Bone growth during rapamycin therapy in young rats

The autophagy-mediated mechanism via TSC1/mTOR signaling pathway in thiram-induced tibial dyschondroplasia of broilers

Thiram is a member of the dithiocarbamate family and is widely used in agriculture, especially in low-income countries. Its residues lead to various diseases, among which tibial dyschondroplasia (TD) in broiler chickens is the most common. Recent studies have also demonstrated that thiram residues may harm human health. Our previous study showed that the activity of the mTOR (mammalian target of rapamycin) signaling pathway has changed after thiram exposure. In the current study, we investigated the effect of autophagy via the mTOR signaling pathway after thiram exposure in vitro and in vivo. Our results showed that thiram inhibited the protein expression of mTOR signaling pathway-related genes such as p-4EBP1 and p-S6K1. The analysis showed a significant increase in the expression of key autophagy-related proteins, including LC3, ULK1, ATG5, and Beclin1. Further investigation proved that the effects of thiram were mediated through the downregulation of mTOR. The mTOR agonist MHY-1485 reverse the upregulation of autophagy caused by thiram in vitro. Moreover, our experiment using knockdown of TSC1 resulted in chondrocytes expressing lower levels of autophagy. In conclusion, our results demonstrate that thiram promotes autophagy via the mTOR signaling pathway in chondrogenesis, providing a potential pharmacological target for the prevention of TD.

Rapamycin does not alter bone microarchitecture or material properties quality in young-adult and aged female C57BL/6 mice

Advancing age is the strongest risk factor for osteoporosis and skeletal fragility. Rapamycin is an FDA-approved immunosuppressant that inhibits the mechanistic target of rapamycin (mTOR) complex, extends lifespan, and protects against aging-related diseases in multiple species; however, the impact of rapamycin on skeletal tissue is incompletely understood. We evaluated the effects of a short-term, low-dosage, interval rapamycin treatment on bone microarchitecture and strength in young-adult (3 mo old) and aged female (20 mo old) C57BL/6 mice. Rapamycin (2 mg/kg body mass) was administered via intraperitoneal injection 1×/5 d for a duration of 8 wk; this treatment regimen has been shown to induce geroprotective effects while minimizing the side effects associated with higher rapamycin dosages and/or more frequent or prolonged delivery schedules. Aged femurs exhibited lower cancellous bone mineral density, volume, trabecular connectivity density and number, higher trabecular thickness and spacing, and lower cortical thickness compared to young-adult mice. Rapamycin had no impact on assessed microCT parameters. Flexural testing of the femur revealed that both yield strength and ultimate strength were lower in aged mice compared to young-adult mice. There were no effects of rapamycin on these or other measures of bone biomechanics. Age, but not rapamycin, altered local and global measures of bone turnover. These data demonstrate that short-term, low-dosage interval rapamycin treatment does not negatively or positively impact the skeleton of young-adult and aged mice.

Osteogenic effects of rapamycin on bone marrow mesenchymal stem cells via inducing autophagy

BACKGROUND

While autophagy is essential for stem cells' self-renewal and differentiation, its effect on bone marrow mesenchymal stem cells (BMSCs) remains unclear. This study aimed to investigate the interaction between autophagy and osteogenic differentiation using rapamycin (RAPA), a classical autophagy agonist with osteo-regulatory effects.

METHODS

Rat BMSC's autophagy was analyzed after osteoinduction (0, 7, 14, and 21 d) by western blotting, immunofluorescence, and real-time quantitative polymerase chain reaction (RT-qPCR). In addition, we evaluated osteogenic differentiation using alizarin red staining, alkaline phosphatase assays, and RT-qPCR/Western blotting quantification of bone sialoprotein, type 1 collagen, alkaline phosphatase, osteopontin, and Runt-related transcription factor 2 mRNA and protein levels.

RESULTS

The BMSC's basal autophagy level gradually decreased during osteogenic differentiation with a decrease in BECN1 level and the lipidated (LC3-II) to unlipidated (LC3-I) microtubule-associated protein 1 light chain 3 ratio and an increase in the expression of selective autophagic target p62. In contrast, it increased with increasing RAPA concentration. Furthermore, while 2 nM RAPA promoted BMSC osteogenic differentiation on days 7 and 14, 5 nM RAPA inhibited osteogenesis on days 14 and 21. Inhibition of autophagy by the inhibitor 3-methyladenine could impair RAPA's osteogenesis-enhancing effect on BMSCs.

CONCLUSIONS

The BMSC's basal autophagy level decreased over time during osteogenic differentiation. However, an appropriate RAPA concentration promoted BMSC osteogenic differentiation via autophagy activation.

Inflammation produced by senescent osteocytes mediates age-related bone loss

Purpose

The molecular mechanisms of age-related bone loss are unclear and without valid drugs yet. The aims of this study were to explore the molecular changes that occur in bone tissue during age-related bone loss, to further clarify the changes in function, and to predict potential therapeutic drugs.

Methods

We collected bone tissues from children, middle-aged individuals, and elderly people for protein sequencing and compared the three groups of proteins pairwise, and the differentially expressed proteins (DEPs) in each group were analyzed by Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG). K-means cluster analysis was then used to screen out proteins that continuously increased/decreased with age. Canonical signaling pathways that were activated or inhibited in bone tissue along with increasing age were identified by Ingenuity Pathway Analysis (IPA). Prediction of potential drugs was performed using the Connectivity Map (CMap). Finally, DEPs from sequencing were verified by Western blot, and the drug treatment effect was verified by quantitative real-time PCR.

Results

The GO and KEGG analyses show that the DEPs were associated with inflammation and bone formation with aging, and the IPA analysis shows that pathways such as IL-8 signaling and acute-phase response signaling were activated, while glycolysis I and EIF2 signaling were inhibited. A total of nine potential drugs were predicted, with rapamycin ranking the highest. In cellular experiments, rapamycin reduced the senescence phenotype produced by the H₂O₂-stimulated osteocyte-like cell MLO-Y4.

Conclusion

With age, inflammatory pathways are activated in bone tissue, and signals that promote bone formation are inhibited. This study contributes to the understanding of the molecular changes that occur in bone tissue during age-related bone loss and provides evidence that rapamycin is a drug of potential clinical value for this disease. The therapeutic effects of the drug are to be further studied in animals.

A novel adiponectin receptor agonist (AdipoAI) ameliorates type 2 diabetes-associated periodontitis by enhancing autophagy in osteoclasts

BACKGROUND AND OBJECTIVE

Type 2 diabetes (T2D)-associated periodontitis is severe and refractory in many cases. Considered an inflammatory disease, T2D predisposes to periodontitis by increasing whole-body inflammation. One mechanism of increased inflammation is thatT2D is mediated by loss of production or function of the anti-inflammatory hormone adiponectin. In our previous report, AdipoRon, an adiponectin receptor agonist, and AdipoAI, a newly discovered, more specific agonist, attenuated T2D-associated inflammation by inhibiting osteoclastogenesis and LPS-induced endotoxemia. Autophagy plays an important role during osteoclast differentiation and function. The impact of AdipoAI on osteoclast function and autophagy involved in osteoclastogenesis is not known. Here, we compare AdipoRon and AdipoAI potency, side effects and mechanism of action in T2D-associated periodontitis.

METHODS

The RAW 264.7 cell line was used for in vitro studies. We analyzed the potential cytotoxicity of AdipoAI using the CCK-8 assay. The anti-osteoclastogenic potential of AdipoAI was studied by real-time qPCR and tartrate-resistant acid phosphatase staining. The actions of AdipoAI involved in autophagy were tested by real-time qPCR, western blot and immunofluorescence staining. In the diet-induced obesity model of T2D, we investigated the impact of AdipoAI on fasting blood glucose, alveolar bone loss, and gingival inflammation in mice with experimental periodontitis.

