Possible roles of insulin signaling in osteoblasts

Relationship between long-term use of a typical antipsychotic medication by Chinese schizophrenia patients and the bone turnover markers serum osteocalcin and β-CrossLaps

BACKGROUND

Increasing evidence shows that schizophrenia patients with long-term exposure to antipsychotic medications have decreased bone mass, which suggests that they are at a high risk of osteoporosis. However, the mechanism underlying this remains unclear. In this study, we selected two bone turnover markers to explore whether atypical antipsychotics can affect bone metabolism and identified possible influencing factors.

METHODS

A total of 116 schizophrenia patients (18-40years old) participated in the study. The subjects included 31 drug-naive first-episode patients and 85 patients who had undergone atypical antipsychotic monotherapy for at least 6months. A total of 71 subjects were assigned as normal controls. Demographic and physical examination data were analyzed for all subjects. The positive and negative syndrome scale (PANSS) was used to assess psychopathology in schizophrenia patients. Levels of the bone turnover markers osteocalcin and β-CrossLaps were measured. Serum prolactin (PRL), lipid, sex hormone, glucose, insulin, and parathyroid hormone levels were also measured.

RESULTS

The serum β-CrossLaps levels of patients who had been treated with atypical antipsychotics were higher compared with those of drug-naive first-episode patients and normal subjects. Atypical antipsychotics, schizophrenia, age, gender, and body mass index, as well as serum levels of PRL, triglyceride, high-density lipoprotein cholesterol, glucose, and testosterone, were significantly associated with serum osteocalcin and β-CrossLaps levels. Serum insulin was only positively associated with serum osteocalcin, whereas estradiol was only negatively associated with serum β-CrossLaps.

CONCLUSION

Patients who had been treated with atypical antipsychotics had accelerated bone resorption. Our findings uncover a link between atypical antipsychotics and bone metabolism, possibly through abnormalities in glucose and lipid metabolism and insulin resistance.

Molecular Mechanisms of Obesity-Induced Osteoporosis and Muscle Atrophy

Obesity and osteoporosis are two alarming health disorders prominent among middle and old age populations, and the numbers of those affected by these two disorders are increasing. It is estimated that more than 600 million adults are obese and over 200 million people have osteoporosis worldwide. Interestingly, both of these abnormalities share some common features including a genetic predisposition, and a common origin: bone marrow mesenchymal stromal cells. Obesity is characterized by the expression of leptin, adiponectin, interleukin 6 (IL-6), interleukin 10 (IL-10), monocyte chemotactic protein-1 (MCP-1), tumor necrosis factor-alpha (TNF-α), macrophage colony stimulating factor (M-CSF), growth hormone (GH), parathyroid hormone (PTH), angiotensin II (Ang II), 5-hydroxy-tryptamine (5-HT), Advance glycation end products (AGE), and myostatin, which exert their effects by modulating the signaling pathways within bone and muscle. Chemical messengers (e.g., TNF-α, IL-6, AGE, leptins) that are upregulated or downregulated as a result of obesity have been shown to act as negative regulators of osteoblasts, osteocytes and muscles, as well as positive regulators of osteoclasts. These additive effects of obesity ultimately increase the risk for osteoporosis and muscle atrophy. The aim of this review is to identify the potential cellular mechanisms through which obesity may facilitate osteoporosis, muscle atrophy and bone fractures.

Insulin receptor substrate-1 time-dependently regulates bone formation by controlling collagen Iα2 expression via miR-342

