Leptin treatment prevents type I diabetic marrow adiposity but not bone loss in mice

Long-Term Follow-up After Ovariectomy Reveals Correlations Between Bone Marrow Adiposity and Trabecular Bone Quality in the Proximal Metaphysis of Tibiae in Rats

This study aimed to evaluate the correlation between BMAT and bone quality, describe the long-term effects of ovariectomy on bone, and investigate BMAT's spatial distribution. Fifteen-months-old female Sprague‒Dawley rats were studied, comparing ovariectomized (OVX, n = 22) and sham-operated (SHAM, n = 11) groups at 6 months. Tibias were analyzed for bone microarchitecture, BMAT (microcomputed tomography), mineral parameters (quantitative backscattered electron imaging), and bone composition (Raman microspectroscopy). The OVX tibias showed severe trabecular bone loss (lower bone volume/total volume, p < 0.001) with increased BMAT (higher adipose volume per marrow volume, p < 0.001), decreased mineral content (lower calcium concentration, p < 0.001), and altered organic components (lower mineral/matrix ratio in new bone, p = 0.03 trabecular surface, p < 0.001 trabecular core). When the data are pooled over both groups (SHAM and OVX), the adipose volume/marrow volume ratio was negatively correlated with bone volume/total volume (r =  - 0.79, p < 0.001) and mineral/matrix ratio (r =  - 0.37, p = 0.04 trabecular surface; r =  - 0.65, p < 0.001 trabecular core) and positively correlated with crystallinity (r = 0.55, p = 0.001 trabecular surface; r = 0.49, p = 0.006 trabecular core). The mineral/matrix ratio of trabecular surface new bone was strongly negatively correlated with the adipose compartment nearest to the bone surface. These findings suggest mechanisms underlying BMAT's role in bone resorption.

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.

Endospanin Is a Candidate for Regulating Leptin Sensitivity

The hypothesis advanced is that endospanin, a highly conserved vesicle traffic protein in vertebrates, regulates leptin sensitivity in bone signaling. The effects of leptin on bones are well-studied but without consensus on whether the increases in leptin signaling stimulate bone gain or loss. The bone response may depend on leptin sensitivity, and endospanin is an established modulator of leptin sensitivity. An argument is advanced to develop zebrafish models for specific leptin signaling pathways. Zebrafish have well-developed molecular tools (e.g., CRISPR) and the advantage of non-destructive sampling of bones in the form of scales. Using these tools, experiments are described to substantiate the role of endospanin in zebrafish bone dynamics.

Aucubin slows the development of osteoporosis by inhibiting osteoclast differentiation via the nuclear factor erythroid 2-related factor 2-mediated antioxidation pathway

CONTEXT

Osteoporosis (OP) is a metabolic disease. We have previously demonstrated that aucubin (AU) has anti-OP effects that are due to its promotion of the formation of osteoblasts.

OBJECTIVES

To investigate the mechanisms of anti-OP effects of AU.

MATERIALS AND METHODS

C57BL/6 mice were randomly divided into control group, 30 mg/kg Dex-induced OP group (OP model group, 15 μg/kg oestradiol-treated positive control group, 5 or 45 mg/kg AU-treated group), and 45 mg/kg AU-alone-treated group. The administration lasted for 7 weeks. Subsequently, 1, 2.5 and 5 µM AU were incubated with 50 ng/mL RANKL-induced RAW264.7 cells for 7 days to observe osteoclast differentiation. The effect of AU was evaluated by analysing tissue lesions, biochemical factor and protein expression.

RESULTS

The LD₅₀ of AU was greater than 45 mg/kg. AU increased the number of trabeculae and reduced the loss of chondrocytes in OP mice. Compared to OP mice, AU-treated mice exhibited decreased serum concentrations of TRAP5b (19.6% to 28.4%), IL-1 (12.2% to 12.6%), IL-6 (12.1%) and ROS (5.9% to 10.7%) and increased serum concentrations of SOD (14.6% to 19.4%) and CAT (17.2% to 27.4%). AU treatment of RANKL-exposed RAW264.7 cells decreased the numbers of multi-nuclear TRAP-positive cells, reversed the over-expression of TRAP5, NFATc1 and CTSK. Furthermore, AU increased the expression of nuclear factor erythroid 2-related factor 2 (Nrf2) and its downstream proteins in RANKL-exposed RAW264.7 cells.

CONCLUSIONS

AU slows the development of OP via Nrf2-mediated antioxidant pathways, indicating the potential use of AU in OP therapy and other types of OP research.

Evaluation of Bone Marrow Texture and Trabecular Changes With Quantitative DCE-MRI and QCT in Alloxan-Induced Diabetic Rabbit Models

PURPOSE

To investigate whether the microvascular permeability of lumbar marrow and bone trabecular changes in early-stage diabetic rabbits can be quantitatively evaluated using dynamic contrast-enhanced magnetic resonance imaging (DCE-MRI), quantitative computed tomography, and texture-analyzed permeability parameter K^trans map of DCE-MRI.

MATERIALS AND METHODS

This prospective study included 24 rabbits that were randomly assigned to diabetic (n = 14) and control (n = 10) groups. All rabbits underwent sagittal MRI of the lumbar region at 0, 4, 8, 12, and 16 weeks after alloxan injection. Pearson correlation coefficient was performed to determine the correlation between permeability parameter and bone mineral density (BMD). Repeated-measures ANOVA was used to analyze the changes in lumbar BMD over time in each group and the texture parameters of diabetic rabbit lumbar marrow at different time points. Mann-Whitney U rank sum test was used to compare the differences of each index between the two groups and calculate the area under the curve (AUC).

RESULTS

BMD was correlated with K^trans , K_ep , and V_e but not with V_p . At weeks 0-16, the BMD of the rabbits in the diabetic and normal groups was not statistically significant, but the change in BMD showed an overall downward trend. For texture analysis, entropy, energy, and Uniformized positive pixel (UPP) parameters extracted from the K^trans map showed significant differences from week 0 to 16 between the two groups. The identification ability at 8-12 weeks was higher than that at 12-16 weeks, and the AUCs were 0.734, 0.766, and 0.734, respectively (P < 0.05 for all).

CONCLUSIONS

The changes in BMD measured using quantitative computed tomography occurred later than those measured using bone trabecular morphometry. Texture analysis parameters based on DCE-MRI quantitative parameter Ktrans map are feasible to identify early changes in lumbar marrow structure in diabetic rabbits.

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 (BMAT) has recently been found to induce osteoclastogenesis by secreting RANKL. Although Type 1 diabetes mellitus (T1DM) has been reported to be associated with increased BMAT and bone loss, little is known about the relationship between BMAT and osteoclasts in T1DM. We studied the role of BMAT in the alterations of osteoclast activities in early-stage T1DM, by using a streptozotocin-induced T1DM mouse model. Our results showed that osteoclast activity was enhanced in the long bones of T1DM mice, accompanied by increased protein expression of RANKL. However, RANKL mRNA levels in bone tissues of T1DM mice remained unchanged. Meanwhile, we found that BMAT was significantly increased in the long bones of T1DM mice, and both mRNA and protein levels of RANKL were elevated in the diabetic BMAT. More importantly, RANKL protein was mainly expressed on the cell membranes of the increased adipocytes, most of which were located next to the metaphyseal region. These results suggest that the enhanced bone resorption in early-stage diabetic mice is induced by RANKL derived from BMAT rather than the bone tissue itself.