RESULTS

AdipoRon inhibited osteoclastogenesis and AdipoAI inhibited osteoclastogenesis at lower doses than AdipoRon without any cytotoxicity. In DIO mice with experimental periodontitis, AdipoAI reduced mouse body weight in 14 days, reducing fasting glucose levels, alveolar bone destruction, osteoclast number along the alveolar bone surface, and decreased the expression of pro-inflammatory factors in periodontal tissues. AdipoAI and AdipoRon also enhanced LC3A/B expression when cultured with RANKL.3-Methyladenine, a known autophagy inhibitor, decreased LC3A/B expression and reversed the inhibition of osteoclastogenesis during AdipoAI treatment.

CONCLUSIONS

Our results demonstrate that AdipoAI ameliorates the severity of T2D-associated periodontitis by enhancing autophagy in osteoclasts at lower doses than AdipoRon without demonstrable side effects. Thus, AdipoAI has pharmaceutical potential for treating diabetes-associated periodontal disease.

Fluoride regulates chondrocyte proliferation and autophagy via PI3K/AKT/mTOR signaling pathway

Fluorine is an essential trace element for human health. However, excessive fluoride intake causes skeletal fluorosis which affects cartilage development. Fluoride inhibited chondrocyte proliferation which is the initial and critical step of endochondral ossification, but the underlying mechanism has not been clearly illustrated. Mammalian target of rapamycin (mTOR) is an important protein kinase which modulates various cellular processes and is believed to be a central regulator of chondrocyte proliferation and autophagy. In this study, we explored the effect of fluoride on the proliferation and autophagy of chondrocytes and the regulatory role of mTOR signaling pathway. Our results suggested that NaF inhibited the protein expressions of proliferating cell nuclear antigen (PCNA) and pS6 in cultured fetal rat tibias. Furthermore, NaF significantly downregulated the expressions of mTOR signaling pathway-related genes, including PI3K, AKT, mTOR, 4EBP1 and S6K1 in mouse ATDC5 chondrogenic cell line. We also found that NaF increased autophagy in ATDC5 cells. The mRNA and protein levels of autophagy-related genes LC3, Beclin1 and p62 were significantly changed after NaF treatment. Further studies demonstrated that MHY1485, a small-molecular mTOR activator, totally reversed fluoride-induced promotion of autophagy. MHY1485 also recovered the downregulation of proliferative chondrocytes markers Sox9 and Type Ⅱ Collagen (Col2a1) induced by fluoride in ATDC5 cells. Taken together, our result demonstrate that fluoride suppressed proliferation and facilitated autophagy via PI3K/AKT/mTOR signaling pathway in chondrogenesis.

Histone methyltransferase SET domain bifurcated 1 negatively regulates parathyroid hormone/parathyroid hormone-related peptide receptor to control chondrocyte proliferation in Meckel's cartilage

OBJECTIVE

The aim of this study is to show that the proliferation of chondrocytes is regulated by SET domain bifurcated 1 (SETDB1) along with the downregulation of parathyroid hormone (PTH)/parathyroid hormone-related peptide (PTHrP) receptor in Meckel's cartilage.

DESIGN

Setdb1 was knocked down or overexpressed in a mouse chondrogenic ATDC5 cells, by transfecting the cells with short interfering RNA against Setdb1 or wild-type Setdb1 expression vector, respectively. Cell proliferation was detected by bromodeoxyuridine incorporation. Setdb1 was conditionally deleted in neural crest cells with Wnt1-Cre (Setdb1 conditional knockout mice). Immunofluorescence staining of paraffin sections of embryonic days 13.5 and 14.5 Setdb1 conditional knockout mice or transfected ATDC5 cells was performed to detect PTH/PTHrP receptor. Protein kinase B (AKT) phosphorylation inhibitor was added to both siRNA-transfected ATDC5 cultures to determine whether AKT activation induces PTH/PTHrP receptor expression after Setdb1 knockdown or vice versa.

RESULTS

Setdb1 knockdown in ATDC5 cells showed increased cell proliferation and parathyroid hormone receptor 1 expression. Contrasting results were observed in the Setdb1-overexpressed wild-type cells. Immunofluorescence staining showed the highly expressed PTH/PTHrP receptor in Setdb1-knocked down ATDC5 cells and in the chondrocytes of Setdb1 conditional knockout embryonic Meckel's cartilage, indicating the negative regulation of SETDB1 on PTH/PTHrP receptor. Strong staining of phosphorylated AKT was observed in Setdb1-knocked down ATDC5 cells. However, the inhibition of AKT phosphorylation significantly reduced both the PTH/PTHrP receptor staining and the Setdb1-knockdown-induced increase in ATDC5 cell proliferation.

CONCLUSIONS

Our findings contribute new insights on SETDB1 function in relation with AKT and PTH/PTHrP receptor during chondrocyte proliferation.

Rapamycin impairs bone accrual in young adult mice independent of Nrf2

Advanced age is the strongest risk factor for osteoporosis. The immunomodulator drug rapamycin extends lifespan in numerous experimental model organisms and is being investigated as a potential therapeutic to slow human aging, but little is known about the effects of rapamycin on bone. We evaluated the impact of rapamycin treatment on bone mass, architecture, and indices of bone turnover in healthy adult (16-20 weeks old at treatment initiation) female wild-type (ICR) and Nrf2^-/- mice, a mouse model of oxidative damage and aging-related disease vulnerability. Rapamycin (4 mg/kg bodyweight) was administered by intraperitoneal injection every other day for 12 weeks. Mice treated with rapamycin exhibited lower femur bone mineral content, bone mineral density, and bone volume compared to vehicle-treated mice. In midshaft femur diaphysis (cortical bone), rapamycin-treated mice had lower cortical volume and thickness, and in the distal femur metaphysis (cancellous bone), rapamycin-treated mice had higher trabecular spacing and lower connectivity density. Mice treated with rapamycin exhibited lower bone volume, bone volume fraction, and trabecular thickness in the 5th lumbar vertebra. Rapamycin-treated mice had lower levels of bone formation in the distal femur metaphysis compared to vehicle-treated mice which occurred co-incidentally with increased serum CTX-1, a marker of global bone resorption. Rapamycin had no impact on tibia inflammatory cytokine gene expression, and we found no independent effects of Nrf2 knockout on bone, nor did we find any interactions between genotype and treatment. These data show that rapamycin may have a negative impact on the skeleton of adult mice that should not be overlooked in the clinical context of its usage as a therapy to retard aging and reduce the incidence of age-related pathologies.

Recent developments in natural and synthetic polymeric drug delivery systems used for the treatment of osteoarthritis

Osteoarthritis (OA), is a common musculoskeletal disorder that will progressively increase in older populations and is expected to be the most dominant cause of disability in the world population by 2030. The progression of OA is controlled by a multi-factorial pathway that has not been completely elucidated and understood yet. However, over the years, research efforts have provided a significant understanding of some of the processes contributing to the progression of OA. Both cartilage and bone degradation processes induce articular cells to produce inflammatory mediators that produce proinflammatory cytokines that block the synthesis of collagen type II and aggrecan, the major components of cartilage. Systemic administration and intraarticular injection of anti-inflammatory agents are the first-line treatments of OA. However, small anti-inflammatory molecules are rapidly cleared from the joint cavity which limits their therapeutic efficacy. To palliate this strong technological drawback, different types of polymeric materials such as microparticles, nanoparticles, and hydrogels, have been examined as drug carriers for the delivery of therapeutic agents to articular joints. The main purpose of this review is to provide a summary of recent developments in natural and synthetic polymeric drug delivery systems for the delivery of anti-inflammatory agents to arthritic joints. Furthermore, this review provides an overview of the design rules that have been proposed so far for the development of drug carriers used in OA therapy. Overall it is difficult to state clearly which polymeric platform is the most efficient one because many advantages and disadvantages could be pointed to both natural and synthetic formulations. That requires further research in the near future.

