The effect of rapamycin on bone growth in rabbits

Miao medicine Gu Yan Xiao tincture inhibits mTOR to stimulate chondrocyte autophagy in a rabbit model of osteoarthritis

ETHNOPHARMACOLOGICAL RELEVANCE

The Gu Yan Xiao tincture, a blend of traditional Chinese herbs, is traditionally used for osteoarthritis and related pain. This study investigated its mechanism of action in order to rationalize and validate its therapeutic use.

AIM OF THE STUDY

This study analyzed, in a rabbit model of knee osteoarthritis, whether and how Gu Yan Xiao tincture exerts therapeutic benefits by modulating chondrocyte autophagy.

MATERIALS AND METHODS

The active constituents within the GYX tincture were identified using liquid chromatography-mass spectrometry. The rabbit model was established by injecting animals with type II collagenase intra-articularly, and the effects of topically applied tincture were examined on osteoarthritis lesions of the knee using histopathology, micro-computed tomography and x-ray imaging. Effects of the tincture were also evaluated on levels of inflammatory cytokines, matrix metalloproteases, and autophagy in chondrocytes. As a positive control, animals were treated with sodium diclofenac.

RESULTS

The tincture mitigated the reduction in joint space, hyperplasia of the synovium and matrix metalloproteases in serum that occurred after injection of type II collagenase in rabbits. These therapeutic effects were associated with inhibition of mTOR and activation of autophagy in articular chondrocytes. Inhibiting mTOR with rapamycin potentiated the therapeutic effects of the tincture, while inhibiting autophagy with 3-methyladenine antagonized them.

CONCLUSIONS

Gu Yan Xiao tincture mitigates tissue injury in a rabbit model of osteoarthritis, at least in part by inhibiting mTOR and thereby promoting autophagy in chondrocytes. These results rationalize the use of the tincture not only against osteoarthritis but also potentially other diseases involving inhibition of autophagy in bones and joints.

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.

mTOR Signaling Pathway in Bone Diseases Associated with Hyperglycemia

The interplay between bone and glucose metabolism has highlighted hyperglycemia as a potential risk factor for bone diseases. With the increasing prevalence of diabetes mellitus worldwide and its subsequent socioeconomic burden, there is a pressing need to develop a better understanding of the molecular mechanisms involved in hyperglycemia-mediated bone metabolism. The mammalian target of rapamycin (mTOR) is a serine/threonine protein kinase that senses extracellular and intracellular signals to regulate numerous biological processes, including cell growth, proliferation, and differentiation. As mounting evidence suggests the involvement of mTOR in diabetic bone disease, we provide a comprehensive review of its effects on bone diseases associated with hyperglycemia. This review summarizes key findings from basic and clinical studies regarding mTOR's roles in regulating bone formation, bone resorption, inflammatory responses, and bone vascularity in hyperglycemia. It also provides valuable insights into future research directions aimed at developing mTOR-targeted therapies for combating diabetic bone diseases.

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.

Crosstalk Between Senescent Bone Cells and the Bone Tissue Microenvironment Influences Bone Fragility During Chronological Age and in Diabetes

Bone is a complex organ serving roles in skeletal support and movement, and is a source of blood cells including adaptive and innate immune cells. Structural and functional integrity is maintained through a balance between bone synthesis and bone degradation, dependent in part on mechanical loading but also on signaling and influences of the tissue microenvironment. Bone structure and the extracellular bone milieu change with age, predisposing to osteoporosis and increased fracture risk, and this is exacerbated in patients with diabetes. Such changes can include loss of bone mineral density, deterioration in micro-architecture, as well as decreased bone flexibility, through alteration of proteinaceous bone support structures, and accumulation of senescent cells. Senescence is a state of proliferation arrest accompanied by marked morphological and metabolic changes. It is driven by cellular stress and serves an important acute tumor suppressive mechanism when followed by immune-mediated senescent cell clearance. However, aging and pathological conditions including diabetes are associated with accumulation of senescent cells that generate a pro-inflammatory and tissue-destructive secretome (the SASP). The SASP impinges on the tissue microenvironment with detrimental local and systemic consequences; senescent cells are thought to contribute to the multimorbidity associated with advanced chronological age. Here, we assess factors that promote bone fragility, in the context both of chronological aging and accelerated aging in progeroid syndromes and in diabetes, including senescence-dependent alterations in the bone tissue microenvironment, and glycation changes to the tissue microenvironment that stimulate RAGE signaling, a process that is accelerated in diabetic patients. Finally, we discuss therapeutic interventions targeting RAGE signaling and cell senescence that show promise in improving bone health in older people and those living with diabetes.

