucocorticoid signaling is also an important determinant of glucose uptake in bone, which influences the whole-body metabolic response to HFD.

MicroRNA-29a in Osteoblasts Represses High-Fat Diet-Mediated Osteoporosis and Body Adiposis through Targeting Leptin

Skeletal tissue involves systemic adipose tissue metabolism and energy expenditure. MicroRNA signaling controls high-fat diet (HFD)-induced bone and fat homeostasis dysregulation remains uncertain. This study revealed that transgenic overexpression of miR-29a under control of osteocalcin promoter in osteoblasts (miR-29aTg) attenuated HFD-mediated body overweight, hyperglycemia, and hypercholesterolemia. HFD-fed miR-29aTg mice showed less bone mass loss, fatty marrow, and visceral fat mass together with increased subscapular brown fat mass than HFD-fed wild-type mice. HFD-induced O₂ underconsumption, respiratory quotient repression, and heat underproduction were attenuated in miR-29aTg mice. In vitro, miR-29a overexpression repressed transcriptomic landscapes of the adipocytokine signaling pathway, fatty acid metabolism, and lipid transport, etc., of bone marrow mesenchymal progenitor cells. Forced miR-29a expression promoted osteogenic differentiation but inhibited adipocyte formation. miR-29a signaling promoted brown/beige adipocyte markers Ucp-1, Pgc-1α, P2rx5, and Pat2 expression and inhibited white adipocyte markers Tcf21 and Hoxc9 expression. The microRNA also reduced peroxisome formation and leptin expression during adipocyte formation and downregulated HFD-induced leptin expression in bone tissue. Taken together, miR-29a controlled leptin signaling and brown/beige adipocyte formation of osteogenic progenitor cells to preserve bone anabolism, which reversed HFD-induced energy underutilization and visceral fat overproduction. This study sheds light on a new molecular mechanism by which bone integrity counteracts HFD-induced whole-body fat overproduction.

Effect of magnetic graphene oxide on cellular behaviors and osteogenesis under a moderate static magnetic field

The biological behaviors of magnetic graphene oxide (MGO) in a static magnetic field (SMF) are unknown. The current study is to investigate the cellular behaviors, osteogenesis and the mechanism in BMSCs treated with MGO combined with an SMF. Results showed that the synthetic MGO particles were bio-compatible and could significantly improve the osteogenesis of BMSCs under SMFs, as verified by elevated alkaline phosphatase activity, mineralized nodule formation, and expressions of mRNA and protein levels. Under SMF at the same intensity, the addition of graphene oxide to Fe₃O₄ could increase the osteogenic ability of BMSCs. The Wnt/β-catenin pathway was indicated to be related to the MGO-driven osteogenic behavior of the BMSCs under SMF. Taken together, our findings suggested that MGO under an SMF could promote osteogenesis in BMSCs through the Wnt/β-catenin pathway and hence should attract more attention for practical applications in bone tissue regeneration.

Abnormal Lipid Profile in Fast-Growing Broilers With Spontaneous Femoral Head Necrosis

This study investigated lipid metabolism in broilers with spontaneous femoral head necrosis (FHN) by determining the levels of markers of the blood biochemistry and bone metabolism. The birds were divided into a normal group and FHN group according to the femoral head scores of 3-, 4-, and 5-week-old chickens with FHN, and a comparative study was conducted. The study showed that spontaneous FHN broilers had a lipid metabolism disorder, hyperlipidemia, and an accumulation of lipid droplets in the femur. In addition, there were significant changes in the bone parameters and blood bone biochemistry markers, and the expression of genes related to lipid metabolism in the femoral head was also significantly increased. Therefore, FHN may result from dyslipidemia, which affects the bone growth and development of broilers.

Endocrine role of bone in the regulation of energy metabolism

Bone mainly functions as a supportive framework for the whole body and is the major regulator of calcium homeostasis and hematopoietic function. Recently, an increasing number of studies have characterized the significance of bone as an endocrine organ, suggesting that bone-derived factors regulate local bone metabolism and metabolic functions. In addition, these factors can regulate global energy homeostasis by altering insulin sensitivity, feeding behavior, and adipocyte commitment. These findings may provide a new pathological mechanism for related metabolic diseases or be used in the diagnosis, treatment, and prevention of metabolic diseases such as osteoporosis, obesity, and diabetes mellitus. In this review, we summarize the regulatory effect of bone and bone-derived factors on energy metabolism and discuss directions for future research.