Overloaded Orthopedic Force Induces Condylar Subchondral Bone Absorption by Stimulating Rat Mesenchymal Stem Cells Differentiating into Osteoclasts via mTOR-Regulated RANKL/OPG Secretion in Osteoblasts

Appropriate orthopedic force led to bone remodeling of mandibular condyle, while overloaded orthopedic force (OOF) induced condylar bone absorption. Bone absorption is ascribed to the imbalanced activities between osteoclasts (OCs) and osteoblasts (OBs), mechanism of which remains unclear. This study aimed to observe the condylar changes induced by OOF by mandible advancement appliance and to further investigate the role of mammalian target of Rapamycin (mTOR) and RANKL/OPG in osteoclastic differentiation of stem cells in vivo and in vitro. In vivo, the results of micro-CT analysis indicated that condylar bone resorption was induced by OOF through mandibular advancement appliance for 2 weeks and worsened time dependently. Morphologically, cartilage thickness was reduced, subchondral cortical bone line appeared not continuous, and subchondral bone exhibited irregular-shaped and owned uneven surface. The bone mineral density (BMD), bone volume/tissue volume (BV/TV), trabecular number (Tb.N), and trabecular thickness (Tb.Th) were decreased accomplished with the increased trabecular separation (Tb.Sp) determined by micro-CT. In addition, based on immunofluorescent labeling, OOF activated both OCs and OBs, but osteoclastogenesis prevailed over osteogenesis. The mTOR activation and ratio of RANKL/OPG in OBs were elevated by OOF. In vitro, the results of western blot and polymerase chain reaction (PCR) consistently suggested that the mTOR and RANKL/OPG ratio were upregulated by overloaded mechanical stretch. Pretreatment with mTOR inhibitor, rapamycin, could attenuate the activation of mTOR and the secretion of RANKL in OBs. Interestingly, based on the Trap staining, the supernatant of OBs exposed to OOF could promote osteoclastic differentiation of mesenchymal stem cells (MSCs), while its role was weakened by inhibition of mTOR in OBs. Collectively, OOF induced condylar bone absorption; in the process, osteoclastogenesis was prominent than osteogenesis. The activation of mTOR and secretion of RANKL/OPG were enhanced by OOF and were involved in promoting MSCs differentiating into OCs.

Autophagy and its significance in periodontal disease

Autophagy is an evolutionarily conserved process essential for cellular homeostasis and human health. As a lysosome-dependent degradation pathway, autophagy acts as a modulator of the pathogenesis of diverse diseases. The relationship between autophagy and oral diseases has been explored in recent years, and there is increasing interest in the role of autophagy in periodontal disease. Periodontal disease is a prevalent chronic inflammatory disorder characterized by the destruction of periodontal tissues. It is initiated through pathogenic bacterial infection and interacts with the host immune defense, leading to inflammation and alveolar bone resorption. In this review, we outline the machinery of autophagy and present an overview of work on the significance of autophagy in regulating pathogen invasion, the immune response, inflammation, and alveolar bone homeostasis of periodontal disease. Existing data provide support for the importance of autophagy as a multi-dimensional regulator in the pathogenesis of periodontal disease and demonstrate the importance of future research on the potential roles of autophagy in periodontal disease.

Bone tissue morphology of rat offspring lactationally exposed to polychlorinated biphenyl 169 and 155

Polychlorinated biphenyls (PCBs) are ubiquitous, persistent, organic pollutants also considered endocrine-disrupting chemicals. Our study examined the effects of lactational exposure to nondioxin-like PCB-155 and/or dioxin-like PCB-169 on longitudinal femur growth at the distal epiphyseal growth plate (EGP) in young rats at three different ages [postnatal days (PNDs) 9, 22, and 42]. After delivery, lactating rats were divided into four groups (PCB-169, PCB-155, PCB-155 + 169, and control) and administered PCBs intraperitoneally. The femurs of offspring were used to estimate growth rate (µm/day), and histomorphometric analysis on the distal femur included the thickness of the EGP and zones of proliferation and hypertrophy with calcification. Stereometry was used to determine trabecular bone volume density. In the PCB-169 and PCB-155 + 169 groups, PCB-169 affected longitudinal bone growth in the early postnatal period by interfering with chondrocytes in the EGP zone of proliferation and, to a lesser extent, the zone of hypertrophy. Morphometric alterations in EGP structure diminished until puberty. A slow growth rate persisted in the PCB-169 group until PND 42, while in the PCB-155 group, a fast growth rate between PNDs 9 to 22 was significantly reduced between PNDs 22 to 42. Sterometric assessment showed decreased trabecular bone volume in the PCB-155 + 169 group compared with that in the control on PND 9 and increased in the PCB-169 group compared with that in the PCB-155 group on PND 42. To summarize, studied PCB congeners exerted congener- and age-dependent effects on femur growth rate and its histomorphometric characteristics.

The Role of Autophagy in Osteoclast Differentiation and Bone Resorption Function

Autophagy is an evolutionary conserved and highly regulated recycling process of cellular wastes. Having a housekeeping role, autophagy through the digestion of domestic cytosolic organelles, proteins, macromolecules, and pathogens, eliminates unnecessary materials and provides nutrients and energy for cell survival and maintenance. The critical role of autophagy and autophagy-related proteins in osteoclast differentiation, bone resorption, and maintenance of bone homeostasis has previously been reported. Increasing evidence reveals that autophagy dysregulation leads to alteration of osteoclast function and enhanced bone loss, which is associated with the onset and progression of osteoporosis. In this review, we briefly consolidate the current state-of-the-art technology regarding the role of autophagy in osteoclast function in both physiologic and pathologic conditions to have a more general view on this issue.

Inflammatory osteolysis is regulated by site-specific ISGylation of the scaffold protein NEMO

Inflammatory osteolysis is governed by exacerbated osteoclastogenesis. Ample evidence points to central role of NF-κB in such pathologic responses, yet the precise mechanisms underpinning specificity of these responses remain unclear. We propose that motifs of the scaffold protein IKKγ/NEMO partly facilitate such functions. As proof-of-principle, we used site-specific mutagenesis to examine the role of NEMO in mediating RANKL-induced signaling in mouse bone marrow macrophages, known as osteoclast precursors. We identified lysine (K)270 as a target regulating RANKL signaling as K270A substitution results in exuberant osteoclastogenesis in vitro and murine inflammatory osteolysis in vivo. Mechanistically, we discovered that K270A mutation disrupts autophagy, stabilizes NEMO, and elevates inflammatory burden. Specifically, K270A directly or indirectly hinders binding of NEMO to ISG15, a ubiquitin-like protein, which we show targets the modified proteins to autophagy-mediated lysosomal degradation. Taken together, our findings suggest that NEMO serves as a toolkit to fine-tune specific signals in physiologic and pathologic conditions.

The human skeleton contains over 200 bones that together act as an internal framework for the body. Over our lifetime, the body constantly removes older bone tissue from the skeleton and replaces it with new bone tissue. This “bone remodeling” also controls how bones are repaired after being damaged by injuries, disease or normal wear and tear.

Cells known as osteoclasts are responsible for breaking down old bone tissue and participate in repairing damaged bone. A cellular pathway known as NF-kB signaling stimulates other cells called “bone marrow macrophages” to become osteoclasts. A certain level of NF-kB signaling is required to maintain a healthy skeleton. However, under certain inflammatory conditions, the level of NF-kB signaling becomes too high causing hyperactive osteoclasts to accumulate and inflict severe bone breakdown. This abnormal osteoclast activity leads to eroded and fragile bones and joints, as is the case in diseases such as rheumatoid arthritis and osteoporosis.

Previous studies have shown that a protein called NEMO is a core component of the NF-kB signal pathway, but the precise role of NEMO in the diseased response remained unclear. Adapala, Swarnkar, Arra et al. have now used site-directed mutagenesis approach to study the role of NEMO in bone marrow macrophages in mice. The experiments showed that one specific site within the NEMO protein, referred to as lysine 270, is crucial for its role in controlling osteoclasts and the breakdown of bone tissue. Mutating NEMO at lysine 270 led to uncontrolled NF-kB signaling in the bone marrow macrophages. Further experiments showed that lysine 270 served as a sensor to allow NEMO to bind another protein called ISG15, which in turn helped to decrease NF-kB signaling and slow down the erosion of the bone.

