Glucocorticoids and osteocyte autophagy

Exploring the interplay between stress mediators and skin microbiota in shaping age-related hallmarks: A review

Psychological stress is a major contributing factor to several health problems (e.g., depression, cardiovascular disease). Around 35 % of the world's population suffers from it, including younger generations. Physiologically, stress manifests through neuroendocrine pathways (Hypothalamic-Pituitary-Adrenal (HPA) axis and Sympathetic-Adrenal-Medullary (SAM) system) which culminate in the production of stress mediators like cortisol, epinephrine and norepinephrine. Stress and its mediators have been associated to body aging, through molecular mechanisms such as telomere attrition, mitochondrial dysfunction, cellular senescence, chronic inflammation, and dysbiosis, among others. Regarding its impact in the skin, stress impacts its structural integrity and physiological function. Despite this review focusing on several hallmarks of aging, emphasis was placed on skin microbiota dysbiosis. In this line, several studies, comprising different age groups, demographic contexts and body sites, have reported skin microbiota alterations associated with aging, and some effects of stress mediators on skin microbiota have also been reviewed in this paper. From a different perspective, since it is not a "traditional" stress mediator, oxytocin, a cortisol antagonist, has been related to glucorticoids inhibition and to display positive effects on cellular aging. This hormone dysregulation has been associated to psychological issues such as depression, whereas its upregulation has been linked to positive social interaction.

Cushing's disease and bone

Bone impairment associated with Cushing's disease (CD) is a complex disorder, mainly involving deterioration of bone quality and resulting in an increased fracture rate, often despite normal bone mineral density. Bone complications are common in patients with CD at the time of diagnosis but may persist even after successful treatment. There is currently no agreement on the optimal diagnostic methods, thresholds for anti-osteoporotic therapy and its timing in CD. In this review, we summarize the current data on the pathophysiology, diagnostic approach and management of bone complications in CD.

Yunnan Baiyao Might Mitigate Periodontitis Bone Destruction by Inhibiting Autophagy and Promoting Osteoblast Differentiation in vivo, ex vivo and in vitro

Background and Objective

Periodontitis is an inflammatory disease that eventually destroys tooth-supporting tissue. Yunnan Baiyao (YNBY), a traditional Chinese medicine compound with haemostatic and anti-inflammatory properties has shown therapeutic potential in several diseases. Our previous study revealed that YNBY suppressed osteoclast differentiation in periodontitis. The purpose of this study is to investigate the influences of YNBY on osteoblasts and explore its potential mechanisms.

Materials and Methods

A rat periodontitis model was established by ligation of maxillary second molars. After the end of modelling, histopathological observation by hematoxylin-eosin (HE) staining and Masson trichrome staining, detection of bone resorption by Micro-CT scanning, detection of osteoclasts by tartrate-resistant acid phosphatase (TRAP) staining, expression of osteocalcin (OCN) and microtubule-associated protein 1 light chain 3 (LC3) by immunohistochemistry. Lipopolysaccharides was used to irritate MC3T3-E1 osteoblastic cells and ex vivo calvarial organ as an in vitro model of inflammation. CCK-8 assay was performed to examine the toxicity of YNBY to MC3T3-E1 osteoblastic cells. Osteogenesis was assessed with alizarin red staining, immunofluorescence staining, Western blot and immunohistochemical staining. Transmission electron microscopy, fluorescent double staining, Western blot and immunohistochemical staining were employed to detect autophagy.

Results

Histological and micro-CT analyses revealed that YNBY gavage reduced bone loss caused by experimental periodontitis and upregulated osteogenic proteins in vivo. YNBY attenuated the production of autophagy-related proteins in periodontitis rats. Additionally, YNBY promoted osteogenesis by inhibiting inflammation-induced autophagy in vitro. Furthermore, YNBY suppressed LPS-mediated bone resorption and promoted the production of osteoblast-related proteins in inflamed calvarial tissues ex vivo.

Conclusion

This study demonstrated, through in vivo, in vitro and ex vivo experiments, that YNBY promoted osteoblast differentiation by suppressing autophagy, which markedly alleviated bone destruction caused by periodontitis.

Prostate cancer and bone: clinical presentation and molecular mechanisms

Prostate cancer (PCa) is an increasingly prevalent health problem in the developed world. Effective treatment options exist for localized PCa, but metastatic PCa has fewer treatment options and shorter patient survival. PCa and bone health are strongly entwined, as PCa commonly metastasizes to the skeleton. Since androgen receptor signaling drives PCa growth, androgen-deprivation therapy whose sequelae reduce bone strength constitutes the foundation of advanced PCa treatment. The homeostatic process of bone remodeling - produced by concerted actions of bone-building osteoblasts, bone-resorbing osteoclasts, and regulatory osteocytes - may also be subverted by PCa to promote metastatic growth. Mechanisms driving skeletal development and homeostasis, such as regional hypoxia or matrix-embedded growth factors, may be subjugated by bone metastatic PCa. In this way, the biology that sustains bone is integrated into adaptive mechanisms for the growth and survival of PCa in bone. Skeletally metastatic PCa is difficult to investigate due to the entwined nature of bone biology and cancer biology. Herein, we survey PCa from origin, presentation, and clinical treatment to bone composition and structure and molecular mediators of PCa metastasis to bone. Our intent is to quickly yet effectively reduce barriers to team science across multiple disciplines that focuses on PCa and metastatic bone disease. We also introduce concepts of tissue engineering as a novel perspective to model, capture, and study complex cancer-microenvironment interactions.

