Sirolimus and tacrolimus rather than cyclosporine A cause bone loss in healthy adult male rats

Local FK506 delivery induces osteogenesis in rat bone defect and rabbit spine fusion models

Bone grafting procedures are commonly used for the repair, regeneration, and fusion of bones in a wide range of orthopaedic surgeries, including large bone defects and spine fusion procedures. Autografts are the clinical gold standard, though recombinant human bone morphogenetic proteins (rhBMPs) are often used, particularly in difficult clinical situations. However, treatment with rhBMPs can have off-target effects and increase surgical costs, adding to patients' already high economic and mental burden. Recent studies have identified that FDA-approved immunosuppressant drug, FK506 (Tacrolimus), can also activate the BMP pathway by binding to its inhibitors. This study tested the hypothesis that FK506, as a standalone treatment, could induce osteogenic differentiation of human mesenchymal stromal cells (hMSCs), as well as functional bone formation in a rat segmental bone defect model and rabbit spinal fusion model. FK506 enhanced osteogenic differentiation and mineralization of hMSCs in vitro. Standalone treatment with FK506 delivered on a collagen sponge produced consistent bone bridging of a critically sized rat femoral defect with functional mechanical properties comparable to naïve bone. In a rabbit single level posterolateral spine fusion model, treatment with FK506 delivered on a collagen sponge successfully fused the L5-L6 vertebrae at rates comparable to rhBMP-2 treatment. These data demonstrate the ability of FK506 to induce bone formation in human cells and two challenging in vivo models, and indicate FK506 can be utilized to treat a variety of spine disorders.

Local FK506 delivery induces osteogenesis in in vivo rat bone defect and rabbit spine fusion models

Bone grafting procedures are commonly used for the repair, regeneration, and fusion of bones in in a wide range of orthopaedic surgeries, including large bone defects and spine fusion procedures. Autografts are the clinical gold standard, though recombinant human bone morphogenetic proteins (rhBMPs) are often used, particularly in difficult clinical situations. However, treatment with rhBMPs can have off-target effects and significantly increase surgical costs, adding to patients' already high economic and mental burden. Recent studies have identified that FDA-approved immunosuppressant drug, FK506 (Tacrolimus), can also activate the BMP pathway by binding to its inhibitors. This study tested the hypothesis that FK506, as a standalone treatment, could induce osteogenic differentiation of human mesenchymal stromal cells (hMSCs), as well as functional bone formation in a rat segmental bone defect model and rabbit spinal fusion model. FK506 potentiated the effect of low dose BMP-2 to enhance osteogenic differentiation and mineralization of hMSCs in vitro. Standalone treatment with FK506 delivered on a collagen sponge, produced consistent bone bridging of a rat critically-sized femoral defect with functional mechanical properties comparable to naïve bone. In a rabbit single level posterolateral spine fusion model, treatment with FK506 delivered on a collagen sponge successfully fused the L5-L6 vertebrae at rates comparable to rhBMP-2 treatment. These data demonstrate the ability of FK506 to induce bone formation in human cells and two challenging in vivo models, and indicate FK506 can be utilized either as a standalone treatment or in conjunction with rhBMP to treat a variety of spine disorders.

RhoA promotes osteoclastogenesis and regulates bone remodeling through mTOR-NFATc1 signaling

BACKGROUND

The cytoskeletal architecture of osteoclasts (OCs) and bone resorption activity must be appropriately controlled for proper bone remodeling, which is associated with osteoporosis. The RhoA protein of GTPase plays a regulatory role in cytoskeletal components and contributes to osteoclast adhesion, podosome positioning, and differentiation. Although osteoclast investigations have traditionally been performed by in vitro analysis, however, the results have been inconsistent, and the significance of RhoA in bone physiology and pathology is still unknown.

METHODS

We generated RhoA knockout mice by specifically deleting RhoA in the osteoclast lineage to understand more about RhoA's involvement in bone remodeling. The function of RhoA in osteoclast differentiation and bone resorption and the mechanisms were assessed using bone marrow macrophages (BMMs) in vitro. The ovariectomized (OVX) mouse model was adopted to examine the pathological effect of RhoA in bone loss.