These findings suggest that site-specific targeting of NEMO, rather than inhibiting the whole NF-kB pathway, may help to reduce the symptoms of bone disease while maintaining the beneficial roles of this essential pathway. However, additional research is required to identify NEMO sites responsible for controlling the inflammatory component.

Autophagy plays an essential role in bone homeostasis

Autophagy is very critical for multiple cellular processes. Autophagy plays a critical role in bone cell differentiation and function.

Redox control of chondrocyte differentiation and chondrogenesis

Chondrogenesis involves the recruitment and migration of mesenchymal cells, mesenchymal condensation, and chondrocyte differentiation and hypertrophy. Multiple factors precisely regulate chondrogenesis. Recent studies have demonstrated that the redox status of chondrocytes plays an essential role in the regulation of chondrocyte differentiation and chondrogenesis. Reactive oxygen species (ROS) and reactive nitrogen species (RNS) are important factors that change the intracellular redox status. Physiological levels of ROS/RNS act as intracellular signals in chondrocytes, and oxidative stress impairs the metabolism of chondrocytes. Under physiological conditions, the balance between ROS/RNS production and elimination ensures that redox-sensitive signalling proteins function correctly. The redox homeostasis of chondrocytes ensures that they respond appropriately to endogenous and exogenous stimuli. This review focuses on the redox regulation of key signalling pathways and transcription factors that control chondrogenesis and chondrocyte differentiation. Additionally, the mechanism by which ROS/RNS regulate signalling proteins and transcription factors in chondrocytes is also reviewed.

The Autophagy in Osteoimmonology: Self-Eating, Maintenance, and Beyond

It has been long realized that the immune and skeletal systems are closely linked. This crosstalk, also known as osteoimmunology, is a primary process required for bone health. For example, the immune system acts as a key regulator in osteoclasts-osteoblasts coupling to maintain the balanced bone remodeling. Osteoimmunology is achieved through many cellular and molecular processes, among which autophagy has recently been found to play an indispensable role. Autophagy is a highly conserved process in eukaryotic cells, by which the cytoplasm components such as dysfunctional organelles are degraded through lysosomes and then returned to the cytosol for reuse. Autophagy is present in all cells at basal levels to maintain homeostasis and to promote cell survival in response to cellular stress conditions such as nutrition deprivation and hypoxia. Autophagy is a required process in immune cell activation/polarization and osteoclast differentiation, which protecting cells from oxidative stress. The essential of autophagy in osteogenesis is its involvement in osteoblast differentiation and mineralization, especially the role of autophagosome in extracellular calcium transportation. The modulatory feature of autophagy in both immune and skeleton systems suggests its crucial roles in osteoimmunology. Furthermore, autophagy also participates in the maintenance of bone marrow hematopoietic stem cell niche. The focus of this review is to highlight the role of autophagy in the immune-skeleton interactions and the effects on bone physiology, as well as the future application in translational research.

Rapamycin promotes osteogenesis under inflammatory conditions

Chronic periodontitis, a common oral disease, usually results in irreversible bone resorption. Bone regeneration is a complex process between bone-forming activity of osteoblasts and bone-resorbing activity of osteoclasts, and still remains a challenge for physicians clinically. A previous study demonstrated that the mechanistic target of rapamycin signaling pathway is involved in osteogenic differentiation of mesenchymal stromal cells. Herein, whether rapamycin could be used to induce osteogenic differentiation of primary bone marrow-derived mesenchymal stem cells (BMSCs) in vitro and promote new bone formation in vivo were evaluated. The results demonstrated that rapamycin alone was not enough to fully induce osteoblast differentiation in vitro and enhanced bone regeneration in vivo. Interestingly, rapamycin in rapamycin plus lipopolysaccharide (LPS)-treated BMSCs significantly increased the gene expression levels of Sp7 transcription factor, runt related transcription factor 2, alkaline phosphatase (ALP) and collagen I (Col I), ALP activity, and calcium nodule at different time points in vitro, indicating that osteoblast differentiation occurs by rapamycin when BMSCs are exposed to LPS simultaneously. It was also demonstrated that rapamycin in rapamycin plus LPS-treated rats promoted bone regeneration in vivo. These results suggest that rapamycin may influence osteoblast differentiation and new bone formation after LPS induces an inflammatory environment. Rapamycin may be used to treat periodontitis associated with bone loss in future clinical practice.

Mammalian target of rapamycin as a therapeutic target in osteoporosis

The mechanistic target of rapamycin (mTOR) plays a key role in sensing and integrating large amounts of environmental cues to regulate organismal growth, homeostasis, and many major cellular processes. Recently, mounting evidences highlight its roles in regulating bone homeostasis, which sheds light on the pathogenesis of osteoporosis. The activation/inhibition of mTOR signaling is reported to positively/negatively regulate bone marrow mesenchymal stem cells (BMSCs)/osteoblasts-mediated bone formation, adipogenic differentiation, osteocytes homeostasis, and osteoclasts-mediated bone resorption, which result in the changes of bone homeostasis, thereby resulting in or protect against osteoporosis. Given the likely importance of mTOR signaling in the pathogenesis of osteoporosis, here we discuss the detailed mechanisms in mTOR machinery and its association with osteoporosis therapy.

mTORC1 Inhibits NF-κB/NFATc1 Signaling and Prevents Osteoclast Precursor Differentiation, In Vitro and In Mice

The mechanistic target of rapamycin complex 1 (mTORC1) is a critical sensor for bone homeostasis and bone formation; however, the role of mTORC1 in osteoclast development and the underlying mechanisms have not yet been fully established. Here, we found that mTORC1 activity declined during osteoclast precursors differentiation in vitro and in vivo. We further targeted deletion of Raptor (mTORC1 key component) or Tsc1 (mTORC1 negative regulator) to constitutively inhibit or activate mTORC1 in osteoclast precursors (monocytes/macrophages), using LyzM-cre mice. Osteoclastic formation was drastically increased in cultures of Raptor deficient bone marrow monocytes/macrophages (BMMs), and Raptor-deficient mice displayed osteopenia with enhanced osteoclastogenesis. Conversely, BMMs lacking Tsc1 exhibited a severe defect in osteoclast-like differentiation and absorptive function, both of which were restored following rapamycin treatment. Importantly, expression of nuclear factor κ-light-chain-enhancer of activated B cells (NF-κB) and nuclear factor of activated T cells, cytoplasmic 1 (NFATc1), transcription factors that are essential for osteoclast differentiation was negatively regulated by mTORC1 in osteoclast lineages. These results provide evidence that mTORC1 plays as a critical role as an osteoclastic differentiation-limiting signal and suggest a potential drawback in treating bone loss-related diseases with mTOR inhibitors clinically. © 2017 American Society for Bone and Mineral Research.

Altered paracrine signaling from the injured knee joint impairs postnatal long bone growth

Regulation of organ growth is a poorly understood process. In the long bones, the growth plates (GPs) drive elongation by generating a scaffold progressively replaced by bone. Although studies have focused on intrinsic GP regulation, classic and recent experiments suggest that local signals also modulate GP function. We devised a genetic mouse model to study extrinsic long bone growth modulation, in which injury is specifically induced in the left hindlimb, such that the right hindlimb serves as an internal control. Remarkably, when only mesenchyme cells surrounding postnatal GPs were killed, left bone growth was nevertheless reduced. GP signaling was impaired by altered paracrine signals from the knee joint, including activation of the injury response and, in neonates, dampened IGF1 production. Importantly, only the combined prevention of both responses rescued neonatal growth. Thus, we identified signals from the knee joint that modulate bone growth and could underlie establishment of body proportions.