Glucocorticoid effects in the regenerating fin reflect tissue homeostasis disturbances in zebrafish by affecting Wnt signaling

As a treatment for various immune-mediated diseases, the use of glucocorticoids as anti-inflammatory and immunosuppressive agents is common practice. However, their use is severely hampered by the risk of the development of adverse effects such as secondary osteoporosis, skin atrophy, and peptic ulcer formation. The exact molecular and cellular mechanisms underlying those adverse effects, which involve most major organ systems, are not yet fully understood. Therefore, their investigation is of great importance to improve treatment regimens for patients. Here, we investigated the effects of the glucocorticoid prednisolone on cell proliferation and Wnt signaling in homeostatic skin and intestinal tissue and compared them to the anti-regenerative effects in zebrafish fin regeneration. We also investigated a potential recovery from the glucocorticoid treatment and the impact of short-term treatment with prednisolone. We identified a dampening effect of prednisolone on Wnt signaling and proliferation in highly proliferative tissues, namely the skin and intestine, as well as reduced fin regenerate length and Wnt reporter activity in the fin. The presence of the Wnt inhibitor Dickkopf1 was enhanced in prednisolone treated skin tissue. A decreased number of mucous producing goblet cells was observed in the intestine of prednisolone treated zebrafish. Unexpectedly, proliferation in bone forming osteoblasts of the skull, homeostatic scales, as well as the brain was not decreased, opposite to the observed effects in the skin, fin, and intestine. Short-term treatment with prednisolone for a few days did not significantly alter fin regenerate length, skin cell proliferation, intestinal leukocyte number and proliferation of intestinal crypt cells. However, it affected the number of mucous-producing goblet cells in the gut. Likewise, discontinuation of prednisolone treatment for a few days saved the skin and intestine from a significant reduction of skin and intestinal cell proliferation, intestinal leukocyte number and regenerate length, but did not rescue goblet cell number. The suppressive effects of glucocorticoids in highly proliferative tissues may be relevant in the context of their therapeutic applications in patients with inflammatory diseases.

Cadmium accelerates autophagy of osteocytes by inhibiting the PI3K/AKT/mTOR signaling pathway

Cadmium (Cd) can damage bone cells and cause osteoporosis. Osteocytes are the most numerous bone cells and also important target cells for Cd-induced osteotoxic damage. Autophagy plays important role in the progression of osteoporosis. However, osteocyte autophagy in Cd-induced bone injury is not well characterized. Thus, we established a Cd-induced bone injury model in BALB/c mice and a cellular damage model in MLO-Y4 cells. Aqueous Cd exposure for 16 months showed an increase in plasma alkaline phosphatase (ALP) activity and increase in urine calcium (Ca) and phosphorus (P) concentrations in vivo. Moreover, expression level of autophagy-related microtubule-associated protein 1A/1B-light chain 3 II (LC3II) and autophagy-related 5 (ATG5) proteins were induced, and the expression of sequestosome-1 (p62) was reduced, along with Cd-induced trabecular bone damage. In addition, Cd inhibited the phosphorylation of mammalian target of rapamycin (mTOR), protein kinase B (AKT), and phosphatidylinositol 3-kinase (PI3K). In vitro, 80 μM Cd concentrations exposure upregulated LC3II protein expression, and downregulated of p62 protein expression. Similarly, we found that treatment with 80 μM Cd resulted in a reduction in the phosphorylation levels of mTOR, AKT, and PI3K. Further experiments revealed that addition of rapamycin, an autophagy inducer, enhanced autophagy and alleviated the Cd-induced damage to MLO-Y4 cells. The findings of our study reveal for the first time that Cd causes damage to both bone and osteocytes, as well as induces autophagy in osteocytes and inhibits PI3K/AKT/mTOR signaling, which could be a protective mechanism against Cd-induced bone injury.

Pathogenic mechanisms of glucocorticoid-induced osteoporosis

Glucocorticoid (GC) is one of the most prescribed medicines to treat various inflammatory and autoimmune diseases. However, high doses and long-term use of GCs lead to multiple adverse effects, particularly glucocorticoid-induced osteoporosis (GIO). Excessive GCs exert detrimental effects on bone cells, including osteoblasts, osteoclasts, and osteocytes, leading to impaired bone formation and resorption. The actions of exogenous GCs are considered to be strongly cell-type and dose dependent. GC excess inhibits the proliferation and differentiation of osteoblasts and enhances the apoptosis of osteoblasts and osteocytes, eventually contributing to reduced bone formation. Effects of GC excess on osteoclasts mainly include enhanced osteoclastogenesis, increased lifespan and number of mature osteoclasts, and diminished osteoclast apoptosis, which result in increased bone resorption. Furthermore, GCs have an impact on the secretion of bone cells, subsequently disturbing the process of osteoblastogenesis and osteoclastogenesis. This review provides timely update and summary of recent discoveries in the field of GIO, with a particular focus on the effects of exogenous GCs on bone cells and the crosstalk among them under GC excess.