Animal source foods, rich in essential amino acids, are important for linear growth and development of young children in low- and middle-income countries

Growth faltering under 5 years of age is unacceptably high worldwide, and even more children, while not stunted, fail to reach their growth potential. The time between conception and 2 years of age is critical for development. The period from 6 to 23 months, when complementary foods are introduced, coincides with a time when growth faltering and delayed neurocognitive developments are most common. Fortunately, this is also the period when diet exercises its greatest influence. Growing up in an adverse environment, with a deficient diet, as typically seen in low‐ and middle‐income countries (LMICs), hampers growth and development of children and prevents them from realising their full developmental and economic future potential. Sufficient nutrient availability and utilisation are paramount to a child's growth and development trajectory, especially in the period after breastfeeding. This review highlights the importance of essential amino acids (EAAs) in early life for linear growth and, likely, neurocognitive development. The paper further discusses signalling through mammalian target of rapamycin complex 1 (mTORC1) as one of the main amino acid (AA)‐sensing hubs and the master regulator of both growth and neurocognitive development. Children in LMICs, despite consuming sufficient total protein, do not meet their EAA requirements due to poor diet diversity and low‐quality dietary protein. AA deficiencies in early life can cause reductions in linear growth and cognition. Ensuring AA adequacy in diets, particularly through inclusion of nutrient‐dense animal source foods from 6 to 23 months, is strongly encouraged in LMICs in order to compensate for less than optimal growth during complementary feeding.

Height and Risk of Vitiligo: A Nationwide Cohort Study

Adult height is linked to the risk of several diseases, but its association with vitiligo has not been established. This study aimed to investigate the relationship between adult height and vitiligo incidence. Korean nationwide claims data from 15,980,754 individuals (20 years of age or older) who received a health checkup during the period 2005–2008, were examined. Subjects were categorized into age- and gender-specific height quintiles. Participants were followed until vitiligo diagnosis or until the end of 2015. The Cox proportional-hazards model for cumulative risk was computed for height categories. During the follow-up period, 29,196 cases (136,020,214 person-years) of newly diagnosed vitiligo were reported. A positive association was found between height and risk of vitiligo in which the hazard ratio between the highest and lowest quintiles of height was 1.36 (95% confidence interval: 1.31–1.42). While more diverse cohort studies are needed, our findings suggest that taller stature increases the risk of vitiligo.

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.

Interactions between Growth of Muscle and Stature: Mechanisms Involved and Their Nutritional Sensitivity to Dietary Protein: The Protein-Stat Revisited