Glucose Metabolism in Osteoblasts in Healthy and Pathophysiological Conditions

Bone tissue in vertebrates is essential to performing movements, to protecting internal organs and to regulating calcium homeostasis. Moreover, bone has also been suggested to contribute to whole-body physiology as an endocrine organ, affecting male fertility; brain development and cognition; and glucose metabolism. A main determinant of bone quality is the constant remodeling carried out by osteoblasts and osteoclasts, a process consuming vast amounts of energy. In turn, clinical conditions associated with impaired glucose metabolism, including type I and type II diabetes and anorexia nervosa, are associated with impaired bone turnover. As osteoblasts are required for collagen synthesis and matrix mineralization, they represent one of the most important targets for pharmacological augmentation of bone mass. To fulfill their function, osteoblasts primarily utilize glucose through aerobic glycolysis, a process which is regulated by various molecular switches and generates adenosine triphosphate rapidly. In this regard, researchers have been investigating the complex processes of energy utilization in osteoblasts in recent years, not only to improve bone turnover in metabolic disease, but also to identify novel treatment options for primary bone diseases. This review focuses on the metabolism of glucose in osteoblasts in physiological and pathophysiological conditions.

The mTORC1 complex in pre-osteoblasts regulates whole-body energy metabolism independently of osteocalcin

Overnutrition causes hyperactivation of mTORC1-dependent negative feedback loops leading to the downregulation of insulin signaling and development of insulin resistance. In osteoblasts (OBs), insulin signaling plays a crucial role in the control of systemic glucose homeostasis. We utilized mice with conditional deletion of Rptor to investigate how the loss of mTORC1 function in OB affects glucose metabolism under normal and overnutrition dietary states. Compared to the controls, chow-fed Rptor_ob^−/− mice had substantially less fat mass and exhibited adipocyte hyperplasia. Remarkably, upon feeding with high-fat diet, mice with pre- and post-natal deletion of Rptor in OBs were protected from diet-induced obesity and exhibited improved glucose metabolism with lower fasting glucose and insulin levels, increased glucose tolerance and insulin sensitivity. This leanness and resistance to weight gain was not attributable to changes in food intake, physical activity or lipid absorption but instead was due to increased energy expenditure and greater whole-body substrate flexibility. RNA-seq revealed an increase in glycolysis and skeletal insulin signaling pathways, which correlated with the potentiation of insulin signaling and increased insulin-dependent glucose uptake in Rptor-knockout osteoblasts. Collectively, these findings point to a critical role for the mTORC1 complex in the skeletal regulation of whole-body glucose metabolism and the skeletal development of insulin resistance.

Genistein prevents bone loss in type 2 diabetic rats induced by streptozotocin

Background

Diabetic osteoporosis has become a severe public health problem in the aging societies. Genistein has been reported to play an important role in preventing and treating metabolic diseases via its anti-inflammatory, antioxidant, anti-estrogenic, and estrogen-like functions.

Objective

We aimed to investigate whether genistein exerts bone-protective effect on diabetic rats induced by 35 mg/kg streptozotocin (STZ) plus a 4-week high-fat diet.

Design

Sprague–Dawley rats were randomly divided into four groups: (1) control group, (2) type 2 diabetes mellitus (T2DM) model group, (3) T2DM with 10 mg/kg genistein, and (4) T2DM with 30 mg/kg genistein. After an 8-week treatment with genistein, the femurs, tibias, and blood were collected from all rats for further analysis.

Results

Genistein at 10 mg/kg showed little effect on diabetic osteoporosis, whereas genistein at 30 mg/kg significantly improved glucose and bone metabolisms compared with diabetic rats. Our results showed that 30 mg/kg genistein significantly increased bone mineral density, serum osteocalcin, and bone alkaline phosphatase. Genistein also effectively lowered fasting blood glucose, tartrate-resistant acid phosphatase 5b, tumor necrosis factor-α, interleukin-6, and numbers of adipocytes and osteoclasts. Compared with the T2DM group, protein levels of receptor activator of nuclear factor κB ligand (RANKL) and peroxisome proliferator-activated receptor-γ (PPAR-γ) were decreased, while protein levels of osteoprotegerin (OPG), β-catenin, and runt-related transcription factor 2 (Runx-2) were increased after genistein intervention.