Oxidative Stress and Inflammation in Osteoporosis: Molecular Mechanisms Involved and the Relationship with microRNAs

Osteoporosis is characterized by the alteration of bone homeostasis due to an imbalance between osteoclastic bone resorption and osteoblastic bone formation. Estrogen deficiency causes bone loss and postmenopausal osteoporosis, the pathogenesis of which also involves oxidative stress, inflammatory processes, and the dysregulation of the expression of microRNAs (miRNAs) that control gene expression at post-transcriptional levels. Oxidative stress, due to an increase in reactive oxygen species (ROS), proinflammatory mediators and altered levels of miRNAs enhance osteoclastogenesis and reduce osteoblastogenesis through mechanisms involving the activation of MAPK and transcription factors. The present review summarizes the principal molecular mechanisms involved in the role of ROS and proinflammatory cytokines on osteoporosis. Moreover, it highlights the interplay among altered miRNA levels, oxidative stress, and an inflammatory state. In fact, ROS, by activating the transcriptional factors, can affect miRNA expression, and miRNAs can regulate ROS production and inflammatory processes. Therefore, the present review should help in identifying targets for the development of new therapeutic approaches to osteoporotic treatment and improve the quality of life of patients.

Cladrin alleviates dexamethasone-induced apoptosis of osteoblasts and promotes bone formation through autophagy induction via AMPK/mTOR signaling

Glucocorticoid-induced osteoporosis (GIOP) is a common clinical consequence that arises due to the extensive usage of glucocorticoids. Cladrin (Clad), a methoxylated isoflavone has been reported to have a bone protecting effect by enhancing osteoblast proliferation and differentiation. However, its consequences on GIOP are not reported yet. This study investigates whether Clad protects against the deleterious effects of Dexamethasone (Dex) on osteoblast and bone. Mice calvarial osteoblasts were treated with Clad and then exposed to Dex to study the effect on osteoblast differentiation, proliferation, and survival. Further, GIOP mice were treated with Clad (5 and 10 mg/kg) doses along with reference standard alendronate (ALN 3 mg/kg) for evaluation of bone protecting effect of Clad. We analyzed bone and vertebral microarchitecture, mechanical strength, and biochemical parameters. We observed that Clad at 10 nM concentration mitigated Dex-induced cytotoxicity and defend osteoblasts against apoptosis. Subsequent results demonstrate that Clad suppressed apoptosis of osteoblast in the presence of Dex by enhancing autophagy in a way that was reliant on the AMP-activated protein kinase (AMPK) and mammalian target of rapamycin (mTOR) pathway. Furthermore, micro-CT scanning, eco MRI results, and serum CTX levels revealed that 12 weeks of Clad treatment prevented bone loss and preserved trabecular bone mass in GIOP animals. We also observed that Clad treated osteoblasts had a lower rate of apoptosis and a greater LC3-II/LC3-I ratio than the Dex group. Our findings show that Clad can protect osteoblasts against glucocorticoids by inducing autophagy via the AMPK/mTOR pathway.

Milk kefir therapy improves the skeletal response to resistance exercise in rats submitted to glucocorticoid-induced osteoporosis

Glucocorticoid-induced osteoporosis (GIO) has emerged as a challenge after long-term glucocorticoids (GCs) administration. Exercise has been an important non-pharmacological option, while medications modulate bone remodeling despite adverse effects. In this way, milk Kefir (MK) therapy stands out as a safe alternative to improve bone metabolism. Our study aimed to investigate the effect of MK associated to resistance exercise on bone loss in rats with GIO. For this, sixty male Wistar rats were divided into 2 groups: normal (N) and subjected to GIO, which was subdivided into 4 groups: control (C), milk kefir therapy (K), Exercise (Ex), and Exercise+K (ExK). GIO was induced by dexamethasone (7 mg/kg - i.m.; 1×/wk, 5 wk). MK was administered daily (1×/day; 0.7 ml/animal) and the climb exercise with load was performed 3×/wk; both for 16 wk. Femur was collected for assessment of bone microarchitecture, quality and metabolism. GIO markedly reduced trabecular bone volume density (BV/TV) (-35 %), trabecular thickness (Tb.Th) (-33 %), mineral content of femur (-26 %) as well as bone collagen content (-56 %). Bone strength and its biomechanical properties given by flexural strength (-81 %), fracture load (-80 %), and the number of osteocytes (-84 %) were lowered after GIO. GCs reduced osteoblast number and function while increased osteoclast number, altering bone remodeling (p < 0.05). On the other hand, ExK significantly improved bone microarchitecture and quality, marked by fractal dimension increase (+38 %), cortical volume (+34 %), BV/TV (+34 %), Tb.Th (+33 %), mineral content and collagen maturity, while reduced the space between trabecula (-34 %). The Ex and ExK increased the number of osteocytes (p < 0.05) and they were able to reverse the lower osteoblast number. Both treatments used alone significantly enhanced bone biomechanical properties, but the ExK showed a more significant improvement. ExK ameliorated bone strength and biomechanics (p < 0.05) and stimulated bone formation and modulated bone remodeling (p < 0.05). MK and exercise administered isolated or in association increased the percentage of collagen bone filling after GIO (p < 0.05), but only ExK improved collagen maturity. Our results showed that MK associated to resistance exercise enhanced bone microarchitecture, quality and metabolism, being therefore an interesting tool to improve skeletal response during GIO.