Reporting Guidelines, Review of Methodological Standards, and Challenges Toward Harmonization in Bone Marrow Adiposity Research. Report of the Methodologies Working Group of the International Bone Marrow Adiposity Society

The interest in bone marrow adiposity (BMA) has increased over the last decade due to its association with, and potential role, in a range of diseases (osteoporosis, diabetes, anorexia, cancer) as well as treatments (corticosteroid, radiation, chemotherapy, thiazolidinediones). However, to advance the field of BMA research, standardization of methods is desirable to increase comparability of study outcomes and foster collaboration. Therefore, at the 2017 annual BMA meeting, the International Bone Marrow Adiposity Society (BMAS) founded a working group to evaluate methodologies in BMA research. All BMAS members could volunteer to participate. The working group members, who are all active preclinical or clinical BMA researchers, searched the literature for articles investigating BMA and discussed the results during personal and telephone conferences. According to the consensus opinion, both based on the review of the literature and on expert opinion, we describe existing methodologies and discuss the challenges and future directions for (1) histomorphometry of bone marrow adipocytes, (2) ex vivo BMA imaging, (3) in vivo BMA imaging, (4) cell isolation, culture, differentiation and in vitro modulation of primary bone marrow adipocytes and bone marrow stromal cell precursors, (5) lineage tracing and in vivo BMA modulation, and (6) BMA biobanking. We identify as accepted standards in BMA research: manual histomorphometry and osmium tetroxide 3D contrast-enhanced μCT for ex vivo quantification, specific MRI sequences (WFI and H-MRS) for in vivo studies, and RT-qPCR with a minimal four gene panel or lipid-based assays for in vitro quantification of bone marrow adipogenesis. Emerging techniques are described which may soon come to complement or substitute these gold standards. Known confounding factors and minimal reporting standards are presented, and their use is encouraged to facilitate comparison across studies. In conclusion, specific BMA methodologies have been developed. However, important challenges remain. In particular, we advocate for the harmonization of methodologies, the precise reporting of known confounding factors, and the identification of methods to modulate BMA independently from other tissues. Wider use of existing animal models with impaired BMA production (e.g., Pfrt^-/-, Kit^W/W-v) and development of specific BMA deletion models would be highly desirable for this purpose.

The Unique Metabolic Characteristics of Bone Marrow Adipose Tissue

Bone marrow adipose tissue (MAT) is distinct from white adipose tissue (WAT) or brown adipose tissue (BAT) for its location, feature and function. As a largely ignored adipose depot, it is situated in bone marrow space and resided with bone tissue side-by-side. MAT is considered not only as a regulator of bone metabolism through paracrine, but also as a functionally particular adipose tissue that may contribute to global metabolism. Adipokines, inflammatory factors and other molecules derived from bone marrow adipocytes may exert systematic effects. In this review, we summary the evidence from several aspects including development, distribution, histological features and phenotype to elaborate the basic characteristics of MAT. We discuss the association between bone metabolism and MAT, and highlight our current understanding of this special adipose tissue. We further demonstrate the probable relationship between MAT and energy metabolism, as well as glucose metabolism. On the basis of preliminary results from animal model and clinical studies, we propose that MAT has its unique secretory and metabolic function, although there is no in-depth study at present.

Effect of a bisphosphonate and selective estrogen receptor modulator on bone remodeling in streptozotocin-induced diabetes and ovariectomized rat model

BACKGROUND CONTEXT

Diabetes and menopause can cause severe osteoporosis. In general, menopause and diabetes can lead to an imbalance in bone turnover, which results in secondary osteoporosis. However, the efficacy of antiresorptive drugs against this form of osteoporosis has not been extensively evaluated.

OBJECTIVE

The aim of this study was to determine the changes in vertebral bone remodeling when postmenopausal osteoporosis is accompanied by diabetes and to compare the efficacy of bisphosphonates and selective estrogen-receptor modulators (SERMs) against these outcomes.

STUDY DESIGN

Streptozotocin-induced diabetic, ovariectomized Sprague-Dawley rats were used as the disease model. Alendronate and raloxifene were used as the bisphosphonate and SERM, respectively.

METHODS

We divided 62 female rats into five groups: (1) control (n=14), (2) DM (diabetes) (n=12), (3) DM+OVX (diabetes+ovariectomy) (n=12), (4) DM+OVX+A (diabetes+ovariectomy+alendronate) (n=12), and (5) DM+OVX+R (diabetes+ovariectomy+raloxifene) (n=12). Serum biochemical markers of bone turnover, including osteocalcin and the C-telopeptide of type I collagen (CTX-1), were analyzed. We measured histomorphometric parameters of the fourth lumbar vertebrae using microcomputed tomography. Mechanical strength was evaluated by a compression test.

RESULTS

In the DM and DM+OVX group, only the levels of osteocalcin significantly decreased compared with those of the control group at 8 weeks after OVX. At 12 weeks, the serum CTX-1 levels in the DM+OVX+A and DM+OVX+R groups were significantly lower than those of the DM+OVX group, but there were no changes in the levels of osteocalcin. Bone mineral density and mechanical strength were higher in the DM+OVX+A and DM+OVX+R groups than in the DM and DM+OVX groups (p<.05).

CONCLUSIONS

Even if postmenopausal osteoporosis is accompanied by diabetes in this animal model, both alendronate and raloxifene seem to show antiresorptive effects, decreased bone turnover rates, and improved bone mechanical strength. Therefore, alendronate and raloxifene are effective in the treatment of osteoporosis even for bone loss caused by DM and postmenopausal osteoporosis.

Novel leptin receptor signaling mutants identify location and sex-dependent modulation of bone density, adiposity, and growth

Leptin, a hormone primarily produced by adipocytes, contributes to the regulation of bone health by modulating bone density, growth and adiposity. Upon leptin binding, multiple sites of the long form of the leptin receptor (LepRb) are phosphorylated to trigger activation of downstream signaling pathways. To address the role of LepRb-signaling pathways in bone health, we compared the effects of three LepRb mutations on bone density, adiposity, and growth in male and female mice. The ∆65 mutation, which lacks the known tyrosine phosphorylation sites, caused obesity and the most dramatic bone phenotype marked by excessive bone adiposity, osteoporosis, and decreased growth, consistent with the phenotype of db/db and ob/ob mice that fully lack leptin receptor signaling. Mutation of LepRb Tyr ₁₁₃₈ , which results in an inability to recruit and phosphorylate signal transducer and activator of transcription 3, also caused obesity, but bone loss and adiposity were more dominant in male mice and no growth defect was observed. In contrast, mutation of LepRb Tyr ₉₈₅ , which blocks SHP2/SOCS3 recruitment to LepRb and contributes to leptin hypersensitivity, promoted increased femur bone density only in male mice, while marrow adiposity and bone growth were not affected. Additional analyses of vertebral trabecular bone volume indicate that only the Tyr ₁₁₃₈ mutant mice exhibit bone loss in vertebrae. Together, our findings suggest that the phosphorylation status of specific sites of the LepRb contribute to the sex- and location-dependent bone responses to leptin. Unraveling the mechanisms by which leptin responses are sex- and location-dependent can contribute to the development of uniquely targeted osteoporosis therapies.

Magnetic Resonance Spectroscopy for Evaluating the Effect of Pulsed Electromagnetic Fields on Marrow Adiposity in Postmenopausal Women With Osteopenia

OBJECTIVE

Pulsed electromagnetic fields (PEMFs) could promote osteogenic differentiation and suppress adipogenic differentiation in bone mesenchymal stem cells ex vivo. However, data on the effect of PEMF on marrow adiposity in humans remain elusive. We aimed to determine the in vivo effect of PEMF on marrow adiposity in postmenopausal women using magnetic resonance spectroscopy.