Childhood growth and its sensitivity to dietary protein is reviewed within a Protein-Stat model of growth regulation. The coordination of growth of muscle and stature is a combination of genetic programming, and of two-way mechanical interactions involving the mechanotransduction of muscle growth through stretching by bone length growth, the core Protein-Stat feature, and the strengthening of bone through muscle contraction via the mechanostat. Thus, growth in bone length is the initiating event and this is always observed. Endocrine and cellular mechanisms of growth in stature are reviewed in terms of the growth hormone-insulin like growth factor-1 (GH-IGF-1) and thyroid axes and the sex hormones, which together mediate endochondral ossification in the growth plate and bone lengthening. Cellular mechanisms of muscle growth during development are then reviewed identifying (a) the difficulties posed by the need to maintain its ultrastructure during myofibre hypertrophy within the extracellular matrix and the concept of muscle as concentric "bags" allowing growth to be conceived as bag enlargement and filling, (b) the cellular and molecular mechanisms involved in the mechanotransduction of satellite and mesenchymal stromal cells, to enable both connective tissue remodelling and provision of new myonuclei to aid myofibre hypertrophy and (c) the implications of myofibre hypertrophy for protein turnover within the myonuclear domain. Experimental data from rodent and avian animal models illustrate likely changes in DNA domain size and protein turnover during developmental and stretch-induced muscle growth and between different muscle fibre types. Growth of muscle in male rats during adulthood suggests that "bag enlargement" is achieved mainly through the action of mesenchymal stromal cells. Current understanding of the nutritional regulation of protein deposition in muscle, deriving from experimental studies in animals and human adults, is reviewed, identifying regulation by amino acids, insulin and myofibre volume changes acting to increase both ribosomal capacity and efficiency of muscle protein synthesis via the mechanistic target of rapamycin complex 1 (mTORC1) and the phenomenon of a "bag-full" inhibitory signal has been identified in human skeletal muscle. The final section deals with the nutritional sensitivity of growth of muscle and stature to dietary protein in children. Growth in length/height as a function of dietary protein intake is described in the context of the breastfed child as the normative growth model, and the "Early Protein Hypothesis" linking high protein intakes in infancy to later adiposity. The extensive paediatric studies on serum IGF-1 and child growth are reviewed but their clinical relevance is of limited value for understanding growth regulation; a role in energy metabolism and homeostasis, acting with insulin to mediate adiposity, is probably more important. Information on the influence of dietary protein on muscle mass per se as opposed to lean body mass is limited but suggests that increased protein intake in children is unable to promote muscle growth in excess of that linked to genotypic growth in length/height. One possible exception is milk protein intake, which cohort and cross-cultural studies suggest can increase height and associated muscle growth, although such effects have yet to be demonstrated by randomised controlled trials.

Promoting bone health in children and adolescents following solid organ transplantation

Solid organ transplantation in children and adolescents provides many benefits through improving critical organ function, including better growth, development, cardiovascular status, and quality of life. Unfortunately, bone status may be adversely affected even when overall status is improving, due to issues with pre-existing bone disease as well as medications and nutritional challenges inherent post-transplantation. For all children and adolescents, bone status entering adulthood is a critical determinant of bone health through adulthood. The overall health and bone status of transplant recipients benefits from attention to regular physical activity, good nutrition, adequate calcium, phosphorous, magnesium and vitamin D intake and avoidance/minimization of soda, extra sodium, and obesity. Many immunosuppressive agents, especially glucocorticoids, can adversely affect bone function and development. Minimizing exposure to "bone-toxic" medications is an important part of promoting bone health in children post-transplantation. Existing guidelines detail how regular monitoring of bone status and biochemical markers can help detect bone abnormalities early and facilitate valuable bone-directed interventions. Attention to calcium and vitamin D supplementation, as well as tapering and withdrawing glucocorticoids as early as possible after transplant, can provide best bone outcomes for these children. Dual-energy X-ray absorptiometry can be useful to detect abnormal bone mass and fracture risk in this population and newer bone assessment methods are being evaluated in children at risk for poor bone outcomes. Newer bone therapies being explored in adults with transplants, particularly bisphosphonates and the RANKL inhibitor denosumab, may offer promise for children with low bone mass post-transplantation.

Effects of inhibiting PDK‑1 expression in bone marrow mesenchymal stem cells on osteoblast differentiation in vitro