Conclusion

Genistein could effectively improve abnormal bone metabolism in STZ-induced diabetic rats; the underlying molecular mechanisms might be related to OPG/RANKL, PPAR-γ, and β-catenin/Runx-2 pathways.

Notoginsenoside R1 promotes differentiation of human alveolar osteoblasts in inflammatory microenvironment through inhibiting NF‑κB pathway and activating Wnt/β‑catenin pathway

Alveolar bone is vital for dental implantation and periodontal treatment. Notoginsenoside R1 (NTR1) may promote the differentiation of human alveolar osteoblasts (HAOBs), but the underlying molecular mechanisms remain unclear. The present study investigated the pro-differentiation function of NTR1 on HAOBs in order to find new methods of dental treatment. HAOBs were surgically obtained from dental patients and the cells were isolated, cultured and identified under an inverted phase contrast microscope. The cells were treated with different concentrations of NTR1 alone or further stimulated by TNF-α. An alkaline phosphate (ALP) activity assay and alizarin red staining were performed to detect ALP activity and mineralization of the cells, respectively. Cell viability was assayed using an MTT assay. The expressions of osteogenic-related factors and the factors associated with the NF-κB and Wnt/β-catenin pathways were examined by reverse transcription-quantitative PCR or western blot analysis. Successfully passaged HAOBs presented blue granules and red calcium deposits after staining. The viability of HAOBs was unchanged following treatment with NTR1 at ≤20 µmol/l and/or TNF-α, but slightly reduced by 40 µmol/l NTR1. TNF-α-induced decreases of calcium nodules and ALP activity were decreased by NTR1 in HAOBs. TNF-α also regulated the expressions of runt-related transcription factor 2, osteopontin (OPN), osteocalcin (OCN), p50, phosphorylated p65, AXIN2, Dickkopf-related protein 1 and β-catenin, while the regulatory effect was reversed by NTR1. NTR1 promoted the differentiation of HAOBs in the TNF-α-induced inflammatory microenvironment through inhibiting the NF-κB pathway and activating the Wnt/β-catenin pathway.

Protective role of osteocalcin in diabetes pathogenesis

In diabetes, metabolic, inflammatory, and stress-associated alterations conduce to ß-cell failure and tissue damage. Osteocalcin is a bone protein with several endocrine functions in different tissues. In this review, we gathered scientific evidence of how osteocalcin could modulate functional disorders that are altered in diabetes in an integrative way. We include adipose tissue, pancreatic function, and oxidative stress aspects. In the first section, we focus on the role of inflammatory mediators and adiponectin in energy homeostasis and insulin sensitivity. In the following section, we discuss the effect of osteocalcin in metabolic and pancreatic function and its association in insulin signaling and in ß-cell proliferation. Finally, we focus on osteocalcin action in oxidative and endoplasmic reticulum stress, and in antioxidant regulation, since ß-cells are well known by its vulnerability to stress damage. These evidences support the notion that osteocalcin could have an important role in diabetes treatment.

Axin 1 knockdown inhibits osteoblastic apoptosis induced by Porphyromonas gingivalis lipopolysaccharide

BACKGROUND

Porphyromonas gingivalis (Pg) is one of the pathogenic bacteria that cause periodontal diseases, lipopolysaccharide (LPS) is the key factor that triggers alveolar bone absorption. This study explored the action of Axin 1 on Pg-LPS-induced osteoblasts injury, so as to search a possible treatment for periodontal diseases.

METHODS

Rat osteoblasts were dealt with Pg-LPS and Axin 1 knockdown alone or in combination. The effect of Pg-LPS and Axin 1 on osteoblast viability and apoptosis were detected by Cell Counting Kit-8 and flow cytometry. The expressions of alkaline phosphatase (ALP) and Axin 1 in processed osteoblasts were measured by western blot (WB) and quantitative real-time polymerase chain reaction (qRT-PCR) assays. Furthermore, the role of Axin 1 knockdown in the levels of inflammatory cytokines and apoptosis-related proteins were also determined.