Insulin promotes bone formation via a well-studied canonical signaling pathway. An adapter in this pathway, insulin-receptor substrate (IRS)-1, has been implicated in the diabetic osteopathy provoked by impaired insulin signaling. To further investigate IRS-1’s role in the bone metabolism, we generated Irs-1-deficient Irs-1^smla/smla mice. These null mice developed a spontaneous mutation that led to an increase in trabecular thickness (Tb.Th) in 12-mo-old, but not in 2-mo-old mice. Analyses of the bone marrow stromal cells (BMSCs) from these mice revealed their differential expression of osteogenesis-related genes and miRNAs. The expression of miR-342, predicted and then proven to target the gene encoding collagen type Iα2 (COL1A2), was reduced in BMSCs derived from Irs-1-null mice. COL1A2 expression was then shown to be age dependent in osteoblasts and BMSCs derived from Irs-1^smla/smla mice. After the induction of osteogenesis in BMSCs, miR-342 expression correlated inversely with that of Col1a2. Further, Col1a2-specific small interfering RNA (siRNA) reduced alkaline phosphatase (ALP) activity and inhibited BMSC differentiation into osteocyte-like cells, both in wild-type (WT) and Irs-1^smla/smla mice. Conversely, in Irs-1^smla/smla osteocytes overexpressing COL1A2, ALP-positive staining was stronger than in WT osteocytes. In summary, we uncovered a temporal regulation of BMSC differentiation/bone formation, controlled via Irs-1/miR-342 mediated regulation of Col1a2 expression.—Guo, Y., Tang, C.-Y., Man, X.-F., Tang, H.-N., Tang, J., Wang, F., Zhou, C.-L., Tan, S.-W., Feng, Y.-Z., Zhou, H.-D. Insulin receptor substrate-1 time-dependently regulates bone formation by controlling collagen Iα2 expression via miR-342.

A new perspective on mechanisms governing skeletal complications in type 1 diabetes

This review focuses on bone mechanobiology in type 1 diabetes (T1D), an area of research on diabetes-associated skeletal complications that is still in its infancy. We first provide a brief overview of the deleterious effects of diabetes on the skeleton and of the knowledge gained from studies with rodent models of T1D. Second, we discuss two specific hallmarks of T1D, low insulin and high glucose, and address the extent to which they affect skeletal health. Third, we highlight the mechanosensitive nature of bone tissue and the importance of mechanical loading for bone health. We also summarize recent advances in bone mechanobiology that implicate osteocytes as the mechanosensors and major regulatory cells in the bone. Finally, we discuss recent evidence indicating that the diabetic bone is "deaf" to mechanical loading and that osteocytes are central players in mechanisms that lead to bone loss in T1D.

Analysis of the Role of Insulin Signaling in Bone Turnover Induced by Fluoride

The role of insulin signaling on the mechanism underlying fluoride induced osteopathology was studied. We analyzed the expression of genes related with bone turnover and insulin signaling in rats treated by varying dose of fluoride with or without streptozotocin (STZ) in vivo. Furthermore, insulin receptor (InR) expression in MC3T3-E1 cells (pre-osteoblast cell line) was interfered with small interfering RNA (siRNA), and genes related with osteoblastic and osteoclastic differentiation were investigated in cells exposed to fluoride in vitro for 2 days. The in vivo study indicated the possible role of insulin in bone lesion induced by excessive amount of fluoride. Fluoride activated the InR and Insulin-like growth factor 1 (IGF1) signaling, which were involved in the mechanism underlying fluoride induced bone turnover. The TGFβ1 and Wnt10/β-catenin pathway took part in the mechanism of bone lesion induced by fluoride, and insulin probably modulated the TGFβ1 and β-catenin to exert action on bone turnover during the development of bone lesion. The in vitro study showed the concomitant decrease of OPG, osterix and OCN with inhibition of InR expression in osteoblast, and three genes still was low in cells co-treated with fluoride and InR siRNA, which suggested that fluoride probably stimulated the expression of OPG, osterix and OCN through InR signaling. In conclusion, insulin played the important role in bone lesion induced by excessive amount of fluoride through mediating InR receptor signaling, and IGF1 signaling probably exerted action on bone turnover caused by overdose of fluoride.

Insulin Resistance Negatively Influences the Muscle-Dependent IGF-1-Bone Mass Relationship in Premenarcheal Girls

CONTEXT

IGF-1 promotes bone growth directly and indirectly through its effects on skeletal muscle. Insulin and IGF-1 share a common cellular signaling process; thus, insulin resistance may influence the IGF-1-muscle-bone relationship.

OBJECTIVE

We sought to determine the effect of insulin resistance on the muscle-dependent relationship between IGF-1 and bone mass in premenarcheal girls.