Osteoblasts are the main functional cells in bone formation, which are responsible for the synthesis, secretion and mineralization of bone matrix. The PI3K/AKT signaling pathway is strongly associated with the differentiation and survival of osteoblasts. The 3-phosphoinositide-dependent protein kinase-1 (PDK-1) protein is considered the master upstream lipid kinase of the PI3K/AKT cascade. The present study aimed to investigate the role of PDK-1 in the process of mouse osteoblast differentiation in vitro. In the BX-912 group, BX-912, a specific inhibitor of PDK-1, was added to osteoblast induction medium (OBM) to treat bone marrow mesenchymal stem cells (BMSCs), whereas the control group was treated with OBM alone. Homozygote PDK1^flox/flox mice were designed and generated, and were used to obtain BMSCs^{PDK1flox/flox}. Subsequently, an adenovirus containing Cre recombinase enzyme (pHBAd-cre-EGFP) was used to disrupt the PDK-1 gene in BMSCs^{PDK1flox/flox}; this served as the pHBAd-cre-EGFP group and the efficiency of the disruption was verified. Western blot analysis demonstrated that the protein expression levels of phosphorylated (p)-PDK1 and p-AKT were gradually increased during the osteoblast differentiation process. Notably, BX-912 treatment and disruption of the PDK-1 gene with pHBAd-cre-EGFP effectively reduced the number of alkaline phosphatase (ALP)-positive cells and the optical density value of ALP activity, as well as the formation of cell mineralization. The mRNA expression levels of PDK-1 in the pHBAd-cre-EGFP group were significantly downregulated compared with those in the empty vector virus group on days 3–7. The mRNA expression levels of the osteoblast-related genes RUNX2, osteocalcin and collagen I were significantly decreased in the BX-912 and pHBAd-cre-EGFP groups on days 7 and 21 compared with those in the control and empty vector virus groups. Overall, the results indicated that BX-912 and disruption of the PDK-1 gene in vitro significantly inhibited the differentiation and maturation of osteoblasts. These experimental results provided an experimental and theoretical basis for the role of PDK-1 in osteoblasts.

Translational Control of Sox9 RNA by mTORC1 Contributes to Skeletogenesis

The mechanistic/mammalian target of rapamycin complex 1 (mTORC1) regulates cellular function in various cell types. Although the role of mTORC1 in skeletogenesis has been investigated previously, here we show a critical role of mTORC1/4E-BPs/SOX9 axis in regulating skeletogenesis through its expression in undifferentiated mesenchymal cells. Inactivation of Raptor, a component of mTORC1, in limb buds before mesenchymal condensations resulted in a marked loss of both cartilage and bone. Mechanistically, we demonstrated that mTORC1 selectively controls the RNA translation of Sox9, which harbors a 5′ terminal oligopyrimidine tract motif, via inhibition of the 4E-BPs. Indeed, introduction of Sox9 or a knockdown of 4E-BP1/2 in undifferentiated mesenchymal cells markedly rescued the deficiency of the condensation observed in Raptor-deficient mice. Furthermore, introduction of the Sox9 transgene rescued phenotypes of deficient skeletal growth in Raptor-deficient mice. These findings highlight a critical role of mTORC1 in mammalian skeletogenesis, at least in part, through translational control of Sox9 RNA.

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mTORC1 controls skeletogenesis both in skeletogenic progenitors and in chondrocytes

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mTORC1/4E-BPs cascade regulates the translation of Sox9 RNA

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SOX9 is a critical mediator in the control of skeletogenesis by mTORC1 in vivo

Activation of mTORC1 in chondrocytes does not affect proliferation or differentiation, but causes the resting zone of the growth plate to become disordered

There are several pitfalls associated with research based on transgenic mice. Here, we describe our interpretation and analysis of mTORC1 activation in growth plate chondrocytes and compare these to a recent publication (Yan et al., Nature Communications 2016, 7:11151). Both laboratories employed TSC1-floxed mice crossed with collagen type 2-driven Cre (Col2-Cre), but drew substantially different conclusions. It was reported that activation of mechanistic target of rapamycin complex 1 (mTORC1) via Tsc1 ablation promotes the hypertrophy of growth plate chondrocytes, whereas we observe only disorganization in the resting zone, with no effect on chondrocyte hypertrophy or proliferation. Here, we present our data and discuss the differences in comparison to the earlier phenotypic characterization of TSC1 ablation in cartilage. Importantly, we detect Col2-Cre activity in non-cartilaginous tissues (including the brain) and discuss it in relation to other studies reporting non-cartilaginous expression of collagen alpha(1) II. Altogether, we conclude that mouse phenotypes following genetic ablation using Col2-Cre should be interpreted with care. We also conclude that activation of mTORC1 by TSC1 ablation in postnatal chondrocytes with inducible Col2-Cre (Col2-CreERt) leads to disorganization of the resting zone but causes no changes in chondrocyte proliferation or differentiation.