RESULTS

Pg-LPS inhibited the viability of osteoblasts and promote apoptosis with concentration and time dependence. ALP expression in Pg-LPS-treated osteoblasts was reduced, while Axin 1 expression was increased. On the one hand, Axin 1 knockdown reversed the Pg-LPS-induced reduction of cell activity and pro-apoptosis effect. On the other hand, Axin 1 knockdown not only improved the ALP activity of Pg-LPS-treated cells, but also reduced the elevation of inflammatory cytokines (TNF-α, IL-1β and IL-6) caused by Pg-LPS. Moreover, Pg-LPS increased the expressions of cleaved Caspase-3 and Bax, and inhibited Bcl-2 expressed, which was rescued by Axin 1 knockdown.

CONCLUSION

Axin 1 knockdown inhibited Pg-LPS-induced osteoblastic apoptosis by regulating the levels of inflammatory cytokines, which may be helpful for the treatment of periodontal diseases.

Corylin, a flavonoid derived from Psoralea Fructus, induces osteoblastic differentiation via estrogen and Wnt/β-catenin signaling pathways

Corylin is a naturally occurring flavonoid isolated from the fruit of Psoralea corylifolia L. (Fabaceae), which is a Chinese medicinal herb in treating osteoporosis. Although a variety of pharmacological activities of corylin have been reported, its osteogenic action and the underlying mechanism in bone development remain unclear. In the present study, the involvement of bone-specific genes in corylininduced differentiated osteoblasts was analyzed by RT-PCR, promoter-reporter assay, and Western blotting. In cultured osteoblasts, corylin-induced cell differentiation and mineralization, as well as increased the expressions of vital biological markers for osteogenesis, such as Runx2, Osterix, Col1, and ALP. Corylin was proposed to have dual pathways in triggering the osteoblastic differentiation. First, the osteogenic function of corylin acted through the activation of Wnt/β-catenin signaling. The nuclear translocation of β-catenin of cultured osteoblasts, as determined by flow cytometry and confocal microscopy, was triggered by applied corylin, and which was blocked by DKK-1, an inhibitor of Wnt/β-catenin signaling. Second, the application of corylin-induced estrogenic response in a dose-dependent manner, and which was blocked by ICI 182 780, an antagonist of estrogen receptor. Furthermore, the activation of Runx2 promoter by corylin was abolished by both DKK-1 and ICI 182,780, indicating that the corylin exhibited its osteogenic effect via estrogen and Wnt/β-catenin signaling pathways. In addition, corylin regulated the metabolic profiles, as well as the membrane potential of mitochondria, in cultured osteoblasts. Corylin also stimulated the osteogenesis in bone micromass derived from mesenchymal progenitor cells. This study demonstrated the osteogenic activities of corylin in osteoblasts and micromass, suggesting that corylin has the potential to be developed as a novel pro-osteogenic agent in targeting for the treatment of osteoblast-mediated osteoporosis.

LGR6 promotes osteogenesis by activating the Wnt/β-catenin signaling pathway

Leucine-rich repeat containing G-protein-coupled receptor 6 (LGR6) is a member of the rhodopsin-like 7-transmembrane domain receptor superfamily and has high homology to LGR4 and LGR5. LGR6 is highly expressed in osteoblastic progenitors, and LGR6-deficient mice show nail and bone regeneration defect. However, the effect of LGR6 on the osteogenic differentiation of osteoblastic progenitors and its underlying mechanisms are largely unknown. In this study, we overexpressed and knockdown LGR6 with lentivirus in the preosteoblastic cell MC3T3-E1 to observe the effect of LGR6 on osteogenic differentiation and explore its possible molecular mechanism. LGR6 overexpression promoted osteogenic differentiation and mineralization by stabilizing β-catenin to potentiate the Wnt/β-catenin signaling pathway in MC3T3-E1 cells. Conversely, LGR6 knockdown inhibited osteogenic differentiation and mineralization by enhancing β-catenin degradation to inactivate the Wnt/β-catenin signaling pathway. These results reveal that LGR6 is highly expressed in osteoblastic progenitors, and promotes osteogenesis by enhancing β-catenin stability to strengthen the Wnt signaling pathway. This study provides an important reference into the exact mechanisms of osteogenic differentiation.

Association between glycosylated hemoglobin A1c and bone biochemical markers in type 2 diabetic postmenopausal women: a cross-sectional study

BACKGROUND

type 2 diabetes mellitus (T2DM) is a complicated disease that can affect bone health, but the change in bone biochemical markers caused by T2DM was controversial, so the aim of this study was to investigate whether there was a discrepancy in the levels of bone biochemical markers between postmenopausal women with T2DM and non-diabetic women and to explore the relationship between the level of glycosylated hemoglobin A1c (HbA1c) and bone biochemical markers in these subjects.