Therapeutic perspectives of exosomes in glucocorticoid-induced osteoarthrosis

Exosomes are widely involved in a variety of physiological and pathological processes. These important roles are also hidden in the physiological processes related to bone. Chondrocytes, osteoblasts, synovial fibroblasts, and bone marrow mesenchymal stem cells produce and secrete exosomes, thereby affecting the biology process of target cells. Furthermore, in the primary pathogenesis of osteoarthrosis induced by steroid hormones, mainly involve glucocorticoid (GC), the exosomes have also widely participated. Therefore, exosomes may also play an important role in glucocorticoid-induced osteoarthrosis and serve as a promising treatment for early intervention of osteoarthrosis in addition to playing a regulatory role in malignant tumors. This review summarizes the previous results on this direction, systematically combs the role and therapeutic potential of exosomes in GC-induced osteoarthrosis, discusses the potential role of exosomes in the treatment and prevention of GC-induced osteoarthrosis, and reveals the current challenges we confronted.

Skeletal Aging

Aging represents the single greatest risk factor for chronic diseases, including osteoporosis, a skeletal fragility syndrome that increases fracture risk. Optimizing bone strength throughout life reduces fracture risk. Factors critical for bone strength include nutrition, physical activity, and vitamin D status, whereas unhealthy lifestyles, illnesses, and certain medications (eg, glucocorticoids) are detrimental. Hormonal status is another important determinant of skeletal health, with sex steroid concentrations, particularly estrogen, having major effects on bone remodeling. Aging exacerbates bone loss in both sexes and results in imbalanced bone resorption relative to formation; it is associated with increased marrow adiposity, osteoblast/osteocyte apoptosis, and accumulation of senescent cells. The mechanisms underlying skeletal aging are as diverse as the factors that determine the strength (and thus fragility) of bone. This review updates our current understanding of the epidemiology, pathophysiology, and treatment of osteoporosis and provides an overview of the underlying hallmark mechanisms that drive skeletal aging.

Bad to the Bone: The Effects of Therapeutic Glucocorticoids on Osteoblasts and Osteocytes

Despite the continued development of specialized immunosuppressive therapies in the form of monoclonal antibodies, glucocorticoids remain a mainstay in the treatment of rheumatological and auto-inflammatory disorders. Therapeutic glucocorticoids are unmatched in the breadth of their immunosuppressive properties and deliver their anti-inflammatory effects at unparalleled speed. However, long-term exposure to therapeutic doses of glucocorticoids decreases bone mass and increases the risk of fractures - particularly in the spine - thus limiting their clinical use. Due to the abundant expression of glucocorticoid receptors across all skeletal cell populations and their respective progenitors, therapeutic glucocorticoids affect skeletal quality through a plethora of cellular targets and molecular mechanisms. However, recent evidence from rodent studies, supported by clinical data, highlights the considerable role of cells of the osteoblast lineage in the pathogenesis of glucocorticoid-induced osteoporosis: it is now appreciated that cells of the osteoblast lineage are key targets of therapeutic glucocorticoids and have an outsized role in mediating their undesirable skeletal effects. As part of this article, we review the molecular mechanisms underpinning the detrimental effects of supraphysiological levels of glucocorticoids on cells of the osteoblast lineage including osteocytes and highlight the clinical implications of recent discoveries in the field.

Biologic and pathologic aspects of osteocytes in the setting of medication-related osteonecrosis of the jaw (MRONJ)