METHODS

Sixty-one postmenopausal women with osteopenia, aged 53 to 85 years, were randomly assigned to receive either PEMF treatment or placebo. The session was performed 3 times per week for 6 months. All women received adequate dietary calcium and vitamin D. Bone mineral density (BMD) by dual-energy x-ray absorptiometry, vertebral marrow fat content by magnetic resonance spectroscopy, and serum biomarkers were evaluated before and after 6 months of treatment.

RESULTS

A total of 27 (87.1%) and 25 (83.3%) women completed the treatment schedule in the PEMF and placebo groups, respectively. After the 6-month treatment, lumbar spine and hip BMD increased by 1.46% to 2.04%, serum bone-specific alkaline phosphatase increased by 3.23%, and C-terminal telopeptides of type 1 collagen decreased by 9.12% in the PEMF group (P < 0.05), whereas the mean percentage changes in BMD and serum biomarkers were not significant in the placebo group. Pulsed electromagnetic field treatment significantly reduced marrow fat fraction by 4.81%. The treatment difference between the 2 groups was -4.43% (95% confidence interval, -3.70% to -5.65%; P = 0.009).

CONCLUSIONS

Pulsed electromagnetic field is an effective physiotherapy in postmenopausal women, and this effect may, at least in part, regulate the amount of fat within the bone marrow. Magnetic resonance spectroscopy may serve as a complementary imaging biomarker for monitoring response to therapy in osteoporosis.

Effect of Leptin on Marrow Adiposity in Ovariectomized Rabbits Assessed by Proton Magnetic Resonance Spectroscopy

OBJECTIVE

Leptin acts to influence bone metabolism through indirect hypothalamic relay and direct peripheral pathways. Leptin enhances the differentiation of mesenchymal stem cells to the osteoblast rather than the adipocyte lineage, but the in vivo impacts of leptin on ovariectomy (OVX)-induced marrow adiposity are poorly understood. In this work, we aimed to address this question.

METHODS

Forty-five female New Zealand rabbits were divided into sham + vehicle, OVX + vehicle, and OVX + leptin for 5 months. Magnetic resonance spectroscopy and dual-energy x-ray absorptiometry were performed to longitudinally evaluate marrow fat fraction and bone density at 0, 2.5, and 5 months, respectively. At the end of experiment, quantitative parameters of marrow adipocytes were assessed by histopathology.

RESULTS

Estrogen-deficient rabbits markedly exhibited expansion of marrow fat in a time-dependent manner, with a variation of marrow fat fraction (+19.7%) at 2.5 months relative to baseline conditions, and it was maintained until 5 months (+49.2%; all P < 0.001), which was accompanied by diminished bone density. Adipocyte diameter, density, and adipocytes area percentage in the OVX controls was increased by 50.7%, 76.3%, and 135.5%, respectively, relative to the sham controls (all P < 0.001). These OVX-induced marrow adiposity and bone loss were partly restored by leptin treatment. Treatment with leptin prevented OVX-induced increases in bone turnover in rabbits.

CONCLUSIONS

Early leptin administration inhibits the adipogenic effect of estrogen deficiency in terms of reverting marrow fat expansion seen in OVX rabbits. Magnetic resonance spectroscopy may be a useful tool for longitudinal and interventional assessments in osteoporosis.

Development, regulation, metabolism and function of bone marrow adipose tissues

Most adipocytes exist in discrete depots throughout the body, notably in well-defined white and brown adipose tissues. However, adipocytes also reside within specialized niches, of which the most abundant is within bone marrow. Whereas bone marrow adipose tissue (BMAT) shares many properties in common with white adipose tissue, the distinct functions of BMAT are reflected by its development, regulation, protein secretion, and lipid composition. In addition to its potential role as a local energy reservoir, BMAT also secretes proteins, including adiponectin, RANK ligand, dipeptidyl peptidase-4, and stem cell factor, which contribute to local marrow niche functions and which may also influence global metabolism. The characteristics of BMAT are also distinct depending on whether marrow adipocytes are contained within yellow or red marrow, as these can be thought of as 'constitutive' and 'regulated', respectively. The rBMAT for instance can be expanded or depleted by myriad factors, including age, nutrition, endocrine status and pharmaceuticals. Herein we review the site specificity, age-related development, regulation and metabolic characteristics of BMAT under various metabolic conditions, including the functional interactions with bone and hematopoietic cells.

High fat diet attenuates hyperglycemia, body composition changes, and bone loss in male streptozotocin-induced type 1 diabetic mice

There is a growing and alarming prevalence of obesity and the metabolic syndrome in type I diabetic patients (T1DM), particularly in adolescence. In general, low bone mass, higher fracture risk, and increased marrow adipose tissue (MAT) are features of diabetic osteopathy in insulin-deficient subjects. On the other hand, type 2 diabetes (T2DM) is associated with normal or high bone mass, a greater risk of peripheral fractures, and no change in MAT. Therefore, we sought to determine the effect of weight gain on bone turnover in insulin-deficient mice. We evaluated the impact of a 6-week high-fat (HFD) rich in medium chain fatty acids or low-fat diet (LFD) on bone mass and MAT in a streptozotocin (STZ)-induced model using male C57BL/6J mice at 8 weeks of age. Dietary intervention was initiated after diabetes confirmation. At the endpoint, lower non-fasting glucose levels were observed in diabetic mice fed with high fat diet compared to diabetic mice fed the low fat diet (STZ-LFD). Compared to euglycemic controls, the STZ-LFD had marked polydipsia and polyphagia, as well as reduced lean mass, fat mass, and bone parameters. Interestingly, STZ-HFD mice had higher bone mass, namely less cortical bone loss and more trabecular bone than STZ-LFD. Thus, we found that a HFD, rich in medium chain fatty acids, protects against bone loss in a T1DM mouse model. Whether this may also translate to T1DM patients who are overweight or obese in respect to maintenance of bone mass remains to be determined through longitudinal studies.

Bone-Fat Interaction

Marrow adipose tissue (MAT) is a recently identified endocrine organ capable of modulating a host of responses. Given its intimate proximity to the bone microenvironment, the impact marrow adipocytes exert on bone has attracted much interest and scientific inquiry. Although many questions and controversies remain about marrow adipocytes, multiple conditions/disease states in which alterations occur have provided clues about their function. The consensus is that MAT is associated inversely with bone density and quality. While further investigation is warranted, MAT has clearly been demonstrated as an active dynamic depot that contributes to bone turnover and overall metabolic homeostasis.

Diabetes and Bone Marrow Adiposity

PURPOSE OF REVIEW

This study aims to describe bone marrow fat changes in diabetes and to discuss the potential role of marrow fat in skeletal fragility.

RECENT FINDINGS

Advances in non-invasive imaging have facilitated marrow fat research in humans. In contrast to animal studies which clearly demonstrate higher levels of marrow fat in diabetes, human studies have shown smaller and less certain differences. Marrow fat has been reported to correlate with A1c, and there may be a distinct marrow lipid saturation profile in diabetes. Greater marrow fat is associated with impaired skeletal health. Marrow fat may be a mediator of skeletal fragility in diabetes. Circulating lipids, growth hormone alterations, visceral adiposity, and hypoleptinemia have been associated with greater marrow fat and may represent potential mechanisms for the putative effects of diabetes on marrow fat, although other factors likely contribute. Additional research is needed to further define the role of marrow fat in diabetic skeletal fragility and to determine whether marrow fat is a therapeutic target.