DESIGN, SETTING, AND PARTICIPANTS

This was a cross-sectional study conducted at a university research center involving 147 girls ages 9 to 11 years.

MAIN OUTCOME MEASURES

Glucose, insulin, and IGF-1 were measured from fasting blood samples. Homeostasis model assessment of insulin resistance (HOMA-IR) was calculated from glucose and insulin. Fat-free soft tissue (FFST) mass and bone mineral content (BMC) were measured by dual-energy x-ray absorptiometry. Our primary outcome was BMC/height.

RESULTS

In our path model, IGF-1 predicted FFST mass (b = 0.018; P = .001), which in turn predicted BMC/height (b = 0.960; P < .001). IGF-1 predicted BMC/height (b = 0.001; P = .002), but not after accounting for the mediator of this relationship, FFST mass. The HOMA-IR by IGF-1 interaction negatively predicted FFST mass (b = -0.044; P = .034). HOMA-IR had a significant and negative effect on the muscle-dependent relationship between IGF-1 and BMC/height (b = -0.151; P = .047).

CONCLUSIONS

Lean body mass is an important intermediary factor in the IGF-1-bone relationship. For this reason, bone development may be compromised indirectly via suboptimal IGF-1-dependent muscle development in insulin-resistant children.

Biochemical bone turnover markers in diabetes mellitus - A systematic review

BACKGROUND

Diabetes mellitus is associated with an increased risk of fractures, which is not explained by bone mineral density. Other markers as bone turnover markers (BTMs) may be useful.

AIM

To assess the relationship between BTMs, diabetes, and fractures.

METHODS

A systematic literature search was conducted in August 2014. The databases searched were Medline at Pubmed and Embase. Medline at Pubmed was searched by "Diabetes Mellitus" (MESH) and "bone turnover markers" and Embase was searched using the Emtree by "Diabetes Mellitus" and "bone turnover", resulting in 611 studies. The eligibility criteria for the studies were to assess BTM in either type 1 diabetes (T1D) or type 2 diabetes (T2D) patients.

RESULTS

Of the 611 eligible studies, removal of duplicates and screening by title and abstract lead to 114 potential studies for full-text review. All these studies were full-text screened for eligibility and 45 studies were included. Two additional studies were added from other sources. Among the 47 studies included there were 1 meta-analysis, 29 cross-sectional studies, 13 randomized controlled trials, and 4 longitudinal studies. Both T1D and T2D were studied. Most studies reported fasting BTM and excluded renal disease.

CONCLUSION

Markers of bone resorption and formation seem to be lower in diabetes patients. Bone specific alkaline phosphatase is normal or increased, which suggests that the matrix becomes hypermineralized in diabetes patients. The BTMs: C-terminal cross-link of collagen, insulin-like growth factor-1, and sclerostin may potentially predict fractures, but longitudinal trials are needed. This article is part of a Special Issue entitled Bone and diabetes.

Sweet Bones: The Pathogenesis of Bone Alteration in Diabetes

Diabetic patients have increased fracture risk. The pathogenesis underlying the status of bone alterations in diabetes mellitus is not completely understood but is multifactorial. The major deficits appear to be related to a deficit in mineralized surface area, a decrement in the rate of mineral apposition, deceased osteoid surface, depressed osteoblast activity, and decreased numbers of osteoclasts due to abnormal insulin signaling pathway. Other prominent features of diabetes mellitus are an increased urinary excretion of calcium and magnesium, accumulation of advanced glycation end products, and oxidative stress leading to sweet bones (altered bone's strength, metabolism, and structure). Every diabetic patient should be assessed for risk factors for fractures and osteoporosis. The pathogenesis of the bone alterations in diabetes mellitus as well as their molecular mechanisms needs further study.