DOI:http://dx.doi.org/10.7554/eLife.27210.001

As bones grow, their size is carefully controlled and coordinated with the growth of the other organs in the body. The mechanisms that control organ size also help the body to recover from injury, and play a key role in controlling body size and proportions. Over the course of evolution, these mechanisms have likely changed to produce the distinct body sizes and proportions seen in humans and other animals.

Despite their importance, it is not well understood how signals from both inside and outside an organ work together to regulate its size. In growth disorders this signaling goes wrong, which can lead to a person having unusual proportions such as a very short stature or having one leg shorter than the other. Currently, most growth disorders that affect leg proportions are treated with painful surgical procedures. Researchers would like to know how bone growth is affected by signals from the surrounding tissues because this could help them to develop new non-invasive treatments for these conditions.

Long bones, for example those in the leg, grow from structures near their ends called growth plates. Roselló-Díez et al. have now engineered mice in which an injury shortly after birth caused cells in the knee in the rear left leg to die off. At the same time, the rear right leg of the mice developed as normal, allowing the growth of the two legs to be compared. Roselló-Díez et al. found that the left leg of these mice grew more slowly than the right leg, even though none of the cells in the growth plate of the left leg bone had been damaged. Further investigation revealed that this was because the injury caused an imbalance between the growth-promoting and growth-restricting signals that are produced by the fat pad and articular cartilage in the knee joint. Restoring the lost balance allowed the left leg bone to grow to a more normal length.

In the future, boosting bone growth signals might provide a way to treat conditions like dwarfism or leg-length discrepancies. Understanding how different tissues influence body proportions could also help researchers to investigate how different animals evolved different body proportions.

DOI:http://dx.doi.org/10.7554/eLife.27210.002

Autophagy mediated CoCrMo particle-induced peri-implant osteolysis by promoting osteoblast apoptosis

Wear particle-induced osteolysis is the leading cause of aseptic loosening, which is the most common reason for THA (total hip arthroplasty) failure and revision surgery. Although existing studies suggest that osteoblast apoptosis induced by wear debris is involved in aseptic loosening, the underlying mechanism linking wear particles to osteoblast apoptosis remains almost totally unknown. In the present study, we investigated the effect of autophagy on osteoblast apoptosis induced by CoCrMo metal particles (CoPs) in vitro and in a calvarial resorption animal model. Our study demonstrated that CoPs stimulated autophagy in osteoblasts and PIO (particle-induced osteolysis) animal models. Both autophagy inhibitor 3-MA (3-methyladenine) and siRNA of Atg5 could dramatically reduce CoPs-induced apoptosis in osteoblasts. Further, inhibition of autophagy with 3-MA ameliorated the severity of osteolysis in PIO animal models. Moreover, 3-MA also prevented osteoblast apoptosis in an antiautophagic way when tested in PIO model. Collectively, these results suggest that autophagy plays a key role in CoPs-induced osteolysis and that targeting autophagy-related pathways may represent a potential therapeutic approach for treating particle-induced peri-implant osteolysis.

Implications of the Interaction Between miRNAs and Autophagy in Osteoporosis

Imbalances between bone formation and resorption are the primary cause of osteoporosis. However, currently, a detailed molecular mechanism of osteoporosis is not available. Autophagy is the conserved process characterized by degrading and recycling aggregated proteins, intracellular pathogens, and damaged organelles. MicroRNAs (miRNAs) are novel regulatory factors that play important roles in numerous cellular processes, including autophagy, through the posttranscriptional regulation of gene expression. Conversely, autophagy plays a role in the regulation of miRNA homeostasis. Recent advances have revealed that both autophagy and miRNAs are involved in the maintenance of bone homoeostasis, whereas the role of the interaction of miRNAs with autophagy in osteoporosis remains unclear. In this paper, we review previous reports on autophagy, miRNAs, and their interaction in osteoporosis.

mTORC1 regulates PTHrP to coordinate chondrocyte growth, proliferation and differentiation

Precise coordination of cell growth, proliferation and differentiation is essential for the development of multicellular organisms. Here, we report that although the mechanistic target of rapamycin complex 1 (mTORC1) activity is required for chondrocyte growth and proliferation, its inactivation is essential for chondrocyte differentiation. Hyperactivation of mTORC1 via TSC1 gene deletion in chondrocytes causes uncoupling of the normal proliferation and differentiation programme within the growth plate, resulting in uncontrolled cell proliferation, and blockage of differentiation and chondrodysplasia in mice. Rapamycin promotes chondrocyte differentiation and restores these defects in mutant mice. Mechanistically, mTORC1 downstream kinase S6K1 interacts with and phosphorylates Gli2, and releases Gli2 from SuFu binding, resulting in nuclear translocation of Gli2 and transcription of parathyroid hormone-related peptide (PTHrP), a key regulator of bone development. Our findings demonstrate that dynamically controlled mTORC1 activity is crucial to coordinate chondrocyte proliferation and differentiation partially through regulating Gli2/PTHrP during endochondral bone development.

Effectors of mTOR-autophagy pathway: targeting cancer, affecting the skeleton

Although some modulators of autophagy are emerging as drugs or supplements for anti-cancer therapy, the effects of these compounds on normal tissues must be examined carefully. Here, I review the role of autophagy in skeletal tissues in this context. First, I briefly review preclinical studies indicating the role of autophagy in cancer, as well as related on-going clinical trials. Thereafter, the role of autophagy in the physiology of skeletal tissues is discussed, with a focus on recent genetic preclinical studies. Specifically, I discuss the mTOR-autophagy pathway in relationship to epiphyseal chondrocytes, articular chondrocytes, osteoblasts, osteocytes and osteoclasts and potential side effects of targeting either mTOR pathway or autophagy in general in connection with anti-cancer therapy. Current preclinical findings indicate that inhibiting autophagy will not seriously reduce bone mass and enhance osteoporosis. However, inhibition of autophagy might damage articular cartilage and cause osteoarthritis, whereas treatment with rapalogs might result in relatively beneficial effects on articular cartilage. Modulation of the mTOR pathway or autophagy during childhood may have an undesirable influence on adult height, as well as acquisition of bone mass.

P62: An emerging oncotarget for osteolytic metastasis

Bone metastasis occurs in the majority of late-stage tumors with poor prognosis. It is mainly classified as osteoblastic metastasis and osteolytic metastasis. The pathogenesis of osteolytic metastasis is a “vicious cycle” between tumor cells and bone cells (primarily the osteoclasts), which is mediated by secretory factors. The P62 adapter protein is a versatile multitasker between tumor cells and bone cells. The overexpression of P62 has been detected among a variety of tumors, playing positive roles in both tumorigenesis and metastasis. Moreover, P62 is an important modulator of the osteoclastogenesis pathway. Therefore, the ability of P62 to modulate tumors and osteoclasts suggests that it may be a feasible oncotarget for bone metastasis, especially for osteolytic metastasis. Recent research has shown that a P62 DNA vaccine triggered effective anti-tumor, anti-metastatic and anti-osteoporotic activities. Growing lines of evidence point to P62 as an emerging oncotarget for osteolytic metastasis. In this review, we outline the different roles of P62 in tumor cells and osteoclasts, focusing on the P62-related signaling pathway in key steps of osteolytic metastasis, including tumorigenesis, metastasis and osteoclastogenesis. Finally, we discuss the newest observations on P62 as an oncotarget for osteolytic metastasis treatment.

