The increase in bone resorption in early-stage type I diabetic mice is induced by RANKL secreted by increased bone marrow adipocytes

Bone Marrow Adipose Tissue as a Critical Regulator of Postmenopausal Osteoporosis - A Concise Review

Postmenopausal osteoporosis (PMOP) is a major health problem affecting millions of women worldwide. PMOP patients are often accompanied by abnormal accumulation of bone marrow adipose tissue (BMAT). BMAT is a critical regulator of bone homeostasis, and an increasing BMAT volume is negatively associated with bone mass reduction or fracture. BMAT regulates bone metabolism via adipokines, cytokines and the immune system, but the specific mechanisms are largely unknown. This review emphasizes the impact of estrogen deficiency on bone homeostasis and BMAT expansion, and the mechanism by which BMAT regulates PMOP, providing a promising strategy for targeting BMAT in preventing and treating PMOP.

Genetically determined type 1 diabetes mellitus and risk of osteoporosis

BACKGROUND

Observational evidence suggests that type 1 diabetes mellitus (T1DM) is associated with the risk of osteoporosis (OP). Nevertheless, it is not apparent whether these correlations indicate a causal relationship. To elucidate the causal relationship, a two-sample Mendelian randomization (MR) analysis was performed.

METHODS

T1DM data was obtained from the large genome-wide association study (GWAS), in which 6683 cases and 12,173 controls from 12 European cohorts were involved. Bone mineral density (BMD) samples at four sites were extracted from the GEnetic Factors for OSteoporosis (GEFOS) consortium, including forearm (FA) (n = 8,143), femoral neck (FN) (n = 32,735), lumbar spine (LS) (n = 28,498), and heel (eBMD) (n = 426,824). The former three samples were from mixed populations and the last one was from European. Inverse variance weighting, MR-Egger, and weighted median tests were used to test the causal relationship between T1DM and OP. A series of sensitivity analyses were then conducted to verify the robustness of the results.

RESULTS

Twenty-three independent SNPs were associated with FN-BMD and LS-BMD, twenty-seven were associated with FA-BMD, and thirty-one were associated with eBMD. Inverse variance-weighted estimates indicated a causal effect of T1DM on FN-BMD (odds ratio (OR) =1.033, 95 % confidence interval (CI): 1.012-1.054, p = 0.002) and LS-BMD (OR = 1.032, 95 % CI: 1.005-1.060, p = 0.022) on OP risk. Other MR methods, including weighted median and MR-Egger, calculated consistent trends. While no significant causation was found between T1DM and the other sites (FA-BMD: OR = 1.008, 95 % CI: 0.975-1.043, p = 0.632; eBMD: OR = 0.993, 95 % CI: 0.985-1.001, p = 0.106). No significant heterogeneity (except for eBMD) or horizontal pleiotropy was found for instrumental variables, suggesting these results were reliable and robust.

CONCLUSIONS

This study shows a causal relationship between T1DM and the risk of some sites of OP (FN-BMD, LS-BMD), allowing for continued research to discover the clinical and experimental mechanisms of T1DM and OP. It also contributes to the recommendation if patients with T1DM need targeted care to promote bone health and timely prevention of osteoporosis.

Associations of marrow fat fraction with MR imaging based trabecular bone microarchitecture in first-time diagnosed type 1 diabetes mellitus

Purpose

To determine whether there are alterations in marrow fat content in individuals first-time diagnosed with type 1 diabetes mellitus (T1DM) and to explore the associations between marrow fat fraction and MRI-based findings in trabecular bone microarchitecture.

Method

A case-control study was conducted, involving adults with first-time diagnosed T1DM (n=35) and age- and sex-matched healthy adults (n=46). Dual-energy X-ray absorptiometry and 3 Tesla-MRI of the proximal tibia were performed to assess trabecular microarchitecture and vertebral marrow fat fraction. Multiple linear regression analysis was used to test the associations of marrow fat fraction with trabecular microarchitecture and bone density while adjusting for potential confounding factors.