Streptozotocin Aggravated Osteopathology and Insulin Induced Osteogenesis Through Co-treatment with Fluoride

The role of insulin in the mechanism underlying the excessive fluoride that causes skeletal lesion was studied. The in vitro bone marrow stem cells (BMSC) collected from Kunming mice were exposed to varying concentrations of fluoride with or without insulin. The cell viability and early differentiation of BMSC co-treated with fluoride and insulin were measured by using cell counting kit-8 and Gomori modified calcium-cobalt method, respectively. We further investigated the in vivo effects of varying dose of fluoride on rats co-treated with streptozotocin (STZ). Wistar rats were divided into six groups which included normal control, 10 mg fluoride/kg day group, 20 mg fluoride/kg day group, STZ control, STZ+10 mg fluoride/kg day group, and STZ+20 mg fluoride/kg day group. The rats were administered with sodium fluoride (NaF) by gavage with water at doses 10 and 20 mg fluoride/kg day for 2 months. In a period of one month, half of rats in every group were treated with streptozotocin (STZ) once through intraperitoneal injection at 52 mg/kg body weight. The serum glucose, HbA1c, and insulin were determined. Bone mineral content and insulin release were assessed. The results showed insulin combined with fluoride stimulated BMSC cell viability in vitro. The bone mineral content reduced in rats treated with higher dose of fluoride and decreased immensely in rat co-treated with fluoride and STZ. Similarly, a combination treatment of a high dose of fluoride and STZ decreased insulin sensitivity and activity. To sum up, these data indicated fluoride influenced insulin release, activity, and sensitivity. Furthermore, the insulin state in vivo interfered in the osteogenesis in turn and implied there was a close relation between insulin and bone pathogenesis in the mechanism of fluoride toxicity.

Integrative Analysis of Transcriptomic and Epigenomic Data to Reveal Regulation Patterns for BMD Variation

Integration of multiple profiling data and construction of functional gene networks may provide additional insights into the molecular mechanisms of complex diseases. Osteoporosis is a worldwide public health problem, but the complex gene-gene interactions, post-transcriptional modifications and regulation of functional networks are still unclear. To gain a comprehensive understanding of osteoporosis etiology, transcriptome gene expression microarray, epigenomic miRNA microarray and methylome sequencing were performed simultaneously in 5 high hip BMD (Bone Mineral Density) subjects and 5 low hip BMD subjects. SPIA (Signaling Pathway Impact Analysis) and PCST (Prize Collecting Steiner Tree) algorithm were used to perform pathway-enrichment analysis and construct the interaction networks. Through integrating the transcriptomic and epigenomic data, firstly we identified 3 genes (FAM50A, ZNF473 and TMEM55B) and one miRNA (hsa-mir-4291) which showed the consistent association evidence from both gene expression and methylation data; secondly in network analysis we identified an interaction network module with 12 genes and 11 miRNAs including AKT1, STAT3, STAT5A, FLT3, hsa-mir-141 and hsa-mir-34a which have been associated with BMD in previous studies. This module revealed the crosstalk among miRNAs, mRNAs and DNA methylation and showed four potential regulatory patterns of gene expression to influence the BMD status. In conclusion, the integration of multiple layers of omics can yield in-depth results than analysis of individual omics data respectively. Integrative analysis from transcriptomics and epigenomic data improves our ability to identify causal genetic factors, and more importantly uncover functional regulation pattern of multi-omics for osteoporosis etiology.

Bone tissue as a systemic endocrine regulator

Bone is a target tissue for hormones, such as the sex steroids, parathormon, vitamin D, calcitonin, glucocorticoids, and thyroid hormones. In the last decade, other "non-classic" hormones that modulate the bone tissue have been identified. While incretins (GIP and GLP-1) inhibit bone remodeling, angiotensin acts to promote remodeling. Bone morphogenetic protein (BMP) has also been found to have anabolic effects on the skeleton by activating bone formation during embryonic development, as well as in the postnatal period of life. Bone has also been identified as an endocrine tissue that produces a number of hormones, that bind to and modulate extra-skeletal receptors. Osteocalcin occupies a central position in this context. It can increase insulin secretion, insulin sensitivity and regulate metabolism of fatty acids. Moreover, osteocalcin also influences phosphate metabolism via osteocyte-derived FGF23 (which targets the kidneys and parathyroid glands to control phosphate reabsorption and metabolism of vitamin D). Finally, osteocalcin stimulates testosterone synthesis in Leydig cells and thus may play some role in male fertility. Further studies are necessary to confirm clinically important roles for skeletal tissue in systemic regulations.