RESULTS

Conditional deletion of RhoA in the osteoclast lineage causes a severe osteopetrosis phenotype, which is attributable to a bone resorption suppression. Further mechanistic studies suggest that RhoA deficiency suppresses Akt-mTOR-NFATc1 signaling during osteoclast differentiation. Additionally, RhoA activation is consistently related to the significant enhancement the osteoclast activity, which culminates in the development of an osteoporotic bone phenotype. Furthermore, in mice, the absence of RhoA in osteoclast precursors prevented occurring OVX-induced bone loss.

CONCLUSION

RhoA promoted osteoclast development via the Akt-mTOR-NFATc1 signaling pathway, resulting a osteoporosis phenotype, and that manipulating RhoA activity might be a therapeutic strategy for osteoporotic bone loss.

The crosstalk between MYC and mTORC1 during osteoclastogenesis

Osteoclasts are bone-resorbing cells that undergo extensive changes in morphology throughout their differentiation. Altered osteoclast differentiation and activity lead to changes in pathological bone resorption. The mammalian target of rapamycin (mTOR) is a kinase, and aberrant mTOR complex 1 (mTORC1) signaling is associated with altered bone homeostasis. The activation of mTORC1 is biphasically regulated during osteoclastogenesis; however, the mechanism behind mTORC1-mediated regulation of osteoclastogenesis and bone resorption is incompletely understood. Here, we found that MYC coordinates the dynamic regulation of mTORC1 activation during osteoclastogenesis. MYC-deficiency blocked the early activation of mTORC1 and also reversed the decreased activity of mTORC1 at the late stage of osteoclastogenesis. The suppression of mTORC1 activity by rapamycin in mature osteoclasts enhances bone resorption activity despite the indispensable role of high mTORC1 activation in osteoclast formation in both mouse and human cells. Mechanistically, MYC induces Growth arrest and DNA damage-inducible protein (GADD34) expression and suppresses mTORC1 activity at the late phase of osteoclastogenesis. Taken together, our findings identify a MYC-GADD34 axis as an upstream regulator of dynamic mTORC1 activation in osteoclastogenesis and highlight the interplay between MYC and mTORC1 pathways in determining osteoclast activity.

Low bone density, vertebral fracture and FRAX score in kidney transplant recipients: A cross-sectional cohort study

Background

Kidney transplantation (KT) recipients are at increased risk of low bone density (LBD) and fractures. In this retrospective study, we investigated bone mineral density (BMD), vertebral fractures, calculated risk for major osteoporotic fractures (MOF), and hip fractures in the KT recipients.

Patients-method

Patients who completed at least one year after KT were included in the analysis. Demographic, clinical, and laboratory data were recorded. Measurements of BMD were performed by dual-energy X-ray absorptiometry. Vertebral fractures were assessed using semi-quantitative criteria with conventional radiography. The ten-year risk for MOF and hip fracture were calculated using the FRAX@ tool with BMD.

Results

One hundred fifty-three KT recipients were included in the study. The population included 77 women. The mean age at evaluation was 46,5±11,9 years. Seventy-eight (50.9%) patients had normal femoral neck BMD while osteoporosis and osteopenia at the femoral neck were present in 12 (7.8%) and 63 (41.1%) of the patients, respectively. Age at evaluation was the risk factor for LBD (OR 1.057; 95% CI 1.024–1.091; p = 0.001). In female KT recipients, LBD was principally affected by menopausal status whereas in males, mammalian target of rapamycin (mTOR) inhibitor use and lower BMI levels were the risk factors. The prevalent vertebral fracture was found in 43.4% of patients. In multivariate analysis, only steroid use (OR 0.121; 95% CI 0.015–0.988; p = 0.049) was found to be associated with prevalent fracture. Among all KT recipients, 1.9% had a high MOF probability (≥20% risk of fracture), and 23.5% had high hip fracture probability (≥3% risk of hip fracture) according to FRAX.