Results

In individuals first-time diagnosed with T1DM, the marrow fat fraction was significantly higher (p < 0.001) compared to healthy controls. T1DM patients also exhibited higher trabecular separation [median (IQR): 2.19 (1.70, 2.68) vs 1.81 (1.62, 2.10), p < 0.001], lower trabecular volume [0.45 (0.30, 0.56) vs 0.53 (0.38, 0.60), p = 0.013], and lower trabecular number [0.37 (0.26, 0.44) vs 0.41 (0.32, 0.47), p = 0.020] compared to controls. However, bone density was similar between the two groups (p = 0.815). In individuals with T1DM, there was an inverse association between marrow fat fraction and trabecular volume (r = -0.69, p < 0.001) as well as trabecular number (r = -0.55, p < 0.001), and a positive association with trabecular separation (r = 0.75, p < 0.001). Marrow fat fraction was independently associated with total trabecular volume (standardized β = -0.21), trabecular number (β = -0.12), and trabecular separation (β = 0.57) of the proximal tibia after adjusting for various factors including age, gender, body mass index, physical activity, smoking status, alcohol consumption, blood glucose, plasma glycated hemoglobin, lipid profile, and bone turnover biomarkers.

Conclusions

Individuals first-time diagnosed with T1DM experience expansion of marrow adiposity, and elevated marrow fat content is associated with MRI-based trabecular microstructure.

Effect of Liraglutide on Osteoporosis in a Rat Model of Type 2 Diabetes Mellitus: A Histological, Immunohistochemical, and Biochemical Study

Diabetic osteoporosis (DOP) is a diabetic complication associated with a significant disability rate. Liraglutide, a glucagon-like peptide-1 receptor agonist, is a promising and innovative drug for type 2 diabetes mellitus (T2DM), with potential therapeutic implications for bone disorders. This investigation examined the impact of liraglutide on osteoporosis in rats with T2DM and studied the influence of vitamin D receptor Bsm1 polymorphism on liraglutide-induced outcomes. Thirty rats were divided into control, T2DM induced by a combination of a high-fat diet and 25 mg/kg streptozotocin, and T2DM-liraglutide (T2DM treated with 0.4 mg/kg/day liraglutide) groups. After 8 weeks of liraglutide treatment, femurs and blood samples were obtained from all rats for subsequent investigations. Diabetes induced a remarkable rise in the serum levels of receptor activator of nuclear factor kappa B ligand (RANKL) and C-telopeptide of type I collagen (CTX-1) associated with a remarkable decline in osteocalcin and osteoprotegerin (OPG). Impaired bone architecture was also demonstrated by light and scanning electron microscopic study. The immune expression of OPG was down-regulated, while RANKL was up-regulated. Interestingly, the administration of liraglutide ameliorated the previous changes induced by diabetes mellitus. In conclusion, liraglutide can prevent DOP, mostly due to liraglutide's ability to increase bone growth, while inhibiting bone resorption.

The role of non-coding RNAs in diabetes-induced osteoporosis

Diabetes mellitus (DM) and osteoporosis are two major health care problems worldwide. Emerging evidence suggests that DM poses a risk for osteoporosis and can contribute to the development of diabetes-induced osteoporosis (DOP). Interestingly, some epidemiological studies suggest that DOP may be at least partially distinct from those skeletal abnormalities associated with old age or postmenopausal osteoporosis. The increasing number of DM patients who also have DOP calls for a discussion of the pathogenesis of DOP and the investigation of drugs to treat DOP. Recently, non-coding RNAs (ncRNAs) have received more attention due to their significant role in cellular functions and bone formation. It is worth noting that ncRNAs have also been demonstrated to participate in the progression of DOP. Meanwhile, nano-delivery systems are considered a promising strategy to treat DOP because of their cellular targeting, sustained release, and controlled release characteristics. Additionally, the utilization of novel technologies such as the CRISPR system has expanded the scope of available options for treating DOP. Hence, this paper explores the functions and regulatory mechanisms of ncRNAs in DOP and highlights the advantages of employing nanoparticle-based drug delivery techniques to treat DOP. Finally, this paper also explores the potential of ncRNAs as diagnostic DOP biomarkers.