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Ablation of Tsc1 using Col2-Cre causes severe developmental abnormalities.

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Col2-Cre is not specific to chondrocytes during early development.

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Mice develop normally when Tsc1 is ablated in chondrocytes postnatally.

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.

Perspective: The Potential Role of Essential Amino Acids and the Mechanistic Target of Rapamycin Complex 1 (mTORC1) Pathway in the Pathogenesis of Child Stunting

Stunting is the best summary measure of chronic malnutrition in children. Approximately one-quarter of children under age 5 worldwide are stunted. Lipid-based or micronutrient supplementation has little to no impact in reducing stunting, which suggests that other critical dietary nutrients are missing. A dietary pattern of poor-quality protein is associated with stunting. Stunted children have significantly lower circulating essential amino acids than do nonstunted children. Inadequate dietary intakes of essential amino acids could adversely affect growth, because amino acids are required for synthesis of proteins. The master growth regulation pathway, the mechanistic target of rapamycin complex 1 (mTORC1) pathway, is exquisitely sensitive to amino acid availability. mTORC1 integrates cues such as nutrients, growth factors, oxygen, and energy to regulate growth of bone, skeletal muscle, nervous system, gastrointestinal tract, hematopoietic cells, immune effector cells, organ size, and whole-body energy balance. mTORC1 represses protein and lipid synthesis and cell and organismal growth when amino acids are deficient. Over the past 4 decades, the main paradigm for child nutrition in developing countries has been micronutrient malnutrition, with relatively less attention paid to protein. In this Perspective, we present the view that essential amino acids and the mTORC1 pathway play a key role in child growth. The current assumption that total dietary protein intake is adequate for growth among most children in developing countries needs re-evaluation.

Pharmacological inhibition of lysosomes activates the MTORC1 signaling pathway in chondrocytes in an autophagy-independent manner

Mechanistic target of rapamycin (serine/threonine kinase) complex 1 (MTORC1) is a protein-signaling complex at the fulcrum of anabolic and catabolic processes, which acts depending on wide-ranging environmental cues. It is generally accepted that lysosomes facilitate MTORC1 activation by generating an internal pool of amino acids. Amino acids activate MTORC1 by stimulating its translocation to the lysosomal membrane where it forms a super-complex involving the lysosomal-membrane-bound vacuolar-type H⁺-ATPase (v-ATPase) proton pump. This translocation and MTORC1 activation require functional lysosomes. Here we found that, in contrast to this well-accepted concept, in epiphyseal chondrocytes inhibition of lysosomal activity by v-ATPase inhibitors bafilomycin A₁ or concanamycin A potently activated MTORC1 signaling. The activity of MTORC1 was visualized by phosphorylated forms of RPS6 (ribosomal protein S6) and EIF4EBP1, 2 well-known downstream targets of MTORC1. Maximal RPS6 phosphorylation was observed at 48-h treatment and reached as high as a 12-fold increase (p < 0.018). This activation of MTORC1 was further confirmed in bone organ culture and promoted potent stimulation of longitudinal growth (p < 0.001). Importantly, the same effect was observed in ATG5 (autophagy-related 5)-deficient bones suggesting a macroautophagy-independent mechanism of MTORC1 inhibition by lysosomes. Thus, our data show that in epiphyseal chondrocytes lysosomes inhibit MTORC1 in a macroautophagy-independent manner and this inhibition likely depends on v-ATPase activity.