Mechanisms involved in altered bone metabolism in diabetes: a narrative review

Many studies have shown that change in metabolism caused by diabetes can influence the bone metabolism in a way that quality and strength of bone is decreased. A 6 times and 2 times increased risk of fracture is reported in patients with type 1 and type 2 diabetes, respectively. There are several mechanisms by which diabetes can affect the bone. The fact that some of these mechanisms are acting in opposite ways opens the door for debate on pathways by which diabetes affects the bones. On the other hand, bone is not a simple organ that only get influence from other organs, but it is an endocrine organ that by secreting the agents such as osteocalcin, adiponectin and visfatin which can affect the insulin sensitivity and metabolism. In this paper we tried to briefly assess the latest finding in this matter.

A T Cell View of the Bone Marrow

The majority of T cells present in the bone marrow (BM) represent an activated/memory phenotype and most of these, if not all, are circulating T cells. Their lodging in the BM keeps them activated, turning the BM microenvironment into a “memory reservoir.” This article will focus on how T cell activation in the BM results in both direct and indirect effects on the hematopoiesis. The hematopoietic stem cell niche will be presented, with its main components and organization, along with the role played by T lymphocytes in basal and pathologic conditions and their effect on the bone remodeling process. Also discussed herein will be how “normal” bone mass peak is achieved only in the presence of an intact adaptive immune system, with T and B cells playing critical roles in this process. Our main hypothesis is that the partnership between T cells and cells of the BM microenvironment orchestrates numerous processes regulating immunity, hematopoiesis, and bone remodeling.

Expansion of Bone Marrow Adipose Tissue During Caloric Restriction Is Associated With Increased Circulating Glucocorticoids and Not With Hypoleptinemia

Bone marrow adipose tissue (MAT) accounts for up to 70% of bone marrow volume in healthy adults and increases further in clinical conditions of altered skeletal or metabolic function. Perhaps most strikingly, and in stark contrast to white adipose tissue, MAT has been found to increase during caloric restriction (CR) in humans and many other species. Hypoleptinemia may drive MAT expansion during CR but this has not been demonstrated conclusively. Indeed, MAT formation and function are poorly understood; hence, the physiological and pathological roles of MAT remain elusive. We recently revealed that MAT contributes to hyperadiponectinemia and systemic adaptations to CR. To further these observations, we have now performed CR studies in rabbits to determine whether CR affects adiponectin production by MAT. Moderate or extensive CR decreased bone mass, white adipose tissue mass, and circulating leptin but, surprisingly, did not cause hyperadiponectinemia or MAT expansion. Although this unexpected finding limited our subsequent MAT characterization, it demonstrates that during CR, bone loss can occur independently of MAT expansion; increased MAT may be required for hyperadiponectinemia; and hypoleptinemia is not sufficient for MAT expansion. We further investigated this relationship in mice. In females, CR increased MAT without decreasing circulating leptin, suggesting that hypoleptinemia is also not necessary for MAT expansion. Finally, circulating glucocorticoids increased during CR in mice but not rabbits, suggesting that glucocorticoids might drive MAT expansion during CR. These observations provide insights into the causes and consequences of CR-associated MAT expansion, knowledge with potential relevance to health and disease.

Region-specific variation in the properties of skeletal adipocytes reveals regulated and constitutive marrow adipose tissues

Marrow adipose tissue (MAT) accumulates in diverse clinical conditions but remains poorly understood. Here we show region-specific variation in MAT adipocyte development, regulation, size, lipid composition, gene expression and genetic determinants. Early MAT formation in mice is conserved, whereas later development is strain dependent. Proximal, but not distal tibial, MAT is lost with 21-day cold exposure. Rat MAT adipocytes from distal sites have an increased proportion of monounsaturated fatty acids and expression of Scd1/Scd2, Cebpa and Cebpb. Humans also have increased distal marrow fat unsaturation. We define proximal 'regulated' MAT (rMAT) as single adipocytes interspersed with active haematopoiesis, whereas distal 'constitutive' MAT (cMAT) has low haematopoiesis, contains larger adipocytes, develops earlier and remains preserved upon systemic challenges. Loss of rMAT occurs in mice with congenital generalized lipodystrophy type 4, whereas both rMAT and cMAT are preserved in mice with congenital generalized lipodystrophy type 3. Consideration of these MAT subpopulations may be important for future studies linking MAT to bone biology, haematopoiesis and whole-body metabolism.

Bisphosphonate treatment of type I diabetic mice prevents early bone loss but accentuates suppression of bone formation

Type I (T1) diabetes is an autoimmune and metabolic disease associated with bone loss. Previous studies demonstrate that T1-diabetes decreases osteoblast activity and viability. Bisphosphonate therapy, commonly used to treat osteoporosis, is demonstrated to inhibit osteoclast activity as well as osteoblast apoptosis. Therefore, we examined the effect of weekly alendronate treatments on T1-diabetes induced osteoblast apoptosis and bone loss. Bone TUNEL assays identified that alendronate therapy prevents the diabetes-induced osteoblast death observed during early stages of diabetes development. Consistent with this, alendronate treatment for 40 days was able to prevent diabetes-induced trabecular bone loss. Alendronate was also able to reduce marrow adiposity in both control diabetic mice compared to untreated mice. Mechanical testing indicated that 40 days of alendronate treatment increased bone stiffness but decreased the work required for fracture in T1-diabetic and alendronate treated mice. Of concern at this later time point, bone formation rate and osteoblast markers, which were already decreased in diabetic mice, were further suppressed in alendronate-treated diabetic mice. Taken together, our results suggest that short-term alendronate treatment can prevent T1-diabetes-induced bone loss in mice, possibly in part by inhibiting diabetes onset associated osteoblast death, while longer treatment enhanced bone density but at the cost of further suppressing bone formation in diabetic mice.

An update on diabetes related skeletal fragility

There are several mechanisms by which diabetes could affect bone mass and strength. These mechanisms include insulin deficiency; hyperglycemia; the accumulation of advanced glycation end products that may influence collagen characteristics; marrow adiposity and bone inflammation. Furthermore, associated diabetic complications and treatment with thaizolidinediones may also increase risk of fracturing. The following article provides its readers with an update on the latest information pertaining to diabetes related bone skeletal fragility. In the authors' opinion, future studies are needed in order to clarify the impact of different aspects of diabetes metabolism, glycemic control, and specific treatments for diabetes on bone. Given that dual energy x-ray absorptiometry is a poor predictor of bone morbidity in this group of patients, there is a need to explore novel approaches for assessing bone quality. It is important that we develop a better understanding of how diabetes affects bone in order to improve our ability to protect bone health and prevent fractures in the growing population of adults with diabetes.

The bone-fat interface: basic and clinical implications of marrow adiposity

Obesity and osteoporosis are two of the most common chronic disorders of the 21st century. Both are accompanied by significant morbidity. The only place in the mammalian organism where bone and fat lie adjacent to each other is in the bone marrow. Marrow adipose tissue is a dynamic depot that probably exists as both constitutive and regulated compartments. Adipocytes secrete cytokines and adipokines that either stimulate or inhibit adjacent osteoblasts. The relationship of marrow adipose tissue to other fat depots is complex and might play very distinct parts in modulation of metabolic homoeostasis, haemopoiesis, and osteogenesis. Understanding of the relationship between bone and fat cells that arise from the same progenitor within the bone marrow niche provides insight into the pathophysiology of age-related osteoporosis, diabetes, and obesity.