Medication-related osteonecrosis of the jaw (MRONJ) is a potentially severe, debilitating condition affecting patients with cancer and patients with osteoporosis who have been treated with powerful antiresorptives (pARs) or angiogenesis inhibitors (AgIs). Oral risk factors associated with the development of MRONJ include tooth extraction and inflammatory dental disease (e.g., periodontitis, periapical infection). In bone tissues, osteocytes play a bidirectional role in which they not only act as the "receiver" of systemic signals from blood vessels, such as hormones and drugs, or local signals from the mineralized matrix as it is deformed, but they also play a critical role as "transmitter" of signals to the cells that execute bone modeling and remodeling (osteoclasts, osteoblasts and lining cells). When the survival capacity of osteocytes is overwhelmed, they can die. Osteocyte death has been associated with several pathological conditions. Whereas the causes and mechanisms of osteocyte death have been studied in conditions like osteonecrosis of the femoral head (ONFH), few studies of the causes and mechanisms of osteocyte death have been done in MRONJ. The three forms of cell death that affect most of the different cells in the body (apoptosis, autophagy, and necrosis) have been recognized in osteocytes. Notably, necroptosis, a form of regulated cell death with "a necrotic cell death phenotype," has also been identified as a form of cell death in osteocytes under certain pathologic conditions. Improving the understanding of osteocyte death in MRONJ may be critical for preventing disease and developing treatment approaches. In this review, we intend to provide insight into the biology of osteocytes, cell death, in general, and osteocyte death, in particular, and discuss hypothetical mechanisms involved in osteocyte death associated with MRONJ.

Autophagy in Femoral Head Necrosis of Broilers Bone Metabolism Parameters and Autophagy-Related Gene Expression in Femoral Head Necrosis Induced by Glucocorticoid in Broilers

Objectives: In this study, the influence of methylprednisolone (MP) and 3-methyladenine (3-MA) on chondrocyte autophagy and bone quality were determined to investigate the mechanisms of femoral head necrosis in broilers.

Methods: Chickens were divided into four groups: control, MP, 3-MA, and 3-MA+MP groups. Blood and bone samples were collected for biochemistry assay and bone quality determination. Cartilage was separated from the femoral head for histopathological analysis and gene expression detection.

Results: The results indicated that MP treatment significantly affected blood levels of alkaline phosphatase, high-density lipoprotein, calcium, phosphorus, bone alkaline phosphatase, and osteocalcin in broilers. Additionally, MP treatment significantly increased blood levels of cholesterol, low-density lipoprotein, triglyceride, carboxy-terminal telopeptide of type-I collagen, and tartrate-resistant acid phosphatase 5. MP treatment also significantly decreased the levels of bone parameters compared with these values in controls, inhibited the expression of collagen-2, aggrecan, and mammalian target of rapamycin, and increased the expression of beclin1 and microtubule-associated protein 1 light chain 3, hypoxia-inducible factor 1 alpha, phosphoinositide 3-kinase, protein kinase B and autophagy-related gene 5 of the femoral head. Furthermore, following co-treatment with 3-MA and MP, 3-MA mitigated the effects of MP.

Conclusions: Our findings demonstrated that autophagy may be involved in the pathogenesis of femoral head necrosis induced by MP in broilers, and this study provides new treatment and prevention ideas for femoral head necrosis caused by glucocorticoids.

Bone remodeling stages under physiological conditions and glucocorticoid in excess: Focus on cellular and molecular mechanisms

This review provides a rationale for the cellular and molecular mechanisms of bone remodeling stages under physiological conditions and glucocorticoids (GCs) in excess. Remodeling is a synchronous process involving bone resorption and formation, proceeding through stages of: (1) resting bone, (2) activation, (3) bone resorption, (4) reversal, (5) formation, (6) termination. Bone remodeling is strictly controlled by local and systemic regulatory signaling molecules. This review presents current data on the interaction of osteoclasts, osteoblasts and osteocytes in bone remodeling and defines the role of osteoprogenitor cells located above the resorption area in the form of canopies and populating resorption cavities. The signaling pathways of proliferation, differentiation, viability, and cell death during remodeling are presented. The study of signaling pathways is critical to understanding bone remodeling under normal and pathological conditions. The main signaling pathways that control bone resorption and formation are RANK / RANKL / OPG; M-CSF – c-FMS; canonical and non-canonical signaling pathways Wnt; Notch; MARK; TGFβ / SMAD; ephrinB1/ephrinB2 – EphB4, TNFα – TNFβ, and Bim – Bax/Bak. Cytokines, growth factors, prostaglandins, parathyroid hormone, vitamin D, calcitonin, and estrogens also act as regulators of bone remodeling. The role of non-encoding microRNAs and long RNAs in the process of bone cell differentiation has been established. MicroRNAs affect many target genes, have both a repressive effect on bone formation and activate osteoblast differentiation in different ways. Excess of glucocorticoids negatively affects all stages of bone remodeling, disrupts molecular signaling, induces apoptosis of osteocytes and osteoblasts in different ways, and increases the life cycle of osteoclasts. Glucocorticoids disrupt the reversal stage, which is critical for the subsequent stages of remodeling. Negative effects of GCs on signaling molecules of the canonical Wingless (WNT)/β-catenin pathway and other signaling pathways impair osteoblastogenesis. Under the influence of excess glucocorticoids biosynthesis of biologically active growth factors is reduced, which leads to a decrease in the expression by osteoblasts of molecules that form the osteoid. Glucocorticoids stimulate the expression of mineralization inhibitor proteins, osteoid mineralization is delayed, which is accompanied by increased local matrix demineralization. Although many signaling pathways involved in bone resorption and formation have been discovered and described, the temporal and spatial mechanisms of their sequential turn-on and turn-off in cell proliferation and differentiation require additional research.