METHODS

A total of 237 type 2 diabetic postmenopausal women visiting the First Affiliated Hospital of Anhui Medical University from January 2017 to October 2018 and 93 healthy postmenopausal women were retrospectively enrolled. The differences in the levels of bone biochemical markers between patients and controls were analyzed by one-way ANOVA or chi-square test. The relationship between HbA1c and bone biochemical markers was analyzed by multivariate regression, forest plot and fitted curve.

RESULTS

Bone formation markers including N-MID osteocalcin and procollagen type 1 amino-terminal pro-peptide (PINP) were decreased in postmenopausal women with T2DM compared to controls (17.42 ± 9.50 vs 23.67 ± 7.58, p < 0.001; 48.47 ± 27.27 vs 65.86 ± 21.06, p < 0.001, respectively), but the bone resorption markers β-crossLaps (β-CTX) was no difference between the two groups (0.57 ± 0.28 vs 0.55 ± 0.21, p = 0.868). Multivariate regression showed that HbA1c was inversely associated with N-MID osteocalcin and PINP after adjusting for age, BMI, menopause^'s years, diabetic duration, TC, TG, HDL-c, LDL-c, creatinine, UA and eGFR. The adjusted coefficients for N-MID osteocalcin and PINP per 1% HbA1c decrease were - 0.71 (- 1.19, - 0.22) and - 1.79 (- 3.30, - 0.28), respectively. A segmentation effect was seen in the fitted curve between HbA1c and β-CTX with an inflection point at 7.4% of HbA1c, the highest quartile of β-CTX (> = 0.74 ng/ml) showed a significantly negative with HbA1c. No significant association was seen between HbA1c and other biochemical markers.

CONCLUSIONS

Our study found that bone formation was inhibited in postmenopausal women with T2DM, but bone resorption was not affected, and poor glycemic control was related to lower levels of bone formation, may increase the risk of bone fracture in postmenopausal women with T2DM.

Bone function, dysfunction and its role in diseases including critical illness

The skeleton is one of the largest organs in the human body. In addition to its conventional functions such as support, movement and protection, the skeleton also contributes to whole body homeostasis and maintenance of multiple important non-bone organs/systems (extraskeletal functions). Both conventional and extraskeletal functions of the skeleton are defined as bone function. Bone-derived factors (BDFs) are key players regulating bone function. In some pathophysiological situations, including diseases affecting bone and/or other organs/systems, the disorders of bone itself and the subsequently impaired functions of extraskeletal organs/systems caused by abnormal bone (impaired extraskeletal functions of bone) are defined as bone dysfunction. In critical illness, which is a health status characterized by the dysfunction or severe damage of one or multiple important organs or systems, the skeleton shows rapid bone loss resulting from bone hyper-resorption and impaired osteoblast function. In addition, the dysfunctions of the skeleton itself are also closely related to the severity and prognosis of critical illness. Therefore, we propose that there is bone dysfunction in critical illness. Some methods to inhibit osteoclast activity or promote osteoblast function by the treatment of bisphosphonates or PTH1-34 benefit the outcome of critical illness, which indicates that enhancing bone function may be a potential novel strategy to improve prognosis of diseases including critical illness.

Apelin-13 deficiency alters cortical bone geometry, organic bone matrix, and inhibits Wnt/β-catenin signaling

Adipokines play key roles in the regulation of obesity, type 2 diabetes mellitus, and bone growth. As a newly discovered hormone in the adipokines family, the precise role of Apelin-13 on bone metabolism is not yet clear. Apelin-13 and 25(OH)D3 expression were detected in freshly isolated serum of healthy individuals and osteoporosis patients with ELISA method. Apelin-13 deficient mice were set up and cortical bone geometry was measured with micro-computed tomography (Micro-CT) at 5 months old, then profile of organic bone matrix genes was detected with quantitative Real-Time PCR (qRT-PCR). Wnt/β-catenin signaling molecules were assayed in primary osteocytes isolated from neonatal calvarias. Apelin-13 and 25(OH)D3 showed decreased expression in osteoporosis patients. Five-month-old Apelin deficient mice exhibited decreased total and bone marrow cavity area and periosteal and endocortical bone surface. Deficiency of Apelin-13 downregulated collagen maturation associated genes (loxl3 and loxl4) and Wnt/β-catenin signaling, while loxl2 was upregulated, all of which indicated that Apelin-13 could play a role in regulating skeletal homeostasis. The decrease in bone formation in Apelin-13 deficient mice is associated with downregulation of organic bone matrix genes and Wnt/β-catenin signaling molecules, all of these indicate that association of Apelin-13 with bone mineral density (BMD) could be mediated by Wnt/β-catenin pathway.