Pathophysiological role of enhanced bone marrow adipogenesis in diabetic complications

Diabetes leads to complications in select organ systems primarily by disrupting the vasculature of the target organs. These complications include both micro- (cardiomyopathy, retinopathy, nephropathy, and neuropathy) and macro-(atherosclerosis) angiopathies. Bone marrow angiopathy is also evident in both experimental models of the disease as well as in human diabetes. In addition to vascular disruption, bone loss and increased marrow adiposity have become hallmarks of the diabetic bone phenotype. Emerging evidence now implicates enhanced marrow adipogenesis and changes to cellular makeup of the marrow in a novel mechanistic link between various secondary complications of diabetes. In this review, we explore the mechanisms of enhanced marrow adipogenesis in diabetes and the link between changes to marrow cellular composition, and disruption and depletion of reparative stem cells.

Effect of the interaction between periodontitis and type 1 diabetes mellitus on alveolar bone, mandibular condyle and tibia

OBJECTIVE

This study examined the effect of the interaction between periodontitis and type 1 diabetes mellitus on alveolar bone, mandibular condyle and tibia in animal models.

MATERIALS AND METHODS

Rats were divided into normal, periodontitis, diabetic and diabetic with periodontitis groups. After injection of streptozotocin to induce diabetes, periodontitis was induced by ligation of both lower-side first molars for 30 days. Alveolar bone loss and trabecular bone volume fraction (BVF) of the mandibular condyle and tibia were estimated via hematoxylin and eosin staining and micro-computed tomography, respectively. Osteoclastogenesis of bone marrow cells isolated from tibia and femur was assayed using tartrate-resistant acid phosphatase staining.

RESULTS

The cemento-enamel junction to the alveolar bone crest distance and ratio of periodontal ligament area in the diabetic with periodontitis group were significantly increased compared to those of the periodontitis group. Mandibular condyle BVF did not differ among groups. The BVF of tibia in the diabetic and diabetic with periodontitis groups was lower than that of the normal and periodontitis groups. Osteoclastogenesis of bone marrow cells in the diabetic groups was higher than that in the non-diabetic groups. However, the BVF of tibia and osteoclastogenesis in the diabetic with periodontitis group were not significantly different than those in the diabetic group.

CONCLUSIONS

Type 1 diabetes mellitus aggravates alveolar bone loss induced by periodontitis, but periodontitis does not alter the mandibular condyle and tibia bone loss induced by diabetes. Alveolar bone, mandibular condyle and tibia may have different responses to bone loss stimuli in the diabetic environment.

Vertebral bone marrow glucose uptake is inversely associated with bone marrow fat in diabetic and healthy pigs: [(18)F]FDG-PET and MRI study

OBJECTIVES

Diabetes induces osteoporosis and during osteoporosis, vertebral bone marrow (VBM) adipose tissue amount increases. The association between this adiposity and bone marrow metabolism is unclear. Here, we compared VBM glucose metabolism and fat content in healthy and diabetic pigs, in vivo, using positron emission tomography (PET), in-phase and out-of-phase magnetic resonance imaging and magnetic resonance proton spectroscopy ((1)H MR spectroscopy).

MATERIALS/METHODS

Eleven pigs (n=11) were used. The intervention group had five diabetic and the control group had six healthy pigs. To measure metabolism, PET-imaging with [(18)F]fluoro-deoxy-glucose ([(18)F]FDG) intravenous tracer was used. 1.5-T MRI with (1)H spectroscopy, in-phase and out-of-phase imaging and chemical TAG analysis of the VBM were performed.

RESULTS

We found a significant inverse correlation between VBM glucose uptake (GU) and VBM fat content (R=-0.800, p<0.01) and TAG concentration assay (R=-0.846, p<0.05). There was a trend, although non-significant, of a linear correlation between VBM (1)H MR spectroscopy and TAG concentration (R=0.661) and (1)H MR spectroscopy and in-phase and out-of-phase MR imaging (R=0.635).

CONCLUSIONS

VBM glucose metabolism coupled with VBM fat content may impact diabetic induced osteoporosis.

The alliance of mesenchymal stem cells, bone, and diabetes

Bone fragility has emerged as a new complication of diabetes. Several mechanisms in diabetes may influence bone homeostasis by impairing the action between osteoblasts, osteoclasts, and osteocytes and/or changing the structural properties of the bone tissue. Some of these mechanisms can potentially alter the fate of mesenchymal stem cells, the initial precursor of the osteoblast. In this review, we describe the main factors that impair bone health in diabetic patients and their clinical impact.

The interrelationship between bone and fat: from cellular see-saw to endocrine reciprocity

The number of mature osteoblasts and marrow adipocytes in bone is influenced by the differentiation of the common mesenchymal progenitor cell towards one phenotype and away from the other. Consequently, factors which promote adipogenesis not only lead to fatty marrow but also inhibit osteoblastogenesis, resulting in decreased osteoblast numbers, diminished bone formation and, potentially, inadequate bone mass and osteoporosis. In addition to osteoblast and bone adipocyte numbers being influenced by this skewing of progenitor cell differentiation towards one phenotype, mature osteoblasts and adipocytes secrete factors which may evoke changes in the cell fate and function of each other. This review examines the endogenous factors, such as PPAR-γ2, Wnt, IGF-1, GH, FGF-2, oestrogen, the GP130 signalling cytokines, vitamin D and glucocorticoids, which regulate the selection between osteoblastogenesis and adipogenesis and the interrelationship between fat and bone. The role of adipokines on bone, such as adiponectin and leptin, as well as adipose-derived oestrogen, is reviewed and the role of bone as an energy regulating endocrine organ is discussed.

Maternal perinatal diet induces developmental programming of bone architecture

Maternal high-fat (HF) diet can alter offspring metabolism via perinatal developmental programming. This study tests the hypothesis that maternal HF diet also induces perinatal programming of offspring bone mass and strength. We compared skeletal acquisition in pups from C57Bl/6J mice fed HF or normal diet from preconception through lactation. Three-week-old male and female pups from HF (HF-N) and normal mothers (N-N) were weaned onto normal diet. Outcomes at 14 and 26 weeks of age included body mass, body composition, whole-body bone mineral content (WBBMC) via peripheral dual-energy X-ray absorptiometry, femoral cortical and trabecular architecture via microcomputed tomography, and glucose tolerance. Female HF-N had normal body mass and glucose tolerance, with lower body fat (%) but higher serum leptin at 14 weeks vs. N-N (P<0.05 for both). WBBMC was 12% lower at 14 weeks and 5% lower at 26 weeks, but trabecular bone volume fraction was 20% higher at 14 weeks in female HF-N vs. N-N (P<0.05 for all). Male HF-N had normal body mass and mildly impaired glucose tolerance, with lower body fat (%) at 14 weeks and lower serum leptin at 26 weeks vs. N-N (P<0.05 for both). Serum insulin was higher at 14 weeks and lower at 26 weeks in HF-N vs. N-N (P<0.05). Trabecular BV/TV was 34% higher and cortical bone area was 6% higher at 14 weeks vs. N-N (P<0.05 for both). These data suggest that maternal HF diet has complex effects on offspring bone, supporting the hypothesis that maternal diet alters postnatal skeletal homeostasis.