17β-estradiol activates mTOR in chondrocytes by AKT-dependent and AKT-independent signaling pathways

To confirm whether 17β-estradiol (E2) activates mammalian target of rapamycin (mTOR) signaling pathway in chondrocytes and in what way activates mTOR. Human immortalized chondrocytes cell lines TC28a2 and C28/I2 were subjected to incubate with or without E2, LY294002 (the inhibitor of PI3K), rapamycin (the inhibitor of mTOR), or E2 in combination with LY294002 or rapamycin. Thereafter, protein levels of S6K1, p-S6K1, protein kinase B (AKT), and p-AKT were determined by Western blot analysis. Matrix metallopeptidase (MMP) 3 or MMP13 mRNA levels were evaluated by quantitative real-time PCR (qRT-PCR). Co-immunoprecipitation and Western blot analysis were performed to verify the interaction between ERα and mTOR. Both p-S6K1 and p-AKT protein levels in TC28a2 and C28/I2E2 cells were significantly increased by incubation with E2 (0.5 h and 1 h) (P < 0.05). Rapamycin did not affect the levels of p-AKT, but were significantly reduced by LY294002 or E2 in combination with LY294002. The levels of p-S6K1 were significantly decreased by incubation with LY294002, but the effect could be reversed by E2 in combination with LY294002. Rabbit anti-mTOR antibody was able to immunoprecipitate ERα after incubation with E2. Moreover, E2 inhibited the mRNA levels of MMP3 and MMP13 by mTOR pathway. E2 actives mTOR in chondrocytes through AKT-dependent and independent ways.

Bone and skeletal muscle: Key players in mechanotransduction and potential overlapping mechanisms

The development and maintenance of skeletal muscle and bone mass is critical for movement, health and issues associated with the quality of life. Skeletal muscle and bone mass are regulated by a variety of factors that include changes in mechanical loading. Moreover, bone mass is, in large part, regulated by muscle-derived mechanical forces and thus by changes in muscle mass/strength. A thorough understanding of the cellular mechanism(s) responsible for mechanotransduction in bone and skeletal muscle is essential for the development of effective exercise and pharmaceutical strategies aimed at increasing, and/or preventing the loss of, mass in these tissues. Thus, in this review we will attempt to summarize the current evidence for the major molecular mechanisms involved in mechanotransduction in skeletal muscle and bone. By examining the differences and similarities in mechanotransduction between these two tissues, it is hoped that this review will stimulate new insights and ideas for future research and promote collaboration between bone and muscle biologists.(1).

Syringetin suppresses osteoclastogenesis mediated by osteoblasts in human lung adenocarcinoma

Bone metastasis in lung cancer results in an unfavorable outcome for patients by not only impairing the quality of life, yet also increasing the cancer-related death rates. In the present study, we discuss a novel treatment strategy that may benefit these patients. Human CD14+ monocytes treated with macrophage-colony stimulating factor (M-CSF)/receptor activator of nuclear factor κB ligand (RANKL) differentiated into osteoclasts, whereas syringetin (SGN), a flavonoid derivative found in both grapes and wine, suppressed the osteoclastogenesis in vitro in a dose-dependent manner. In addition, SGN inhibited osteoclast formation induced by human lung adenocarcinoma A549 and CL1-5 cells. The associated signaling transduction pathway in osteoclastogenesis and SGN inhibition was found to be via the AKT/mammalian target of rapamycin (mTOR) signaling pathway. Blocking AKT and mTOR by respective inhibitors significantly decreased lung adenocarcinoma-mediated osteoclastogenesis. Moreover, SGN regulated the lung adenocarcinoma-mediated interaction between osteoblasts and osteoclasts by suppressing the stimulatory effect of lung adenocarcinoma on M-CSF and RANKL production in osteoblasts, and reversing the inhibitory effect of the lung adenocarcinoma on OPG production in osteoblasts. The present study has two novel findings. It is the first to illustrate lung adenocarcinoma-mediated interaction between osteoblasts and osteoclasts, leading to osteolytic bone metastasis. It also reveals that SGN, a flavonoid derivative, directly inhibits osteoclastogenesis and reverses lung adenocarcinoma-mediated osteoclastogenesis. In conclusion, the present study suggests that SGN, a natural compound, prevents and treats bone metastasis in patients with lung cancer.

mTOR: a potential therapeutic target in osteoarthritis?

Osteoarthritis (OA) is a chronic degenerative joint disease characterized by the progressive loss of articular cartilage, remodeling of the subchondral bone, and synovial inflammation. Mammalian target of rapamycin (mTOR) is a serine/threonine protein kinase that controls critical cellular processes such as growth, proliferation, and protein synthesis. Recent studies suggest that mTOR plays a vital role in cartilage growth and development and in altering the articular cartilage homeostasis as well as contributing to the process of cartilage degeneration associated with OA. Both pharmacological inhibition and genetic deletion of mTOR have been shown to reduce the severity of OA in preclinical mouse models. In this review article, we discuss the roles of mTOR in cartilage development, in maintaining articular cartilage homeostasis, and its potential as an OA therapeutic target.

Wedelolactone inhibits breast cancer-induced osteoclastogenesis by decreasing Akt/mTOR signaling

The bone is the most common metastatic site of breast cancer. Bone metastasis causes pain, pathologic fractures, and severely reduces the quality of life. Breast cancer causes osteolytic bone metastasis, which is dependent on osteoclast-mediated bone resorption. While current treatments rely on palliative anti-resorptive agents, there is a need to develop a drug based on potential alternative therapies. This study is the first to determine that wedelolactone (WDL), a natural coumarin isolated from plants, can inhibit breast cancer-mediated osteoclastogenesis. Osteoclasts were generated from human CD14(+) monocytes cultured with M-CSF/RANKL and WDL suppressed human osteoclast differentiation and activity in vitro in a dose-dependent manner. Moreover, WDL inhibited the upregulation of osteoclasts stimulated by MDA‑MB‑231 breast cancer cells. The activity of WDL on osteoclasts and breast cancer-mediated osteoclastogenesis was associated with the inhibition of Akt/mammalian target of the rapamycin signaling pathway (mTOR). Blocking Akt and mTOR by specific inhibitors significantly decreased osteoclast differentiation and bone resorption. Furthermore, WDL regulated breast cancer-enhanced interaction of osteoblasts and osteoclasts by decreasing M-CSF expression in MDA‑MB‑231-stimulated osteoblasts. Thus, this study suggests that WDL may be a potential natural agent for preventing and treating bone destruction in patients with bone metastasis due to breast cancer.

mTOR inhibitors in pediatric kidney transplantation

The mammalian target of the rapamycin (mTOR) inhibitors sirolimus and everolimus are increasingly being used in pediatric kidney transplantation in different combinations and doses. Several studies have shown beneficial effects of using mTOR inhibitors in children after pediatric renal transplantation. A switch to a low-dose calcineurin inhibitor (CNI) and mTOR inhibitor has been proven to stabilize the glomerular filtration rate. Additionally, de novo studies using a low-dose CNI and an mTOR inhibitor have shown good graft survival and a low number of rejections. Side effects of mTOR inhibitors, such as hyperlipidemia, wound healing problems, and proteinuria, mainly occur if high doses are given and if treatment is not combined with a CNI. Lower doses of mTOR inhibitors do not result in growth impairment or reduced testosterone levels. Treatment with mTOR inhibitors is also associated with a lower number of viral infections, especially cytomegalovirus. Due to their antiproliferative effect, mTOR inhibitors could theoretically reduce the risk of post-transplant lymphoproliferative disease. mTOR inhibitors, especially in combination with low-dose CNIs, can safely be used in children after kidney transplantation as de novo therapy or for conversion from CNI- and mycophenolate mofetil-based regimens.

Longitudinal growth on an everolimus- versus an MMF-based steroid-free immunosuppressive regimen in paediatric renal transplant recipients

Concerns have been raised that mammalian target of rapamycin inhibitors in pediatric transplant recipients might interfere with longitudinal bone growth by inhibition of growth factor signaling and growth plate chondrocyte proliferation. We therefore undertook a prospective nested, case-control study on longitudinal growth over 2 years in steroid-free pediatric renal transplant recipients. Fourteen patients on a steroid-free maintenance immunosuppressive regimen consisting of low-dose everolimus (EVR) in conjunction with low-dose cyclosporine (CsA) were compared to a matched cohort of 14 steroid-free patients on a standard dose mycophenolate mofetil (MMF) regimen in conjunction with a standard dose calcineurin inhibitor (CNI). The mean change in height standard deviation (SD) score in the first study year was 0.31 ± 0.71 SD score in the EVR group compared to 0.31 ± 0.64 SD score in the MMF group (P = 0.20). For the entire study period of 2 years, the change in height SD score in the EVR group was 0.43 ± 0.81 SDS compared to 0.75 ± 0.85 SDS in the MMF group (P = 0.32). The percentage of prepubertal patients experiencing catch-up growth, defined as an increase in height SD score ≥0.5 in 2 years, was similar in the EVR group (5/8, 65%) and the MMF group (6/8, 75%; P = 1.00). Longitudinal growth over 2 years in steroid-free pediatric patients on low-dose EVR and CsA is not different to that of a matched steroid-free control group on an immunosuppressive regimen with standard-dose CNI and MMF. Hence, low-dose EVR does not appear to negatively impact short-term growth in pediatric renal transplant recipients.