Conclusion

Exploring the prevalence of LBD and vertebral fracture and the risk factors would help clinicians to modify long-term follow-up strategies. Furthermore, the high hip fracture risk probability in our cohort suggested that there is a need for longitudinal studies to confirm the validity of the FRAX tool in the transplant population.

A collagen-based hydrogel containing tacrolimus for bone tissue engineering

Bone tissue engineering aims to develop bone graft structure that can heal bone defects without using autografts or allografts. The current study was conducted to promote bone regeneration using a collagen type I hydrogel containing tacrolimus. For this purpose, different amounts of tacrolimus (10 μg/ml, 100 μg/ml, and 1000 μg/ml) were loaded into the hydrogel. The resulting drug-loaded hydrogels were characterized for their porosity, swelling capacity, weight loss, drug release, blood compatibility, and cell proliferation (MTT). For functional analysis, the developed hydrogel surrounded by a film made of gelatin and polycaprolactone (PCL) was administrated in the calvarias defect of Wistar rats. The results indicated that the hydrogel has a porosity of 89.2 ± 12.5% and an appropriate swelling, drug release, and blood compatibility behavior. The in vitro results indicated that the collagen hydrogel containing 1000 μg tacrolimus was adequate in terms of cell proliferation. Finally, in vivo studies provided some evidence of the potential of the developed hydrogel for bone healing.

FK506 prevented bone loss in streptozotocin-induced diabetic rats via enhancing osteogenesis and inhibiting adipogenesis

Background

Type 1 diabetes mellitus (DM) is associated with severe osteoporosis, which is still a great challenge in the clinic. This work aimed to investigate the skeletal effects of FK506 in a rat model of streptozocin induced type 1 DM.

Methods

Rats were divided into three groups: control (CTL), DM rats and DM rats treated with FK506. Dual energy X-ray absorption, micro-computed tomography, bone mechanics and bone histology were used for skeletal analysis. Bone marrow adipocytes infiltrations were detected by oil red O stain and H&E stain. In addition, the protein expression of adipocyte-specific makers (PPAR-γ, C/EBP-αβ), osteoblast-specific markers (Runx2, Osterix) and nuclear translocation of β-catenin in femurs were determined by western blot.

Results

In the study, bone mineral density of femurs and lumbar vertebras in diabetic rats were increased after FK506 administration. FK506 treatment resulted in higher cancellous bone volume but had no significant effect on cortical bones in diabetic rats. The ultimate force and work to failure were increased in DM+FK506 group, while they were reduced in the DM group. Compared with the CTL, the infiltration of bone marrow adipocytes was significantly increased in the DM group, which was reduced after the treatment of FK506. Besides, the expression levels of Runx2 and Osterix were up-regulated, and that of PPAR-γ and C/EBP-α were down-regulated in diabetic rats after FK506 treatment. In addition, the nuclear translocation of β-catenin protein levels were increased in diabetic rats after the treatment of FK506.

Conclusions

Our study indicated that FK506 could alleviate bone loss in diabetic rats. This effects could be due to the results of enhancing osteogenesis and inhibiting adipogenesis, which might be regulated by activation the nuclear translocation of β-catenin.

Effects of Cyclosporine, Tacrolimus, and Rapamycin on Osteoblasts

PURPOSE

One factor that can contribute to severe bone loss after transplantation is the direct action of immunosuppressants on bone cells. The aim of this work was to study the effects of cyclosporine (CsA), tacrolimus (FK-506), and rapamycin (RAPA) on the release of three local factors directly implicated in bone-remodeling regulation and apoptosis of human osteoblasts: interleukin (IL)-6, osteoprotegerin, and receptor activator of nuclear factor κβ (RANKL).

BASIC PROCEDURES

Human osteoblasts were obtained from five different patients who underwent orthopedic surgery. These cells were treated with what are considered to be a clinically high dose and an acceptable dose of each immunosuppressant-RAPA 50 ng/mL and 12 ng/mL, FK-506 20 ng/mL and 5 ng/mL, CsA 1000 ng/mL and 250 ng/mL-or vehicle. Apoptotic cell death was quantified using flow cytometry of DNA content in permeabilized, propidium iodide-stained cells. IL-6 was measured using enzyme-linked immunosorbent assay (ELISA; Quantikine Human IL6, R&D Systems, Minneapolis, Minn, United States). Messenger RNA (mRNA) expression of osteoprotegerin, RANKL, and IL-6 was measured using quantitative RT-PCR.