Bone: Another potential target to treat, prevent and predict diabetes

Type 2 diabetes mellitus is now a worldwide health problem with increasing prevalence. Mounting efforts have been made to treat, prevent and predict this chronic disease. In recent years, increasing evidence from mice and clinical studies suggests that bone-derived molecules modulate glucose metabolism. This review aims to summarize our current understanding of the interplay between bone and glucose metabolism and to highlight potential new means of therapeutic intervention. The first molecule recognized as a link between bone and glucose metabolism is osteocalcin (OCN), which functions in its active form, that is, undercarboxylated OCN (ucOC). ucOC acts in promoting insulin expression and secretion, facilitating insulin sensitivity, and favouring glucose and fatty acid uptake and utilization. A second bone-derived molecule, lipocalin2, functions in suppressing appetite in mice through its action on the hypothalamus. Osteocytes, the most abundant cells in bone matrix, are suggested to act on the browning of white adipose tissue and energy expenditure through secretion of bone morphogenetic protein 7 and sclerostin. The involvement of bone resorption in glucose homeostasis has also been examined. However, there is evidence indicating the implication of the receptor activator of nuclear factor κ-B ligand, neuropeptide Y, and other known and unidentified bone-derived factors that function in glucose homeostasis. We summarize recent advances and the rationale for treating, preventing and predicting diabetes by skeleton intervention.

Strontium inhibits osteoclastogenesis by enhancing LRP6 and β-catenin-mediated OPG targeted by miR-181d-5p

Strontium is a drug with the bone formation and anti-resorption effects on bone. The underlying mechanisms for the dual effect of strontium on bone metabolism, especially for the anti-resorption effects remain unknown. Thus, we aim to investigate the mechanisms of effects of strontium on osteoclastogenesis. Firstly, we found that strontium decreased the levels of important biomarkers of receptor activator of nuclear factor kappa-B ligand (RANKL) which induced osteoclast differentiation, indicating that strontium might directly inhibit osteoclast differentiation. Next, we revealed that strontium enhanced Low Density Lipoprotein Receptor-Related Protein 6 (LRP6)/β-catenin/osteoprotegerin (OPG) signaling pathway in MC3T3-E1 cells. The signaling pathway may negatively regulate osteoclastogenesis. Thus, strontium indirectly inhibited RANKL induced osteoclast differentiation. Finally, we revealed that OPG was targeted by miR-181d-5p as determined by luciferase reporter assay and downregulated by miR-181d-5p at both mRNA and protein levels as determined by western blot.

Regulation of Energy Metabolism by Bone-Derived Hormones

Like many other organs, bone can act as an endocrine organ through the secretion of bone-specific hormones or "osteokines." At least two osteokines are implicated in the control of glucose and energy metabolism: osteocalcin (OCN) and lipocalin-2 (LCN2). OCN stimulates the production and secretion of insulin by the pancreatic β-cells, but also favors adaptation to exercise by stimulating glucose and fatty acid (FA) utilization by the muscle. Both of these OCN functions are mediated by the G-protein-coupled receptor GPRC6A. In contrast, LCN2 influences energy metabolism by activating appetite-suppressing signaling in the brain. This action of LCN2 occurs through its binding to the melanocortin 4 receptor (MC4R) in the paraventricular nucleus of the hypothalamus (PVN) and ventromedial neurons of the hypothalamus.