The role of wnt signaling in diabetes-induced osteoporosis

Osteoporosis, a chronic complication of diabetes mellitus, is characterized by a reduction in bone mass, destruction of bone microarchitecture, decreased bone strength, and increased bone fragility. Because of its insidious onset, osteoporosis renders patients highly susceptible to pathological fractures, leading to increased disability and mortality rates. However, the specific pathogenesis of osteoporosis induced by chronic hyperglycemia has not yet been fully elucidated. But it is currently known that the disruption of Wnt signaling triggered by chronic hyperglycemia is involved in the pathogenesis of diabetic osteoporosis. There are two main types of Wnt signaling pathways, the canonical Wnt signaling pathway (β-catenin-dependent) and the non-canonical Wnt signaling pathway (non-β-catenin-dependent), both of which play an important role in regulating the balance between bone formation and bone resorption. Therefore, this review systematically describes the effects of abnormal Wnt pathway signaling on bone homeostasis under hyperglycemia, hoping to reveal the relationship between Wnt signaling and diabetic osteoporosis to further improve understanding of this disease.

Diabetes and bone

Globally, one in 11 adults has diabetes mellitus of which 90% have type 2 diabetes. The numbers for osteoporosis are no less staggering: 1 in 3 women has a fracture after menopause, and the same is true for 1 in 5 men after the age of 50 years. Aging is associated with several physiological changes that cause insulin resistance and impaired insulin secretion, which in turn lead to hyperglycemia. The negative balance between bone resorption and formation is a natural process that appears after the fourth decade of life and lasts for the following decades, eroding the bone structure and increasing the risk of fractures. Not incidentally, it has been acknowledged that diabetes mellitus, regardless of whether type 1 or 2, is associated with an increased risk of fracture. The nuances that differentiate bone damage in the two main forms of diabetes are part of the intrinsic heterogeneity of diabetes, which is enhanced when associated with a condition as complex as osteoporosis. This narrative review addresses the main parameters related to the increased risk of fractures in individuals with diabetes, and the mutual factors affecting the treatment of diabetes mellitus and osteoporosis.

Effect of chronic lithium on mechanical sensitivity and trabecular bone loss induced by type-1 diabetes mellitus in mice

Type-1 diabetes mellitus (T1DM) is a chronic condition characterized by long-term hyperglycemia that results in several complications such as painful peripheral neuropathy, bone deterioration, and increased risk of bone fractures. Lithium, a first-line therapy for bipolar disorder, has become an attractive agent for attenuating peripheral neuropathy and menopause-induced bone loss. Therefore, our aim was to determine the effect of chronic lithium treatment on mechanical hypersensitivity and trabecular bone loss induced by T1DM in mice. T1DM was induced in male C57BL/6J mice by intraperitoneal injection of streptozotocin (STZ, 50 mg/kg/day, for 5 consecutive days). 12 weeks after T1DM-induction, mice received a daily intraperitoneal injection of vehicle, 30 or 60 mg/kg lithium (as LiCl) for 6 weeks. Throughout the treatment period, blood glucose levels and mechanical sensitivity were evaluated every 2 weeks. After lithium treatment, the femur and L5 vertebra were harvested for microcomputed tomography (microCT) analysis. T1DM mice showed significant hyperglycemia, mechanical hypersensitivity, and significant trabecular bone loss as compared with the control group. Chronic lithium treatment did not revert the hindpaw mechanical hypersensitivity nor hyperglycemia associated to T1DM induced by STZ. In contrast, microCT analysis revealed that lithium reverted, in a dose-dependent manner, the loss of trabecular bone associated to T1DM induced by STZ at both the distal femur and L5 vertebra. Lithium treatment by itself did not affect any trabecular bone parameter in non-diabetic mice.

Bone fragility in diabetes: novel concepts and clinical implications

Increased fracture risk represents an emerging and severe complication of diabetes. The resulting prolonged immobility and hospitalisations can lead to substantial morbidity and mortality. In type 1 diabetes, bone mass and bone strength are reduced, resulting in up to a five-times greater risk of fractures throughout life. In type 2 diabetes, fracture risk is increased despite a normal bone mass. Conventional dual-energy x-ray absorptiometry might underestimate fracture risk, but can be improved by applying specific adjustments. Bone fragility in diabetes can result from cellular abnormalities, matrix interactions, immune and vascular changes, and musculoskeletal maladaptation to chronic hyperglycaemia. This Review summarises how the bone microenvironment responds to type 1 and type 2 diabetes, and the mechanisms underlying fragility fractures. We describe the value of novel imaging technologies and the clinical utility of biomarkers, and discuss current and future therapeutic approaches that protect bone health in people with diabetes.