Understanding leptin-dependent regulation of skeletal homeostasis

Despite growing evidence for adipose tissue regulation of bone mass, the role of the adipokine leptin in bone remodeling remains controversial. The majority of in vitro studies suggest leptin enhances osteoblastic proliferation and differentiation while inhibiting adipogenic differentiation from marrow stromal cells. Alternatively, some evidence demonstrates either no effect or a pro-apoptotic action of leptin on stromal cells. Similarly, in vivo work has demonstrated both positive and negative effects of leptin on bone mass. Most of the literature supports the idea that leptin suppresses bone mass by acting in the brainstem to reduce serotonin-dependent sympathetic signaling from the ventromedial hypothalamus to bone. However, other studies have found partly or entirely contrasting actions of leptin. Recently one study found a significant effect of surgery alone with intracerebroventricular administration of leptin, a technique crucial for understanding centrally-mediated leptin regulation of bone. Thus, two mainstream hypotheses for the role of leptin on bone emerge: 1) direct regulation through increased osteoblast proliferation and differentiation and 2) indirect suppression of bone formation through a hypothalamic relay. At the present time, it remains unclear whether these effects are relevant in only extreme circumstances (i.e. models with complete deficiency) or play an important homeostatic role in the regulation of peak bone acquisition and skeletal remodeling. Ultimately, determining the actions of leptin on the skeleton will be critical for understanding how the obesity epidemic may be impacting the prevalence of osteoporosis.

Amelioration of type I diabetes-induced osteoporosis by parathyroid hormone is associated with improved osteoblast survival

Type 1 diabetic osteoporosis results from impaired osteoblast activity and death. Therefore, anti-resorptive treatments may not effectively treat bone loss in this patient population. Intermittent parathyroid hormone (PTH) treatment stimulates bone remodeling and increases bone density in healthy subjects. However, PTH effects may be limited in patients with diseases that interfere with its signaling. Here, we examined the ability of 8 and 40 µg/kg intermittent PTH to counteract diabetic bone loss. PTH treatment reduced fat pad mass and blood glucose levels in non-diabetic PTH-treated mice, consistent with PTH-affecting glucose homeostasis. However, PTH treatment did not significantly affect general body parameters, including the blood glucose levels, of type 1 diabetic mice. We found that the high dose of PTH significantly increased tibial trabecular bone density parameters in control and diabetic mice, and the lower dose elevated trabecular bone parameters in diabetic mice. The increased bone density was due to increased mineral apposition and osteoblast surface, all of which are defective in type 1 diabetes. PTH treatment suppressed osteoblast apoptosis in diabetic bone, which could further contribute to the bone-enhancing effects. In addition, PTH treatment (40 µg/kg) reversed preexisting bone loss from diabetes. We conclude that intermittent PTH may increase type 1 diabetic trabecular bone volume through its anabolic effects on osteoblasts.

Human bone marrow adiposity is linked with serum lipid levels not T1-diabetes

Increased marrow adiposity is often associated with bone loss. Little is known about the regulation of marrow adiposity in humans. Marrow adiposity is increased in several mouse models including type I (T1)-diabetic mice, which also display bone loss. However, the impact of metabolic disease on marrow adiposity in humans has yet to be examined. This study measured bone marrow adiposity levels with iterative decomposition of water and fat with echo asymmetry and least-squares estimation magnetic resonance imaging and determined their relationship with T1-diabetes, bone mineral density (BMD), and serum lipid levels. Participants were adult T1-diabetic patients (glycosylated hemoglobin averaging 7.70%±0.4%) and age- and body-mass-index-matched nondiabetic subjects. Consistent with previous reports, serum osteocalcin levels were lower in subjects with T1-diabetes compared to controls (reaching statistical significance in females) and negatively correlated with disease duration (r=-0.50, P<.01). Furthermore, femur neck BMD inversely correlated with diabetes severity (r=-0.417, P<.05). While marrow adiposity was not altered by T1-diabetes, there was a striking positive correlation between vertebral, femur, and tibia marrow adiposity and serum lipid levels (low-density lipoprotein, total cholesterol, cholesterol:high-density lipoprotein ratio, and triglyceride; r≥0.383), reaching a significance of P<.001 in some comparisons. Marrow adiposity also displayed strong intrasubject correlations at multiple bone sites (r≥0.411, P<.05), increased with age (r=0.410, P<.05 at vertebral sites), and was reciprocally related to bone density (r≥-0.378, P<.05). Taken together, our data suggest that marrow adiposity may be an indicator of elevated serum lipid levels and decreased bone density.

Low dose aspirin therapy decreases blood glucose levels but does not prevent type i diabetes-induced bone loss

BACKGROUND

Diabetes is strongly associated with increased fracture risk. During T1-diabetes onset, levels of blood glucose and pro-inflammatory cytokines (including TNFα) are increased. At the same time, levels of osteoblast markers are rapidly decreased and stay decreased 40 days later at which point bone loss is clearly evident. Inflammation is known to suppress bone formation and induce bone loss. Previous co-culture studies indicate that diabetic bone is inflamed and diabetic bone marrow is capable of enhancing osteoblast death in vitro. Here we investigate a commonly used non-steroidal anti-inflammatory drug, aspirin, to prevent T1-diabetic bone loss in vivo.

METHODS

We induced diabetes in 16-week-old male C57BL/6 mice and administered aspirin in the drinking water.

RESULTS

Our results demonstrate that aspirin therapy reduced diabetic mouse non-fasting blood glucose levels to less than 400 mg/dl, but did not prevent trabecular and cortical bone loss. In control mice, aspirin treatment increased bone formation markers but did not affect markers of bone resorption or bone density/volume. In diabetic mice, bone formation markers and bone density/volume are decreased and unaltered by aspirin treatment. Bone resorption markers, however, are increased and 2-way ANOVA analysis demonstrates an interaction between aspirin treatment and diabetes (p<0.007). Aspirin treatment did not prevent the previously reported diabetes-induced marrow adiposity.

CONCLUSION

Taken together, our results suggest that low dose aspirin therapy does not negatively impact bone density in control and diabetic mice, but could potentially increase bone resorption in T1-diabetic mice.

Caspase-2 deficiency protects mice from diabetes-induced marrow adiposity

Type I (T1) diabetes is an autoimmune and metabolic disease associated with bone loss. Bone formation and density are decreased in T1-diabetic mice. Correspondingly, the number of TUNEL positive, dying osteoblasts increases in bones of T1-diabetic mice. Moreover, two known mediators of osteoblast death, TNFα and ROS, are increased in T1-diabetic bone. TNFα and oxidative stress are known to activate caspase-2, a factor involved in the extrinsic apoptotic pathway. Therefore, we investigated the requirement of caspase-2 for diabetes-induced osteoblast death and bone loss. Diabetes was induced in 16-week old C57BL/6 caspase-2 deficient mice and their wild type littermates and markers of osteoblast death, bone formation and resorption, and marrow adiposity were examined. Despite its involvement in extrinsic cell death, deficiency of caspase-2 did not prevent or reduce diabetes-induced osteoblast death as evidenced by a twofold increase in TUNEL positive osteoblasts in both mouse genotypes. Similarly, deficiency of caspase-2 did not prevent T1-diabetes induced bone loss in trabecular bone (BV/TV decreased by 30 and 50%, respectively) and cortical bone (decreased cortical thickness and area with increased marrow area). Interestingly, at this age, differences in bone parameters were not seen between genotypes. However, caspase-2 deficiency attenuated diabetes-induced bone marrow adiposity and adipocyte gene expression. Taken together, our data suggest that caspase-2 deficiency may play a role in promoting marrow adiposity under stress or disease conditions, but it is not required for T1-diabetes induced bone loss.

Why does starvation make bones fat?