Role of osteoprotegerin/receptor activator of nuclear factor kappa B/receptor activator of nuclear factor kappa B ligand axis in nonalcoholic fatty liver disease

Concomitantly with the increase in the prevalences of overweight/obesity, nonalcoholic fatty liver disease (NAFLD) has worldwide become the main cause of chronic liver disease in both adults and children. Patients with fatty liver display features of metabolic syndrome (MetS), like insulin resistance (IR), glucose intolerance, hypertension and dyslipidemia. Recently, epidemiological studies have linked obesity, MetS, and NAFLD to decreased bone mineral density and osteoporosis, highlighting an intricate interplay among bone, adipose tissue, and liver. Osteoprotegerin (OPG), an important symbol of the receptor activator of nuclear factor-B ligand/receptor activator of nuclear factor kappa B/OPG system activation, typically considered for its role in bone metabolism, may also play critical roles in the initiation and perpetuation of obesity-related comorbidities. Clinical data have indicated that OPG concentrations are associated with hypertension, left ventricular hypertrophy, vascular calcification, endothelial dysfunction, and severity of liver damage in chronic hepatitis C. Nonetheless, the relationship between circulating OPG and IR as a key feature of MetS as well as between OPG and NAFLD remains uncertain. Thus, the aims of the present review are to provide the existent knowledge on these associations and to discuss briefly the underlying mechanisms linking OPG and NAFLD.

Insulin resistance and bone: a biological partnership

Despite a clear association between type 2 diabetes (T2D) and fracture risk, the pathogenesis of bone fragility in T2D has not been clearly elucidated. Insulin resistance is the primary defect in T2D. Insulin signalling regulates both bone formation and bone resorption, but whether insulin resistance can affect bone has not been established. On the other hand, evidence exists that bone might play a role in the regulation of glucose metabolism. This article reviews the available experimental and clinical evidence on the interplay between bone and insulin resistance. Interestingly, a bilateral relationship between bone and insulin resistance seems to exist that unites them in a biological partnership.

Vhl deletion in osteoblasts boosts cellular glycolysis and improves global glucose metabolism

The skeleton has emerged as an important regulator of systemic glucose homeostasis, with osteocalcin and insulin representing prime mediators of the interplay between bone and energy metabolism. However, genetic evidence indicates that osteoblasts can influence global energy metabolism through additional, as yet unknown, mechanisms. Here, we report that constitutive or postnatally induced deletion of the hypoxia signaling pathway component von Hippel-Lindau (VHL) in skeletal osteolineage cells of mice led to high bone mass as well as hypoglycemia and increased glucose tolerance, not accounted for by osteocalcin or insulin. In vitro and in vivo data indicated that Vhl-deficient osteoblasts displayed massively increased glucose uptake and glycolysis associated with upregulated HIF-target gene expression, resembling the Warburg effect that typifies cancer cells. Overall, the glucose consumption by the skeleton was increased in the mutant mice, as revealed by 18F-FDG radioactive tracer experiments. Moreover, the glycemia levels correlated inversely with the level of skeletal glucose uptake, and pharmacological treatment with the glycolysis inhibitor dichloroacetate (DCA), which restored glucose metabolism in Vhl-deficient osteogenic cells in vitro, prevented the development of the systemic metabolic phenotype in the mutant mice. Altogether, these findings reveal a novel link between cellular glucose metabolism in osteoblasts and whole-body glucose homeostasis, controlled by local hypoxia signaling in the skeleton.

β-Catenin Directs Long-Chain Fatty Acid Catabolism in the Osteoblasts of Male Mice

Wnt-initiated signaling through a frizzled receptor and the low-density lipoprotein-related receptor-5 coreceptor instructs key anabolic events during skeletal development, homeostasis, and repair. Recent studies indicate that Wnt signaling also regulates the intermediary metabolism of osteoblastic cells, inducing glucose consumption in osteoprogenitors and fatty acid utilization in mature osteoblasts. In this study, we examined the role of the canonical Wnt-signaling target, β-catenin, in the control of osteoblast metabolism. In vitro, Wnt ligands and agonists that stimulated β-catenin activation in osteoblasts enhanced fatty acid catabolism, whereas genetic ablation of β-catenin dramatically reduced oleate oxidation concomitant with reduced osteoblast maturation and increased glycolytic metabolism. Temporal ablation of β-catenin expression in osteoblasts in vivo produced the expected low-bone-mass phenotype and also led to an increase in white adipose tissue mass, dyslipidemia, and impaired insulin sensitivity. Because the expression levels of enzymatic mediators of fatty acid β-oxidation are reduced in the skeleton of β-catenin mutants, these results further confirm the role of the osteoblast in lipid metabolism and indicate that the influence of Wnt signaling on fatty acid utilization proceeds via its canonical signaling pathway.