Bone marrow adiposity during pathologic bone loss: molecular mechanisms underlying the cellular events

Bone marrow (BM) is a heterogeneous niche where bone marrow stromal cells (BMSCs), osteoblasts, osteoclasts, adipocytes, hematopoietic cells, and immune cells coexist. The cellular composition of BM changes with various pathophysiological states. A reduction in osteoblast number and a concomitant increase in adipocyte number in aging and pathological conditions put bone marrow adipose tissue (BMAT) into spotlight. Accumulating evidence strongly supports that an overwhelming production of BMAT is a major contributor to bone loss disorders. Therefore, BMAT-targeted therapy can be an efficient and feasible intervention for osteoporosis. However, compared to blocking bone-destroying molecules produced by BMAT, suppressing BMAT formation is theoretically a more effective and fundamental approach in treating osteoporotic bone diseases. Thus, a deep insight into the molecular basis underlying increased BM adiposity during pathologic bone loss is critical to formulate strategies for therapeutically manipulating BMAT. In this review, we comprehensively summarize the molecular mechanisms involved in adipocyte differentiation of BMSCs as well as the interaction between bone marrow adipocytes and osteoclasts. More importantly, we further discuss the potential clinical implications of therapeutically targeting the upstream of BMAT formation in bone loss diseases.

Mechanisms of altered bone remodeling in children with type 1 diabetes

Bone loss associated with type 1 diabetes mellitus (T1DM) begins at the onset of the disease, already in childhood, determining a lower bone mass peak and hence a greater risk of osteoporosis and fractures later in life. The mechanisms underlying diabetic bone fragility are not yet completely understood. Hyperglycemia and insulin deficiency can affect the bone cells functions, as well as the bone marrow fat, thus impairing the bone strength, geometry, and microarchitecture. Several factors, like insulin and growth hormone/insulin-like growth factor 1, can control bone marrow mesenchymal stem cell commitment, and the receptor activator of nuclear factor-κB ligand/osteoprotegerin and Wnt-b catenin pathways can impair bone turnover. Some myokines may have a key role in regulating metabolic control and improving bone mass in T1DM subjects. The aim of this review is to provide an overview of the current knowledge of the mechanisms underlying altered bone remodeling in children affected by T1DM.

Alternative regulatory mechanism for the maintenance of bone homeostasis via STAT5-mediated regulation of the differentiation of BMSCs into adipocytes

STAT5 is a transcription factor that is activated by various cytokines, hormones, and growth factors. Activated STAT5 is then translocated to the nucleus and regulates the transcription of target genes, affecting several biological processes. Several studies have investigated the role of STAT5 in adipogenesis, but unfortunately, its role in adipogenesis remains controversial. In the present study, we generated adipocyte-specific Stat5 conditional knockout (cKO) (Stat5^fl/fl;Apn-cre) mice to investigate the role of STAT5 in the adipogenesis of bone marrow mesenchymal stem cells (BMSCs). BMSC adipogenesis was significantly inhibited upon overexpression of constitutively active STAT5A, while it was enhanced in the absence of Stat5 in vitro. In vivo adipose staining and histological analyses revealed increased adipose volume in the bone marrow of Stat5 cKO mice. ATF3 is the target of STAT5 during STAT5-mediated inhibition of adipogenesis, and its transcription is regulated by the binding of STAT5 to the Atf3 promoter. ATF3 overexpression was sufficient to suppress the enhanced adipogenesis of Stat5-deficient adipocytes, and Atf3 silencing abolished the STAT5-mediated inhibition of adipogenesis. Stat5 cKO mice exhibited reduced bone volume due to an increase in the osteoclast number, and coculture of bone marrow-derived macrophages with Stat5 cKO adipocytes resulted in enhanced osteoclastogenesis, suggesting that an increase in the adipocyte number may contribute to bone loss. In summary, this study shows that STAT5 is a negative regulator of BMSC adipogenesis and contributes to bone homeostasis via direct and indirect regulation of osteoclast differentiation; therefore, it may be a leading target for the treatment of both obesity and bone loss-related diseases.

