Switching fat from the periphery to bone marrow: why in Type I diabetes?

Pathophysiology of bone remodelling cycle: Role of immune system and lipids

Osteoporosis is the most common skeletal disease worldwide, characterized by low bone mineral density, resulting in weaker bones, and an increased risk of fragility fractures. The maintenance of bone mass relies on the precise balance between bone synthesis and resorption. The close relationship between the immune and skeletal systems, called "osteoimmunology", was coined to identify these overlapping "scientific worlds", and its function resides in the evaluation of the mutual effects of the skeletal and immune systems at the molecular and cellular levels, in both physiological and pathological states. Lipids play an essential role in skeletal metabolism and bone health. Indeed, bone marrow and its skeletal components demand a dramatic amount of daily energy to control hematopoietic turnover, acquire and maintain bone mass, and actively being involved in whole-body metabolism. Statins, the main therapeutic agents in lowering plasma cholesterol levels, are able to promote osteoblastogenesis and inhibit osteoclastogenesis. This review is meant to provide an updated overview of the pathophysiology of bone remodelling cycle, focusing on the interplay between bone, immune system and lipids. Novel therapeutic strategies for the management of osteoporosis are also discussed.

Unveiling the signal valve specifically tuning the TGF-β1 suppression of osteogenesis: mediation through a SMAD1-SMAD2 complex

BACKGROUND

TGF-β1 is the most abundant cytokine in bone, in which it serves as a vital factor to interdict adipogenesis and osteogenesis of bone marrow-derived mesenchymal stem cells (BM-MSCs). However, how TGF-β1 concurrently manipulates differentiation into these two distinct lineages remains elusive.

METHODS

Treatments with ligands or inhibitors followed by biochemical characterization, reporter assay, quantitative PCR and induced differentiation were applied to MSC line or primary BM-MSCs for signaling dissection. In vivo adipogenesis and ex vivo culture of bone explants were used to verify the functions of different SMAD complexes. Ingenuity Pathway Analysis, and analysis of transcriptomic datasets from human BM-MSCs in combination with hierarchical clustering and STRING assay were used to decipher the interplaying co-repressors. Mouse models of chronic and acute bone loss followed by biochemical assays and micro-computed tomography demonstrated the bone effects when functionally blocking the critical co-repressor HDAC1.

RESULTS

Distinct from the TGF-β1 inhibition on adipogenesis through canonical SMAD2/3 signaling, we clarified that TGF-β1 suppresses osteogenesis by inducing the formation of previously unidentified mixed SMADs mainly composed of SMAD1 and SMAD2, in which SMAD2 recruits more TGF-β1-induced co-repressors including HDAC1, TGIF1 and ATF3, whereas SMAD1 allows directing the whole transcriptional suppression complex to the cis-elements of osteogenic genes. Depletion of the cross-activation to the mixed SMADs dismantled specifically the TGF-β1 suppression on osteogenesis without affecting its inhibition on adipogenesis. Such phenomena can be reproduced via knockdown of co-repressors such as Hdac1 or addition of HDAC1 inhibitors in TGF-β1-treated MSCs. In either the chronic or the acute bone loss model, we demonstrated that the TGF-β signaling was augmented in the bone niche during osteolysis, whereas administration of HDAC1 inhibitors significantly improved bone quality.

CONCLUSION

This study identifies a new signal valve through which TGF-β1 can inhibit osteogenesis specifically. Functional interruption of this valve can tilt the seesaw balance of BM-MSC differentiation towards osteogenesis, highlighting the interplaying co-repressors, such as HDAC1, as promising therapeutic targets to combat diverse degenerative orthopedic diseases.

Lipids in the Bone Marrow: An Evolving Perspective

Because of heavy energy demands to maintain bone homeostasis, the skeletal system is closely tied to whole-body metabolism via neuronal and hormonal mediators. Glucose, amino acids, and fatty acids are the chief fuel sources for bone resident cells during its remodeling. Lipids, which can be mobilized from intracellular depots in the bone marrow, can be a potent source of fatty acids. Thus, while it has been suggested that adipocytes in the bone marrow act as "filler" and are detrimental to skeletal homeostasis, we propose that marrow lipids are, in fact, essential for proper bone functioning. As such, we examine the prevailing evidence regarding the storage, use, and export of lipids within the skeletal niche, including from both in vitro and in vivo model systems. We also highlight the numerous challenges that remain to fully appreciate the relationship of lipid turnover to skeletal homeostasis.

Does Oxidative Stress Play a Role in Altered Characteristics of Diabetic Bone? A Systematic Review

Diabetes mellitus (DM) has been associated with increased bone fracture rates, impaired bone regeneration, delayed bone healing, and depressed osteogenesis. However, the plausible pathogenic mechanisms remain incompletely understood. The aim of the present systematic review was to investigate whether oxidative stress (OS) plays a role in altered characteristics of diabetic bone under in vivo conditions. An electronic search of the MEDLINE (via PubMed) and Embase databases was performed. In vivo animal studies involving DM and providing information regarding assessment of OS markers combined with analyses of bone histology/histomorphometry parameters were selected. A descriptive analysis of selected articles was performed. Ten studies were included in the present review. Both bone formation and bone resorption parameters were significantly decreased in the diabetic groups of animals compared to the healthy groups. This finding was consistent regardless of different animal/bone models employed or different evaluation periods. A statistically significant increase in systemic and/or local OS status was also emphasised in the diabetic groups in comparison to the healthy ones. Markers of OS were associated with histological and/or histomorphometric parameters, including decreased trabecular bone and osteoid volumes, suppressed bone formation, defective bone mineralisation, and reduced osteoclastic activity, in diabetic animals. Additionally, insulin and antioxidative treatment proved to be efficient in reversing the deleterious effects of high glucose and associated OS. The present findings support the hypotheses that OS in the diabetic condition contributes at least partially to defective bone features, and that antioxidative supplementation can be a valuable adjunctive strategy in treating diabetic bone disease, accelerating bone healing, and improving osteointegration.

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.

RAGE supports parathyroid hormone-induced gains in femoral trabecular bone

Parathyroid hormone (PTH) restores bone mass to the osteopenic skeleton, but significant questions remain as to the underlying mechanisms. The receptor for advanced glycation end products (RAGE) is a multiligand receptor of the immunoglobulin superfamily; however, recent studies indicate a role in bone physiology. We investigated the significance of RAGE to hormone-induced increases in bone by treating 10-wk-old female Rage-knockout (KO) and wild-type (WT) mice with human PTH-(1-34) at 30 microg.kg(-1).day(-1) or vehicle control, 7 days/wk, for 7 wk. PTH produced equivalent relative gains in bone mineral density (BMD) and bone mineral content (BMC) throughout the skeleton in both genotypes. PTH-mediated relative increases in cortical area of the midshaft femur were not compromised in the null mice. However, the hormone-induced gain in femoral cancellous bone was significantly attenuated in Rage-KO mice. The loss of RAGE impaired PTH-mediated increases in femoral cancellous bone volume, connectivity density, and trabecular number but did not impact increases in trabecular thickness or decreases in trabecular spacing. Disabling RAGE reduced femoral expression of bone formation genes, but their relative PTH-responsiveness was not impaired. Neutralizing RAGE did not attenuate vertebral cancellous bone response to hormone. Rage-null mice exhibited an attenuated accrual rate of bone mass, with the exception of the spine, and an enhanced accrual rate of fat mass. We conclude that RAGE is necessary for key aspects of the skeleton's response to anabolic PTH. Specifically, RAGE is required for hormone-mediated improvement of femoral trabecular architecture but not intrinsically necessary for increasing cortical thickness.