Combined treatment with platelet-rich plasma and insulin favours chondrogenic and osteogenic differentiation of human adipose-derived stem cells in three-dimensional collagen scaffolds

Osteochondral lesions due to injury or other pathology commonly result in the development of osteoarthritis and progressive joint destruction. Bioengineered scaffolds are widely studied for regenerative surgery strategies in osteochondral defect management, also combining the use of stem cells, growth factors and hormones. The utility in tissue engineering of human adipose-derived stem cells (ASCs) isolated from adipose tissue has been widely noted. Autologous platelet-rich plasma (PRP) represents an alternative strategy in regenerative medicine for the local release of endogenous growth factors and hormones. Here we compared the effects of three-dimensional (3D) collagen type I scaffold culture and combined treatment with PRP and human recombinant insulin on the chondro-/osteogenic differentiation of ASCs. Histochemical and biomolecular analyses demonstrated that chondro-/osteogenic differentiation was increased in ASC-populated 3D collagen scaffolds compared with two-dimensional (2D) plastic dish culture. Chondro-/osteogenic differentiation was further enhanced in the presence of combined PRP (5% v/v) and insulin (100 nm) treatment. In addition, chondro-/osteogenic differentiation associated with the contraction of ASC-populated 3D collagen scaffold and increased β1/β3-integrin expression. Inhibition studies demonstrated that PRP/insulin-induced chondro-/osteogenic differentiation is independent of insulin-like growth factor 1 receptor (IGF-1R) and mammalian target of rapamycin (mTOR) signalling; IGF-R1/mTOR inhibition even enhanced ASC chondro-/osteogenic differentiation. Our findings underline that 3D collagen scaffold culture in association with platelet-derived growth factors and insulin favour the chondro-/osteogenic differentiation of ASCs, suggesting new translational applications in regenerative medicine for the management of osteochondral defects. Copyright © 2016 John Wiley & Sons, Ltd.

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.

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).

Fluocinolone Acetonide Is a Potent Synergistic Factor of TGF-β3-Associated Chondrogenesis of Bone Marrow-Derived Mesenchymal Stem Cells for Articular Surface Regeneration

Articular cartilage repair remains a challenging problem. Based on a high-throughput screening and functional analysis, we found that fluocinolone acetonide (FA) in combination with transforming growth factor beta 3 (TGF-β3) strongly potentiated chondrogenic differentiation of human bone marrow-derived mesenchymal stem cells (hBMSCs). In an in vivo cartilage defect model in knee joints of immunocompromised mice, transplantation of FA/TGF-β3-treated hBMSCs could completely repair the articular surface. Analysis of the intracellular pathways revealed that FA enhanced TGF-β3-induced phosphorylation of Smad2 and Smad3. Additionally, we performed a pathway array and found that FA activates the mTORC1/AKT pathway. Chemical inhibition of mTORC1 with rapamycin substantially suppressed FA effect, and inhibition of AKT completely repressed chondrogenesis of hBMSCs. Inhibition of glucocorticoid receptor with mifepristone also suppressed FA effect, suggesting that FA involves binding to the glucocorticoid receptor. Comparative analysis with other glucocorticoids (triamcinolone acetonide [TA] and dexamethasone [DEX]) revealed the unique ability of FA to repair articular cartilage surgical defects. Analysis of intracellular pathways showed that the mTORC1/AKT pathway and the glucocorticoid receptor was highly activated with FA and TA, but to a lesser extent with DEX. Collectively, these results show a unique ability of FA to enhance TGF-β3-associated chondrogenesis, and suggest that the FA/TGF-β3 combination may be used as major inducer of chondrogenesis in vitro. Additionally, FA/TGF-β3 could be potentially applied in a clinical setting to increase the efficiency of regenerative approaches based on chondrogenic differentiation of stem cells.

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.