Investigate the influence of microplastics weathering on their heavy metals uptake in stormwater

Plastic debris as the main portion of urban litters could be transported via storm runoff to the water resources. In this study the influence of microplastics (MPs) weathering on their Pb²⁺ and Zn²⁺ uptake in stormwater was examined. Low-density polyethylene (LDPE) and polyethylene terephthalate (PET) MPs were subjected to weathering through mechanical interaction with a mixture of silt/sand, and in synthetic stormwater. The surface analysis revealed significant physio-chemistry alterations of LDPE MPs due to the silt/sand weathering. However, this weathering mostly resulted in the surface morphology alterations of PET MPs. The kinetics of heavy metals adsorptions onto the new and stormwater weathered LDPE MPs were best described by pseudo 1st and 2nd models, respectively. Despite increasing Pb²⁺ uptake by weathered PET MPs, Zn²⁺ uptake by both new and weathered PET MPs was below the detection limit. Both Pb²⁺ and Zn²⁺ were released from new and silt/sand weathered LDPE MPs during five days exposure to the synthetic stormwater. This study underscores the critical role of plastic type and weathering conditions on heavy metal transport by MPs from the urban environment to the water resources.

Connexin Gap Junctions and Hemichannels Link Oxidative Stress to Skeletal Physiology and Pathology

PURPOSE OF REVIEW

The goal of this review is to provide an overview of the impact and underlying mechanism of oxidative stress on connexin channel function, and their roles in skeletal aging, estrogen deficiency, and glucocorticoid excess associated bone loss.

RECENT FINDINGS

Connexin hemichannel opening is increased under oxidative stress conditions, which confers a cell protective role against oxidative stress-induced cell death. Oxidative stress acts as a key contributor to aging, estrogen deficiency, and glucocorticoid excess-induced osteoporosis and impairs osteocytic network and connexin gap junction communication. This paper reviews the current knowledge for the role of oxidative stress and connexin channels in the pathogenesis of osteoporosis and physiological and pathological responses of connexin channels to oxidative stress. Oxidative stress decreases osteocyte viability and impairs the balance of anabolic and catabolic responses. Connexin 43 (Cx43) channels play a critical role in bone remodeling, mechanotransduction, and survival of osteocytes. Under oxidative stress conditions, there is a consistent reduction of Cx43 expression, while the opening of Cx43 hemichannels protects osteocytes against cell injury caused by oxidative stress.

Glucocorticoid-induced autophagy and apoptosis in bone

Glucocorticoids are widely prescribed to treat various allergic and autoimmune diseases; however, long-term use results in glucocorticoid-induced osteoporosis, characterized by consistent changes in bone remodeling with decreased bone formation as well as increased bone resorption. Not only bone mass but also bone quality decrease, resulting in an increased incidence of fractures. The primary role of autophagy is to clear up damaged cellular components such as long-lived proteins and organelles, thus participating in the conservation of different cells. Apoptosis is the physiological death of cells, and plays a crucial role in the stability of the environment inside a tissue. Available basic and clinical studies indicate that autophagy and apoptosis induced by glucocorticoids can regulate bone metabolism through complex mechanisms. In this review, we summarize the relationship between apoptosis, autophagy and bone metabolism related to glucocorticoids, providing a theoretical basis for therapeutic targets to rescue bone mass and bone quality in glucocorticoid-induced osteoporosis.

The Osteocyte: New Insights

Osteocytes are an ancient cell, appearing in fossilized skeletal remains of early fish and dinosaurs. Despite its relative high abundance, even in the context of nonskeletal cells, the osteocyte is perhaps among the least studied cells in all of vertebrate biology. Osteocytes are cells embedded in bone, able to modify their surrounding extracellular matrix via specialized molecular remodeling mechanisms that are independent of the bone forming osteoblasts and bone-resorbing osteoclasts. Osteocytes communicate with osteoclasts and osteoblasts via distinct signaling molecules that include the RankL/OPG axis and the Sost/Dkk1/Wnt axis, among others. Osteocytes also extend their influence beyond the local bone environment by functioning as an endocrine cell that controls phosphate reabsorption in the kidney, insulin secretion in the pancreas, and skeletal muscle function. These cells are also finely tuned sensors of mechanical stimulation to coordinate with effector cells to adjust bone mass, size, and shape to conform to mechanical demands.