Body fat, or adipose tissue, is a crucial energetic buffer against starvation in humans and other mammals, and reserves of white adipose tissue (WAT) rise and fall in parallel with food intake. Much less is known about the function of bone marrow adipose tissue (BMAT), which are fat cells found in bone marrow. BMAT mass actually increases during starvation, even as other fat depots are being mobilized for energy. This review considers several possible reasons for this poorly understood phenomenon. Is BMAT a passive filler that occupies spaces left by dying bone cells, a pathological consequence of suppressed bone formation, or potentially an adaptation for surviving starvation? These possibilities are evaluated in terms of the effects of starvation on the body, particularly the skeleton, and the mechanisms involved in storing and metabolizing BMAT during negative energy balance.

The preventive effects of pulsed electromagnetic fields on diabetic bone loss in streptozotocin-treated rats

The present study was the first report demonstrating that pulsed electromagnetic field (PEMF) could partially prevent bone strength and architecture deterioration and improve the impaired bone formation in streptozotocin-induced diabetic rats. The findings indicated that PEMF might become a potential additive method for inhibiting diabetic osteopenia or osteoporosis.

INTRODUCTION

Diabetes mellitus (DM) can cause various musculoskeletal abnormalities. Optimal therapeutic methods for diabetic bone complication are still lacking. It is essential to develop more effective and safe therapeutic methods for diabetic bone disorders. Pulsed electromagnetic field (PEMF) as an alternative noninvasive method has proven to be effective for treating fracture healing and osteoporosis in non-diabetic conditions. However, the issue about the therapeutic effects of PEMF on diabetic bone complication has not been previously investigated.

METHODS

We herein systematically evaluated the preventive effects of PEMF on diabetic bone loss in streptozotocin-treated rats. Two similar experiments were conducted. In each experiment, 16 diabetic and eight non-diabetic rats were equally assigned to the control, DM, and DM + PEMF group. DM + PEMF group was subjected to daily 8-h PEMF exposure for 8 weeks.

RESULTS

In experiment 1, three-point bending test suggested that PEMF improved the biomechanical quality of diabetic bone tissues, evidenced by increased maximum load, stiffness, and energy absorption. Microcomputed tomography analysis demonstrated that DM-induced bone architecture deterioration was partially reversed by PEMF, evidenced by increased Tb.N, Tb.Th, BV/TV, and Conn.D and reduced Tb.Sp and SMI. Serum OC analysis indicated that PEMF partially prevented DM-induced decrease in bone formation. In experiment 2, no significant difference in the bone resorption marker TRACP5b was observed. These biochemical findings were further supported by the dynamic bone histomorphometric parameters BFR/BS and Oc.N/BS.

CONCLUSIONS

The results demonstrated that PEMF could partially prevent DM-induced bone strength and architecture deterioration and improve the impaired bone formation. PEMF might become a potential additive method for inhibiting diabetic osteoporosis.

Alteration of vitamin D metabolic enzyme expression and calcium transporter abundance in kidney involved in type 1 diabetes-induced bone loss

This study aimed to delineate the mechanism involved in type 1 diabetes-induced bone loss. The results revealed the alteration of vitamin D metabolic enzyme expression and the downregulation of renal calcium transporter abundance in type 1 diabetic mice.

INTRODUCTION

The purpose of this study was to investigate the changes of the expression of vitamin D metabolic enzymes and transcellular calcium-transporting proteins in kidneys from mice with experimentally induced diabetes.

METHODS

Male DBA/2J mice were injected with either vehicle (control) or streptozotocin (STZ) daily for five consecutive days. Bone mineral density was measured by peripheral quantitative computerized tomography, and bone histomorphology was analyzed by Safranin O staining. Real-time PCR and Western blotting were applied to determine the expression of target genes and proteins.

RESULTS

Type 1 diabetes produced high urinary calcium excretion and loss of trabecular bone measured at the proximal metaphysis of the tibia and the distal femur. Bone loss was associated with deterioration of trabecular bone microstructure. Quantified PCR results showed that mRNA expression level in the kidney of diabetic mice for 25-hydroxyvitamin D-24-hydroxylase was downregulated at week 10, while those for 25-hydroxyvitamin D-1α-hydroxylase were upregulated at week 20. In addition, mRNA expression levels for renal transient receptor potential V6, plasma membrane Ca-ATPase (PMCA)1b, and vitamin D receptor (VDR) genes were decreased in STZ-treated mice. Western blot analysis showed that protein expression of PMCA1b and VDR was significantly decreased in kidneys from STZ-treated mice compared to that of controls.

CONCLUSIONS

The limitation in this study is the lack of vitamin D, parathyroid hormone, and phosphorus levels in serum. However, the present study supports the conclusion that the underlying mechanism contributing to type 1 diabetes-associated bone loss may be alterations of vitamin D metabolic enzyme expression and associated decreases in expression of renal calcium transporters.

CCAAT/enhancer binding protein β-deficiency enhances type 1 diabetic bone phenotype by increasing marrow adiposity and bone resorption

Bone loss in type 1 diabetes is accompanied by increased marrow fat, which could directly reduce osteoblast activity or result from altered bone marrow mesenchymal cell lineage selection (adipocyte vs. osteoblast). CCAAT/enhancer binding protein beta (C/EBPβ) is an important regulator of both adipocyte and osteoblast differentiation. C/EBPβ-null mice have delayed bone formation and defective lipid accumulation in brown adipose tissue. To examine the balance of C/EBPβ functions in the diabetic context, we induced type 1 diabetes in C/EBPβ-null (knockout, KO) mice. We found that C/EBPβ deficiency actually enhanced the diabetic bone phenotype. While KO mice had reduced peripheral fat mass compared with wild-type mice, they had 5-fold more marrow adipocytes than diabetic wild-type mice. The enhanced marrow adiposity may be attributed to compensation by C/EBPδ, peroxisome proliferator-activated receptor-γ2, and C/EBPα. Concurrently, we observed reduced bone density. Relative to genotype controls, trabecular bone volume fraction loss was escalated in diabetic KO mice (-48%) compared with changes in diabetic wild-type mice (-22%). Despite greater bone loss, osteoblast markers were not further suppressed in diabetic KO mice. Instead, osteoclast markers were increased in the KO diabetic mice. Thus, C/EBPβ deficiency increases diabetes-induced bone marrow (not peripheral) adipose depot mass, and promotes additional bone loss through stimulating bone resorption. C/EBPβ-deficiency also reduced bone stiffness and diabetes exacerbated this (two-way ANOVA P < 0.02). We conclude that C/EBPβ alone is not responsible for the bone vs. fat phenotype switch observed in T1 diabetes and that suppression of CEBPβ levels may further bone loss and decrease bone stiffness by increasing bone resorption.

The bone marrow microenvironment contributes to type I diabetes induced osteoblast death

Type I diabetes increases an individual's risk for bone loss and fracture, predominantly through suppression of osteoblast activity (bone formation). During diabetes onset, levels of blood glucose and pro-inflammatory cytokines (including tumor necrosis factor α (TNFα)) increased. At the same time, levels of osteoblast markers are rapidly decreased and stay decreased chronically (i.e., 40 days later) at which point bone loss is clearly evident. We hypothesized that early bone marrow inflammation can promote osteoblast death and hence reduced osteoblast markers. Indeed, examination of type I diabetic mouse bones demonstrates a greater than twofold increase in osteoblast TUNEL staining and increased expression of pro-apoptotic factors. Osteoblast death was amplified in both pharmacologic and spontaneous diabetic mouse models. Given the known signaling and inter-relationships between marrow cells and osteoblasts, we examined the role of diabetic marrow in causing the osteoblast death. Co-culture studies demonstrate that compared to control marrow cells, diabetic bone marrow cells increase osteoblast (MC3T3 and bone marrow derived) caspase 3 activity and the ratio of Bax/Bcl-2 expression. Mouse blood glucose levels positively correlated with bone marrow induced osteoblast death and negatively correlated with osteocalcin expression in bone, suggesting a relationship between type I diabetes, bone marrow and osteoblast death. TNF expression was elevated in diabetic marrow (but not co-cultured osteoblasts); therefore, we treated co-cultures with TNFα neutralizing antibodies. The antibody protected osteoblasts from bone marrow induced death. Taken together, our findings implicate the bone marrow microenvironment and TNFα in mediating osteoblast death and contributing to type I diabetic bone loss.