Differences in Mammalian target of rapamycin gene expression in the peripheral blood and articular cartilages of osteoarthritic patients and disease activity

The gene expression of mTOR, autophagy-related ULK1, caspase 3, CDK-inhibitor p21, and TNF α was measured in the peripheral blood of osteoarthritic (OA) patients at different stages of the disease aiming to establish a gene expression profile that might indicate the activity of the disease and joint destruction. Whole blood of 65 OA outpatients, 27 end-stage OA patients, 27 healthy volunteers, and knee articular cartilages of 28 end-stage OA patients and 26 healthy subjects were examined. OA outpatients were subjected to clinical testing, ultrasonography, and radiographic and WOMAC scoring. Protein levels of p70-S6K, p21, and caspase 3 were quantified by ELISA. Gene expression was measured using real-time RT-PCR. Upregulation of mTOR gene expression was observed in PBMCs of 42 OA outpatients ("High mTOR expression subset") and in PBMCs and articular cartilages of all end-stage OA patients. A positive correlation between mTOR gene expression in PBMCs and cartilage was observed in the end-stage OA patients. 23 OA outpatients in the "Low mTOR expression subset" exhibited significantly lower mTOR gene expression in PBMCs compared to healthy controls. These "Low mTOR" subset subjects experienced significantly more pain upon walking, and standing and increased total joint stiffness versus "High mTOR" subset, while the latter more often exhibited synovitis. The protein concentrations of p70-S6K, p21, and caspase 3 in PBMCs were significantly lower in the "Low" subset versus "High" subset and end-stage subjects. Increases in the expression of mTOR in PBMCs of OA patients are related to disease activity, being associated with synovitis more than with pain.

Autophagy: a new player in skeletal maintenance?

Imbalances between bone resorption and formation lie at the root of disorders such as osteoporosis, Paget's disease of bone (PDB), and osteopetrosis. Recently, genetic and functional studies have implicated proteins involved in autophagic protein degradation as important mediators of bone cell function in normal physiology and in pathology. Autophagy is the conserved process whereby aggregated proteins, intracellular pathogens, and damaged organelles are degraded and recycled. This process is important both for normal cellular quality control and in response to environmental or internal stressors, particularly in terminally-differentiated cells. Autophagic structures can also act as hubs for the spatial organization of recycling and synthetic process in secretory cells. Alterations to autophagy (reduction, hyperactivation, or impairment) are associated with a number of disorders, including neurodegenerative diseases and cancers, and are now being implicated in maintenance of skeletal homoeostasis. Here, we introduce the topic of autophagy, describe the new findings that are starting to emerge from the bone field, and consider the therapeutic potential of modifying this pathway for the treatment of age-related bone disorders.

Erythropoietin mediated bone formation is regulated by mTOR signaling

The role of erythropoietin (Epo) and Epo/Epo receptor (EpoR) signaling pathways for production of red blood cells are well established. However, little is known about Epo/EpoR signaling in non-hematopoietic cells. Recently, we demonstrated that Epo activates JAK/STAT signaling in hematopoietic stem cells (HSCs), leading to the production of bone morphogenetic protein 2 (BMP2) and bone formation and that Epo also directly activates mesenchymal cells to form osteoblasts in vitro. In this study, we investigated the effects of mTOR signaling on Epo-mediated osteoblastogenesis and osteoclastogenesis. We found that mTOR inhibition by rapamycin blocks Epo-dependent and -independent osteoblastic phenotypes in human bone marrow stromal cells (hBMSCs) and ST2 cells, respectively. Furthermore, we found that rapamycin inhibits Epo-dependent and -independent osteoclastogenesis in mouse bone marrow mononuclear cells and Raw264.7 cells. Finally, we demonstrated that Epo increases NFATc1 expression and decreases cathepsin K expression in an mTOR-independent manner, resulting in an increase of osteoclast numbers and a decrease in resorption activity. Taken together, these results strongly indicate that mTOR signaling plays an important role in Epo-mediated bone homeostasis.

Short-term treatment with rapamycin and dietary restriction have overlapping and distinctive effects in young mice

Because rapamycin, an inhibitor of the nutrient sensor mammalian target of rapamycin, and dietary restriction both increase life span of mice, it has been hypothesized that they act through similar mechanisms. To test this hypothesis, we compared various biological parameters in dietary restriction mice (40% food restriction) and mice fed rapamycin (14 ppm). Both treatments led to a significant reduction in mammalian target of rapamycin signaling and a corresponding increase in autophagy. However, we observed striking differences in fat mass, insulin sensitivity, and expression of cell cycle and sirtuin genes in mice fed rapamycin compared with dietary restriction. Thus, although both treatments lead to significant downregulation of mammalian target of rapamycin signaling, these two manipulations have quite different effects on other physiological functions suggesting that they might increase life span through a common pathway as well as pathways that are altered differently by dietary restriction and rapamycin.

Growth hormone improves growth retardation induced by rapamycin without blocking its antiproliferative and antiangiogenic effects on rat growth plate

Rapamycin, an immunosuppressant agent used in renal transplantation with antitumoral properties, has been reported to impair longitudinal growth in young individuals. As growth hormone (GH) can be used to treat growth retardation in transplanted children, we aimed this study to find out the effect of GH therapy in a model of young rat with growth retardation induced by rapamycin administration. Three groups of 4-week-old rats treated with vehicle (C), daily injections of rapamycin alone (RAPA) or in combination with GH (RGH) at pharmacological doses for 1 week were compared. GH treatment caused a 20% increase in both growth velocity and body length in RGH animals when compared with RAPA group. GH treatment did not increase circulating levels of insulin-like growth factor I, a systemic mediator of GH actions. Instead, GH promoted the maturation and hypertrophy of growth plate chondrocytes, an effect likely related to AKT and ERK1/2 mediated inactivation of GSK3β, increase of glycogen deposits and stabilization of β-catenin. Interestingly, GH did not interfere with the antiproliferative and antiangiogenic activities of rapamycin in the growth plate and did not cause changes in chondrocyte autophagy markers. In summary, these findings indicate that GH administration improves longitudinal growth in rapamycin-treated rats by specifically acting on the process of growth plate chondrocyte hypertrophy but not by counteracting the effects of rapamycin on proliferation and angiogenesis.

Instruction of mesenchymal cell fate by the transcription factor C/EBPβ

The transcription factor CCAAT/enhancer binding protein beta (C/EBPβ) plays a role in the differentiation of a large variety of cell types. C/EBPβ was initially described as an early inducer of adipocyte differentiation, however, recent data have shown that this is not the only mesenchymal cell lineage where C/EBPβ has an instructive function. Mouse models and tissue culture studies have now established a regulatory role of C/EBPβ in osteoblast and in chondrocyte differentiation. These three different cell lineages are derived from the same precursor, the mesenchymal stem cell (MSC). This review will focus on the emerging role of C/EBPβ and its different protein isoforms in various mesenchymal cell lineages and its function in adipocyte, chondrocyte and osteoblast differentiation. Moreover, the mesenchymal stem cell has attracted the attention of regenerative medicine in recent years, and the possible role of C/EBPβ in this respect will be discussed.