Osteocyte apoptosis: the roles and key molecular mechanisms in resorption-related bone diseases

Vital osteocytes have been well known to function as an important orchestrator in the preservation of robustness and fidelity of the bone remodeling process. Nevertheless, some key pathological factors, such as sex steroid deficiency and excess glucocorticoids, and so on, are implicated in inducing a bulk of apoptotic osteocytes, subsequently resulting in resorption-related bone loss. As much, osteocyte apoptosis, under homeostatic conditions, is in an optimal state of balance tightly controlled by pro- and anti-apoptotic mechanism pathways. Importantly, there exist many essential signaling proteins in the process of osteocyte apoptosis, which has a crucial role in maintaining a homeostatic environment. While increasing in vitro and in vivo studies have established, in part, key signaling pathways and cross-talk mechanism on osteocyte apoptosis, intrinsic and complex mechanism underlying osteocyte apoptosis occurs in various states of pathologies remains ill-defined. In this review, we discuss not only essential pro- and anti-apoptotic signaling pathways and key biomarkers involved in these key mechanisms under different pathological agents, but also the pivotal role of apoptotic osteocytes in osteoclastogenesis-triggered bone loss, hopefully shedding new light on the attractive and proper actions of pharmacotherapeutics of targeting apoptosis and ensuing resorption-related bone diseases such as osteoporosis and fragility fractures.

Targeting autophagy in osteoporosis: From pathophysiology to potential therapy

Osteoporosis is a highly prevalent disorder characterized by the loss of bone mass and microarchitecture deterioration of bone tissue, attributed to various factors, including menopause (primary), aging (primary) and adverse effects of relevant medications (secondary). In recent decades, knowledge regarding the etiological mechanisms underpinning osteoporosis emphasizes that bone cellular homeostasis, including the maintenance of cell functions, differentiation, and the response to stress, is tightly regulated by autophagy, which is a cell survival mechanism for eliminating and recycling damaged proteins and organelles. With the important roles in the maintenance of cellular homeostasis and organ function, autophagy has emerged as a potential target for the prevention and treatment of osteoporosis. In this review, we update and discuss the pathophysiology of autophagy in normal bone cell life cycle and metabolism. Then, the alternations of autophagy in primary and secondary osteoporosis, and the accompanied pathological process are discussed. Finally, we discuss current strategies, limitations, and challenges involved in targeting relevant pathways and propose strategies by which such hurdles may be circumvented in the future for their translation into clinical validations and applications for the prevention and treatment of osteoporosis.

Blocking glucocorticoid signaling in osteoblasts and osteocytes prevents mechanical unloading-induced cortical bone loss

Bone loss has been supposed to be the greatest damage to the health of astronauts. It is generally believed that the mechanical unloading induced by microgravity is the main cause of bone loss. However, besides mechanical unloading, many evidences from animal models and spaceflight missions indicate that microgravity conditions can cause some stress reactions and elevated endogenous glucocorticoid (GC) levels. High levels of GCs can lead to bone loss. This study aimed to investigate whether elevated GC levels are involved in hindlimb unloading (HLU)-induced bone loss in mice. Col2.3-11β-hydroxysteroid dehydrogenase type 2 (Col2.3-11β-HSD2) transgenic mice which are characterized by specific blocking GC signaling in mature osteoblasts and osteocytes were used. Male 14-week-old Col2.3-11β-HSD2 transgenic mice and wild type littermates were tail-suspended or kept under ambulatory conditions. At the endpoint, the tibias were examined by micro-computed tomography and histomorphometry, and bone turnover was analyzed by serum biochemistry, histochemistry staining, immunohistochemistry, and real-time PCR. Mice exposed to unloading occurred a significant increase in serum GC concentrations. Compared with non-unloaded controls, HLU led to a severe damage in cortical bone microstructure and bone strength of the tibia in wild type mice but not transgenic littermates. Osteoblast activity and bone formation were inhibited, whereas osteoclast activity and bone resorption were promoted in the tibial cortical bone of wild type mice following HLU, features absented in transgenic mice. Furthermore, HLU resulted in a significant increase in the number of sclerostin-producing and receptor activator of nuclear factor-κ B ligand (RANKL)-positive osteocytes, and apoptotic osteoblasts and osteocytes in wild type mice of unloading but not in unloaded transgenic mice. In conclusion, cortical bone loss during HLU is mediated through enhancing GC signaling in osteoblasts and osteocytes and subsequently restraining bone formation and activating bone resorption. It suggests that elevated GC levels play an important role in cortical bone loss in response to mechanical unloading.

Dose-related histopathology and bone remodeling characteristics of the knee articular cartilage and subchondral bone induced by glucocorticoids in rats

The aim of the current study was to investigate histopathological changes and bone remodeling in the knee articular cartilage and subchondral bone in rats following treatment with glucocorticoids. A total of 30 3-month-old female Sprague-Dawley rats were randomly divided into either a vehicle control group or one of three experimental groups wherein dexamethasone (Dex) was administered at a dose of 1.0, 2.5 or 5.0 mg/kg (Dex1.0, Dex2.5 and Dex5.0, respectively), for 8 weeks. Articular cartilage and the epiphyseal subchondral bone of the proximal tibias were evaluated by histopathology or for bone remodeling using histomorphometry. No histological changes were identified in the knee articular cartilage but the bone formation rate of the subchondral bone was lower in the Dex1.0 group compared with that of the control group. Compared with the control and the Dex1.0 group, the width of the articular cartilage and the subchondral plate were larger, with abnormal morphology and increased apoptosis of chondrocytes, decreased cell/matrix volume ratio in the cartilage and fewer blood vessels in the subchondral plate in the Dex2.5 and Dex5.0 groups. A higher Dex dose resulted in more severe inhibition of bone formation, a greater number of apoptotic osteocytes and constrained bone resorption. All microstructure parameters indicated no significant changes in the Dex2.5 group but exhibited deterioration in the Dex5.0 group compared with the normal and Dex1.0 group. There were no significant differences in morphological changes, or in static and dynamic bone indices between the Dex2.5 and Dex5.0 groups. In conclusion, long-term glucocorticoid use induced dose-related histopathological changes in the knee articular cartilage, along with unbalanced bone remodeling and osteopenia in the subchondral bone. The degree of damage to the articular cartilage was milder and transformed from compensation to degeneration at higher doses.