The multiple faces of autoimmune-mediated bone loss

Inflammation perturbs normal bone homeostasis and is known to induce bone loss, as it promotes both local cartilage degradation and local and systemic bone destruction by osteoclasts, as well as inhibits bone formation by osteoblasts. Thus, not surprisingly, inflammatory autoimmune diseases often lead to local and/or general bone loss. However, the mechanisms that target the bone in autoimmune disease are complex and diverse, as they range from a direct attack on the bone and cartilage by the immune cells to indirect consequences of disturbances of the systemic control of bone remodeling. This Review discusses current understanding of the mechanisms of autoimmune-mediated bone loss in view of new insight from two new fields of research: osteoimmunology, which analyzes the direct effect of immune cells on bone, and the integrative metabolism approach, which established the existence of neuroendocrine loops that regulate bone remodeling.

Novel insights into the relationship between diabetes and osteoporosis

Only three decades ago adipose tissue was considered inert, with little relationship to insulin resistance. Similarly, bone has long been thought of purely in its structural context. In the last decade, emerging evidence has revealed important endocrine roles for both bone and adipose tissue. The interaction between these two tissues is remarkable. Bone marrow mesenchymal stem cells give rise to both osteoblasts and adipocytes. Leptin and adiponectin, two adipokines secreted by fat tissue, control energy homeostasis, but also have complex actions on the skeleton. In turn, the activities of bone cells are not limited to their bone remodelling activities but also to modulation of adipose cell sensitivity and insulin secretion. This review will discuss these new insights linking bone remodelling to the control of fat metabolism and the association between diabetes mellitus and osteoporosis.

Apolipoprotein E-dependent inverse regulation of vertebral bone and adipose tissue mass in C57Bl/6 mice: modulation by diet-induced obesity

The long prevailing view that obesity is generally associated with beneficial effects on the skeleton has recently been challenged. Apolipoprotein E (apoE) is known to influence both adipose tissue and bone. The goal of the current study was to examine the impact of apoE on the development of fat mass and bone mass in mice under conditions of diet-induced obesity (DIO). Four week-old male C57BL/6 (WT) and apoE-deficient (apoE(-/-)) mice received a control or a diabetogenic high-fat diet (HFD) for 16 weeks. The control-fed apoE(-/-) animals displayed less total fat mass and higher lumbar trabecular bone volume (BV/TV) than WT controls. When stressed with HFD to induce obesity, apoE(-/-) mice had a lower body weight, lower serum glucose, insulin and leptin levels and accumulated less white adipose tissue mass at all sites including bone marrow. While WT animals showed no significant change in BV/TV and bone formation rate (BFR), apoE deficiency led to a decrease of BV/TV and BFR when stressed with HFD. Bone resorption parameters were not affected by HFD in either genotype. Taken together, under normal dietary conditions, apoE-deficient mice acquire less fat mass and more bone mass than WT littermates. When stressed with HFD to develop DIO, the difference of total body fat mass becomes larger and the difference of bone mass smaller between the genotypes. We conclude that apoE is involved in an inverse regulation of bone mass and fat mass in growing mice and that this effect is modulated by diet-induced obesity.

Caloric restriction leads to high marrow adiposity and low bone mass in growing mice

The effects of caloric restriction (CR) on the skeleton are well studied in adult rodents and include lower cortical bone mass but higher trabecular bone volume. Much less is known about how CR affects bone mass in young, rapidly growing animals. This is an important problem because low caloric intake during skeletal acquisition in humans, as in anorexia nervosa, is associated with low bone mass, increased fracture risk, and osteoporosis in adulthood. To explore this question, we tested the effect of caloric restriction on bone mass and microarchitecture during rapid skeletal growth in young mice. At 3 weeks of age, we weaned male C57Bl/6J mice onto 30% caloric restriction (10% kcal/fat) or normal diet (10% kcal/fat). Outcomes at 6 (n = 4/group) and 12 weeks of age (n = 8/group) included body mass, femur length, serum leptin and insulin-like growth factor 1 (IGF-1) values, whole-body bone mineral density (WBBMD, g/cm(2)), cortical and trabecular bone architecture at the midshaft and distal femur, bone formation and cellularity, and marrow fat measurement. Compared with the normal diet, CR mice had 52% and 88% lower serum leptin and 33% and 39% lower serum IGF-1 at 6 and 12 weeks of age (p < .05 for all). CR mice were smaller, with lower bone mineral density, trabecular, and cortical bone properties. Bone-formation indices were lower, whereas bone-resorption indices were higher (p < .01 for all) in CR versus normal diet mice. Despite having lower percent of body fat, bone marrow adiposity was elevated dramatically in CR versus normal diet mice (p < .05). Thus we conclude that caloric restriction in young, growing mice is associated with impaired skeletal acquisition, low leptin and IGF-1 levels, and high marrow adiposity. These results support the hypothesis that caloric restriction during rapid skeletal growth is deleterious to cortical and trabecular bone mass and architecture, in contrast to potential skeletal benefits of CR in aging animals.

Streptozotocin, type I diabetes severity and bone

As many as 50% of adults with type I (T1) diabetes exhibit bone loss and are at increased risk for fractures. Therapeutic development to prevent bone loss and/or restore lost bone in T1 diabetic patients requires knowledge of the molecular mechanisms accounting for the bone pathology. Because cell culture models alone cannot fully address the systemic/metabolic complexity of T1 diabetes, animal models are critical. A variety of models exist including spontaneous and pharmacologically induced T1 diabetic rodents. In this paper, we discuss the streptozotocin (STZ)-induced T1 diabetic mouse model and examine dose-dependent effects on disease severity and bone. Five daily injections of either 40 or 60 mg/kg STZ induce bone pathologies similar to spontaneously diabetic mouse and rat models and to human T1 diabetic bone pathology. Specifically, bone volume, mineral apposition rate, and osteocalcin serum and tibia messenger RNA levels are decreased. In contrast, bone marrow adiposity and aP2 expression are increased with either dose. However, high-dose STZ caused a more rapid elevation of blood glucose levels and a greater magnitude of change in body mass, fat pad mass, and bone gene expression (osteocalcin, aP2). An increase in cathepsin K and in the ratio of RANKL/OPG was noted in high-dose STZ mice, suggesting the possibility that severe diabetes could increase osteoclast activity, something not seen with lower doses. This may contribute to some of the disparity between existing studies regarding the role of osteoclasts in diabetic bone pathology. Examination of kidney and liver toxicity indicate that the high STZ dose causes some liver inflammation. In summary, the multiple low-dose STZ mouse model exhibits a similar bone phenotype to spontaneous models, has low toxicity, and serves as a useful tool for examining mechanisms of T1 diabetic bone loss.