A protein connected with bone maintenance and fat cell differentiation could provide a novel therapeutic target for both obesity and osteoporosis. The processes of healthy bone remodeling and fat cell (adipocyte) differentiation from bone marrow stem cells (BMSCs) are intrinsically connected. The transcription factor protein STAT5 plays roles in maintaining bone homeostasis and adipocyte differentiation, but its role in the latter is unclear. Nacksung Kim at Chonnam National University Medical School in Gwangju, South Korea, and co-workers examined the role of STAT5 in mice. Mice without the Stat5 gene had increased fat tissue in their bone marrow, suggesting increased BMSC differentiation into adipocytes. The mice also had reduced bone mass due to increased numbers of bone-degrading cells. Further investigations showed that STAT5 regulates the differentiation of BMSCs into adipocytes via activation of a regulatory gene.

Effects of normobaric cyclic hypoxia exposure on mesenchymal stem-cell differentiation-pilot study on bone parameters in elderly

BACKGROUND

Mesenchymal stem cells (MSC) of bone marrow are the progenitor of osteoblasts and adipocytes. MSC tend to differentiate into adipocytes, instead of osteoblasts, with aging. This favors the loss of bone mass and development of osteoporosis. Hypoxia induces hypoxia inducible factor 1α gene encoding transcription factor, which regulates the expression of genes related to energy metabolism and angiogenesis. That allows a better adaptation to low O2 conditions. Sustained hypoxia has negative effects on bone metabolism, favoring bone resorption. Yet, surprisingly, cyclic hypoxia (CH), short times of hypoxia followed by long times in normoxia, can modulate MSC differentiation and improve bone health in aging.

AIM

To evaluate the CH effect on MSC differentiation, and whether it improves bone mineral density in elderly.

METHODS

MSC cultures were induced to differentiate into osteoblasts or adipocytes, in CH (3% O2 for 1, 2 or 4 h, 4 d a week). Extracellular-matrix mineralization and lipid-droplet formation were studied in MSC induced to differentiate into osteoblast or adipocytes, respectively. In addition, gene expression of marker genes, for osteogenesis or adipogenesis, have been quantified by quantitative real time polymerase chain reaction. The in vivo studies with elderly (> 75 years old; n = 10) were carried out in a hypoxia chamber, simulating an altitude of 2500 m above sea level, or in normoxia, for 18 wk (36 CH sessions of 16 min each). Percentages of fat mass and bone mineral density from whole body, trunk and right proximal femur (femoral, femoral neck and trochanter) were assessed, using dual-energy X-ray absorptiometry.

RESULTS

CH (4 h of hypoxic exposure) inhibited extracellular matrix mineralization and lipid-droplet formation in MSC induced to differentiate into osteoblasts or adipocytes, respectively. However, both parameters were not significantly affected by the other shorter hypoxia times assessed. The longest periods of hypoxia downregulated the expression of genes related to extracellular matrix formation, in MSC induced to differentiate into osteoblasts. Interestingly, osteocalcin (associated to energy metabolism) was upregulated. Vascular endothelial growth factor an expression and low-density lipoprotein receptor related protein 5/6/dickkopf Wnt signaling pathway inhibitor 1 (associated to Wnt/β-catenin pathway activation) increased in osteoblasts. Yet, they decreased in adipocytes after CH treatments, mainly with the longest hypoxia times. However, the same CH treatments increased the osteoprotegerin/receptor activator for nuclear factor kappa B ligand ratio in both cell types. An increase in total bone mineral density was observed in elderly people exposed to CH, but not in specific regions. The percentage of fat did not vary between groups.

CONCLUSION

CH may have positive effects on bone health in the elderly, due to its possible inhibitory effect on bone resorption, by increasing the osteoprotegerin / receptor activator for nuclear factor kappa B ligand ratio.