Mechanical activation of mammalian target of rapamycin pathway is required for cartilage development

Mechanical stress regulates development by modulating cell signaling and gene expression. However, the cytoplasmic components mediating mechanotransduction remain unclear. In this study, elimination of muscle contraction during chicken embryonic development resulted in a reduction in the activity of mammalian target of rapamycin (mTOR) in the cartilaginous growth plate. Inhibition of mTOR activity led to significant inhibition of chondrocyte proliferation, cartilage tissue growth, and expression of chondrogenic genes, including Indian hedgehog (Ihh), a critical mediator of mechanotransduction. Conversely, cyclic loading (1 Hz, 5% matrix deformation) of embryonic chicken growth plate chondrocytes in 3-dimensional (3D) collagen scaffolding induced sustained activation of mTOR. Mechanical activation of mTOR occurred in serum-free medium, indicating that it is independent of growth factor or nutrients. Treatment of chondrocytes with Rapa abolished mechanical activation of cell proliferation and Ihh gene expression. Cyclic loading of chondroprogenitor cells deficient in SH2-containing protein tyrosine phosphatase 2 (Shp2) further enhanced mechanical activation of mTOR, cell proliferation, and chondrogenic gene expression. This result suggests that Shp2 is an antagonist of mechanotransduction through inhibition of mTOR activity. Our data demonstrate that mechanical activation of mTOR is necessary for cell proliferation, chondrogenesis, and cartilage growth during bone development, and that mTOR is an essential mechanotransduction component modulated by Shp2 in the cytoplasm.

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.

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.

TNFα-mediated apoptosis in human osteoarthritic chondrocytes sensitized by PI3K-NF-κB inhibitor, not mTOR inhibitor

To investigate apoptosis of osteoarthritic (OA) chondrocytes stimulated with different inhibitors targeting tumor necrosis factor-alpha (TNFα) pathway, we isolated first passage chondrocytes from OA patients and then treated them with the inhibitors in combination with TNFα, and then collected the stimulated chondrocytes for Western blotting. Chondrocytes from OA patients expressed cleaved caspase-3 and PARP, suggesting apoptotic background. We here, validated that 10 ng/ml of TNFα couldn't induce more chondrocytes apoptosis. PI3K inhibitor LY294002 or NF-κB inhibitor CAPE, but not mTOR inhibitor rapamycin and MEK1/2 inhibitor U0126 in combination with TNFα could facilitate apoptosis. CAPE-induced more apoptosis could be explained by c-FLIP downregulation more than cIAP1 upregulation. And, we showed the first time that PI3K-NF-κB pathway, but not mTOR pathway could prevent chondrocytes apoptosis induced by a pro-apoptotic factor TNFα and call for attention while trying to inhibit NF-κB as a therapeutic target.

NVP-BEZ235, a dual pan class I PI3 kinase and mTOR inhibitor, promotes osteogenic differentiation in human mesenchymal stromal cells

Osteoblasts are bone-forming cells derived from mesenchymal stromal cells (MSCs) that reside within the bone marrow. In response to a variety of factors, MSCs proliferate and differentiate into mature, functional osteoblasts. Several studies have shown previously that suppression of the PI3K and mTOR signaling pathways in these cells strongly promotes osteogenic differentiation, which suggests that inhibitors of these pathways may be useful as anabolic bone agents. In this study we examined the effect of BEZ235, a newly developed dual PI3K and mTOR inhibitor currently in phase I-II clinical trials for advanced solid tumors, on osteogenic differentiation and function using primary MSC cultures. Under osteoinductive conditions, BEZ235 strongly promotes osteogenic differentiation, as evidenced by an increase in mineralized matrix production, an upregulation of genes involved in osteogenesis, including bone morphogenetic proteins (BMP2, -4, and -6) and transforming growth factor β1 (TGF-β1) superfamily members (TGFB1, TGFB2, and INHBE), and increased activation of SMAD signaling molecules. In addition, BEZ235 enhances de novo bone formation in calvarial organotypic cultures. Using pharmacologic inhibitors to delineate mechanism, our studies reveal that suppression of mTOR and, to a much lesser extent PI3K p110α, mediates the osteogenic effects of BEZ235. As confirmation, shRNA-mediated knockdown of mTOR enhances osteogenic differentiation and function in SAOS-2 osteoblast-like cells. Taken together, our findings suggest that BEZ235 may be useful in treating PI3K/mTOR-dependent tumors associated with bone loss, such as the hematologic malignancy multiple myeloma.