The skeleton: a multi-functional complex organ: new insights into osteoblasts and their role in bone formation: the central role of PI3Kinase

Studies on bone development, formation and turnover have grown exponentially over the last decade in part because of the utility of genetic models. One area that has received considerable attention has been the phosphatidylinositol 3-kinase (PI3K) signaling pathway, which has emerged as a major survival network for osteoblasts. Genetic engineering has enabled investigators to study downstream effectors of PI3K by directly overexpressing activated forms of AKT in cells of the skeletal lineage or deleting Pten that leads to a constitutively active AKT. The results from these studies have provided novel insights into bone development and remodeling, critical processes in the lifelong maintenance of skeletal health. This paper reviews those data in relation to recent advances in osteoblast biology and their potential relevance to chronic disorders of the skeleton and their treatment.

Growth of kidney-transplanted pediatric patients treated with sirolimus

Experimental findings indicate that sirolimus (SRL) inhibits longitudinal growth by mechanisms potentially related to its inhibitory effects on both cell proliferation and expression of vascular endothelial growth factor (VEGF). The aim of this study was to investigate the growth pattern of kidney-transplanted children treated with SRL in a multicenter observational clinical study. Height, change in height SD (Δ height) and growth velocity of pediatric patients with renal transplant were calculated at 0, 6, 12, and 24 months after starting SRL. Controls of kidney-transplanted children not treated with SRL were matched by age, gender, renal function, and dose of corticosteroids. Sixty-eight children (34 SRL, 34 controls) were enrolled in the study. Nephrotoxicity was the most frequent indication to start therapy with SRL. SRL exerted an adverse effect on growth as demonstrated by significantly lower (p < 0.05) growth velocity (cm/year) and smaller change in height SD in the SRL group after 6 (4.08 vs. 6.56 and -0.05 vs. 0.14), 12 (4.44 vs. 6.11 and -0.03 vs. 0.28) and 24 (4.53 vs. 6.03 and -0.04 vs. 0.53) months of treatment. This study suggests that SRL therapy may interfere with growth of kidney-transplanted children. This undesirable effect needs to be taken into account when considering a switch to SRL and confirmed in further prospective trials including larger number of patients.

De novo therapy with everolimus, low-dose ciclosporine A, basiliximab and steroid elimination in pediatric kidney transplantation

The number of acute rejections and infections after pediatric kidney transplantation (KTX) could not be reduced in the last years. To reduce these events, we investigated a new immunosuppressive protocol in a prospective trial. After KTX, 20 children (median age 12 years, range 1-17) were initially treated with Basiliximab, ciclosporine A (CsA) (trough-level = C0 200-250 ng/mL) and prednisolone. After 2 weeks, CsA dose was reduced to 50% (C0 75-100 ng/mL, after 6 months: 50-75 ng/mL) and everolimus (1.6 mg/m²) /day) was started (C0 3-6 ng/mL). Six months after KTX prednisolone was set to alternate dose and stopped 3 months later. All 20 protocol biopsies 6 months after KTX showed no acute rejection or borderline findings. Indication biopsies resulted in no acute rejections and two borderline findings. Mean glomerular filtration rate (GFR) 1 year after KTX was 71 ± 25 mL/min/1.73 m². Without cytomegalovirus (CMV)-prophylaxis, only two primary CMV infections were seen despite a donor/recipient-CMV-constellation pos./neg. in 10/20 children. In pediatric KTX, de novo immunosuppression with low-dose CsA, everolimus and steroid withdrawal after 9 months led to promising results according to numbers of acute rejections and infections. Further follow up is needed. Future larger trials will have to confirm our findings.

Biology and pathology of Rho GTPase, PI-3 kinase-Akt, and MAP kinase signaling pathways in chondrocytes

Chondrocytes provide the framework for the developing skeleton and regulate long-bone growth through the activity of the growth plate. Chondrocytes in the articular cartilage, found at the ends of bones in diarthroidial joints, are responsible for maintenance of the tissue through synthesis and degradation of the extracellular matrix. The processes of growth, differentiation, cell death and matrix remodeling are regulated by a network of cell signaling pathways in response to a variety of extracellular stimuli. These stimuli consist of soluble ligands, including growth factors and cytokines, extracellular matrix proteins, and mechanical factors that act in concert to regulate chondrocyte function through a variety of canonical and non-canonical signaling pathways. Key chondrocyte signaling pathways include, but are not limited to, the p38, JNK and ERK MAP kinases, the PI-3 kinase-Akt pathway, the Jak-STAT pathway, Rho GTPases and Wnt-beta-catenin and Smad pathways. Modulation of the activity of any of these pathways has been associated with various pathological states in cartilage. This review focuses on the Rho GTPases, the PI-3 kinase-Akt pathway, and some selected aspects of MAP kinase signaling. Most studies to date have examined these pathways in isolation but it is becoming clear that there is significant cross-talk among the pathways and that the overall effects on chondrocyte function depend on the balance in activity of multiple signaling proteins.

Phospholipase D signaling pathway is involved in lung cancer-derived IL-8 increased osteoclastogenesis

Bone is a frequent target of lung cancer metastasis, which is associated with significant morbidity and a dismal prognosis. This study analyzed the soluble factors secreted by lung cancer cells, which are responsible for increasing osteoclast differentiation. Addition of recombinant human interleukin-8 (rhIL-8), present in large amounts in A549-conditioned medium (CM) and NCI-H460-CM, mimicked the inductive effect of A549-CM and NCI-H460-CM on osteoclastogenesis. In contrast, depletion of interleukin-8 (IL-8) from A549-CM and NCI-H460-CM decreased the osteoclastogenesis-inductive properties of A549-CM and NCI-H460-CM. Induction of osteoclast differentiation by lung cancer-derived-CM and rhIL-8 was associated with increased phospholipase D (PLD) activation, and the activations of protein kinase C (PKC) alpha/betaII, extracellular signal-regulated kinase (ERK) 1/2 and AKT/the mammalian target of rapamycin (mTOR). Blocking PLD by a specific inhibitor significantly decreased osteoclast formation by inhibiting PKCs activation and subsequently attenuating the phosphorylation of ERK1/2. PLD inhibitor also completely decreased AKT and mTOR phosphorylation, whereas phosphatidylinositol-3-kinase (PI3K) inhibitor only partially decreased mTOR phosphorylation, suggesting that mTOR activation by PLD is through both PI3K/AKT-dependent and PI3K/AKT-independent manner. In addition, blocking AKT and ERK1/2 by a specific inhibitor also suppressed lung cancer-derived-CM and rhIL-8-induced osteoclast differentiation. Moreover, treatment of peripheral blood mononuclear cells with sera from invasive lung cancer patients increased the formation of osteoclasts. Our study suggests that IL-8 or IL-8-mediated PLD/PKC/ERK1/2 or PLD/AKT signaling is an attractive therapeutic target for osteolytic bone metastases in lung cancer patients.

Growth failure associated with sirolimus: case report

An 11-year-old girl, who was a renal transplant recipient, developed linear growth failure associated in time with sirolimus (SRL) treatment. After 5 years of functional graft [creatinine clearance (CCr) 90 ml/min per 1.73 m(2) body surface area], she developed acute renal failure due to calcineurin inhibitor-related hemolytic uremic syndrome, and cyclosporine A was replaced by SRL. Before the drug change, she had been growing normally (5.5 cm/year) and had reached the 33.9 percentile (P) of height (z-height -0.41), similar to her target. Two years later, her height had decreased to P 6th (z-height -1.54), as her growth velocity had diminished to 2.2 cm/year, despite optimal renal function (CCr 68 ml/min per 1.73 m(2)). Human recombinant growth hormone was needed to promote her catch-up growth and achieve the P 49th of height (z-height -0.03). SRL may have deleterious effects on growing children due its characteristic anti-proliferative and anti-angiogenic properties. Pediatric transplant recipients' linear growth should be cautiously monitored while they are being given SRL.