MAIN FINDINGS

A significant increase in IL-6 (mRNA and released protein) was observed in the presence of FK-506 and RAPA. Addition of RAPA to the cultures of osteoblasts produced a significant increase in the OPG/RANKL ratio. A significant increase in osteoblast apoptosis was observed in the cells treated with FK-506 and RAPA 24 hours after the addition of immunosuppressants. CsA did not produce any significant changes in osteoblasts.

PRINCIPAL CONCLUSIONS

These results suggest that an increase in osteoblast apoptosis by osteoblasts may be one of the mechanisms by which bone loss occurs after RAPA and FK-506 treatments.

mTORC1 impedes osteoclast differentiation via calcineurin and NFATc1

Rapamycins are immunosuppressant and anti-cancer drugs that inhibit the kinase mTOR. Clinically, they often cause bone pain, bone necrosis, and high bone turnover, yet the mechanisms are unclear. Here we show that mTORC1 activity is high in osteoclast precursors but downregulated upon RANKL treatment. Loss-of-function genetic models reveal that while early Raptor deletion in hematopoietic stem cells blunts osteoclastogenesis due to compromised proliferation/survival, late Raptor deletion in osteoclast precursors instead augments osteoclastogenesis. Gain-of-function genetic models by TSC1 deletion in HSCs or osteoclast precursors cause constitutive mTORC1 activation, impairing osteoclastogenesis. Pharmacologically, rapamycin treatment at low but clinically relevant doses exacerbates osteoclast differentiation and bone resorption, leading to bone loss. Mechanistically, RANKL inactivates mTORC1 via calcineurin-mediated mTORC1 dephosphorylation, consequently activating NFATc1 by reducing mTORC1-mediated NFATc1 phosphorylation. These findings uncover biphasic roles of mTORC1 in osteoclastogenesis, dosage-dependent effects of rapamycin on bone, and a previously unrecognized calcineurin-mTORC1-NFATc1 phosphorylation-regulatory signaling cascade.

Understanding Bone Damage After Kidney Transplantation: A Retrospective Monocentric Cross Sectional Analysis

BACKGROUND

Kidney transplantation (KT) immunosuppression may induce bone tissue damage with bone mineral density (BMD) loss increasing bone fractures risk. Steroid therapy is considered the major player, but others factors are still under review.

PATIENTS AND METHODS

We designed an observational retrospective cohort study to evaluate bone damage after KT. The prevalence of osteopenia, osteoporosis, bone fractures, and the associated risk factors were investigated. The following parameters were recorded before transplantation and at the last follow-up: demographic indexes, cumulative steroid dose (CSD), dialytic and transplantologic age, previous nephropathy, femoral and lumbar BMD, fractures, immunosuppressors, calcemia, phosphoremia, rejection episodes, estimated glomerular filtration rate, and parathyroid hormone and vitamin D levels. Stata software (Stata Corporation, College Station, Texas, United States) was used for the statistical analysis, to perform the Fisher's exact test, Kruskal-Wallis test, Student t test, as well as univariate and multivariate analyses.

RESULTS

The analyzed cohort was composed of 297 patients (65.3% males and 34.7% females). Sixty percent of KT patients had normal BMD, 24% had osteopenia, and 15% had osteoporosis. Twelve percent were victims of bone fractures (8.4% minor, 2% femoral, and 1.7% vertebral). A significant correlation (P <.05) was observed for both osteopenia and osteoporosis with menopause, transplantologic age, CSD, previous glomerulonephritis, and mammalian target of rapamycin (mTOR) inhibitors treatment (imTOR).

CONCLUSION

This study confirms the correlation between CSD (both before and after transplantation) and post-transplantation bone damage. It also shows that a large fraction of these patients had normal BMD related with a low steroid dose in our protocols. This correlation between imTOR assumption and osteoporosis deserves attention and warrants further in vitro analyses to be performed.