Glucocorticoid-Induced Osteoporosis: New Insights into the Pathophysiology and Treatments

PURPOSE OF THIS REVIEW

The goal of the review is to provide an updated understanding of the pathophysiology of glucocorticoid-induced osteoporosis and treatment recommendations.

RECENT FINDINGS

Glucocorticoids reduce osteoblast and osteocyte lifespan and activity and reduce the vascularity of the bone that together may explain the greater reductions in bone strength than those of bone mass. Treatments with parathyroid hormone fragments appear to reverse glucocorticoid-induced bone loss and fracture risk partially through maintaining bone vascularity and bone strength. This review identifies how glucocorticoid anti-osteogenic and vascular effects together may reduce bone strength. It also provides guidance to clinicians on rationale treatment for glucocorticoid-induced osteoporosis.

Glucocorticoid-Induced Autophagy Protects Osteocytes Against Oxidative Stress Through Activation of MAPK/ERK Signaling

Autophagy confers protective or detrimental effects on cells depending on the cellular context. We showed here that oxidative stress‐induced cell death in osteocytic MLO‐Y4 cells coincided with decreased autophagy. Decreased autophagy was also observed in osteocytes of superoxide dismutase 1‐ (SOD1‐) deficient mice. Oxidative stress‐induced osteocyte death was exacerbated by an autophagy inhibitor, chloroquine, suggesting a protective function of basal autophagy levels against oxidative stress‐induced cell death. Pretreatment with dexamethasone reduced the susceptibility of osteocytes to oxidative stress‐induced cell death and conferred protection against TNFα/cycloheximide‐induced cell death. Inhibition of MAPK/ERK attenuated the formation of autophagosome, leading to increased osteocyte cell death. Taken together, our results suggest that autophagy, induced by moderate levels of glucocorticoids, leads to the preconditioning of osteocytes and conveys a novel cell‐protective function against cell death induced by oxidative stress and other insults. © 2018 The Authors. JBMR Plus is published by Wiley Periodicals, Inc. on behalf of the American Society for Bone and Mineral Research.

Autophagy as a target for glucocorticoid-induced osteoporosis therapy

Autophagy takes part in regulating the eukaryotic cells function and the progression of numerous diseases, but its clinical utility has not been fully developed yet. Recently, mounting evidences highlight an important correlation between autophagy and bone homeostasis, mediated by osteoclasts, osteocytes, bone marrow mesenchymal stem cells, and osteoblasts, and autophagy plays a vital role in the pathogenesis of glucocorticoid-induced osteoporosis (GIOP). The combinations of autophagy activators/inhibitors with anti-GIOP first-line drugs or some new autophagy-based manipulators, such as regulation of B cell lymphoma 2 family proteins and caspase-dependent clearance of autophagy-related gene proteins, are likely to be the promising approaches for GIOP clinical treatments. In view of the important role of autophagy in the pathogenesis of GIOP, here we review the potential mechanisms about the impacts of autophagy in GIOP and its association with GIOP therapy.

Role of nutritional vitamin D in osteoporosis treatment

Osteoporosis is a systemic skeletal disorder characterized by a decrease in bone mass and microarchitectural deterioration of bone tissue. The World Health Organization has defined osteoporosis as a decrease in bone mass (50%) and bony quality (50%). Vitamin D, a steroid hormone, is crucial for skeletal health and in mineral metabolism. Its direct action on osteoblasts and osteoclasts and interaction with nonskeletal tissues help in maintaining a balance between bone turnover and bone growth. Vitamin D affects the activity of osteoblasts, osteoclasts, and osteocytes, suggesting that it affects bone formation, bone resorption, and bone quality. At physiological concentrations, active vitamin D maintains a normal rate of bone resorption and formation through the RANKL/OPG signal. However, active vitamin D at pharmacological concentration inhibits bone resorption at a higher rate than that of bone formation, which influences the bone quality and quantity. Nutritional vitamin D rather than active vitamin D activates osteoblasts and maintains serum 25(OH)D₃ concentration. Despite many unanswered questions, much data support nutritional vitamin D use in osteoporosis patients. This article emphasizes the role of nutritional vitamin D replacement in different turnover status (high or low bone turnover disorders) of osteoporosis together with either anti-resorptive (Bisphosphonate, Denosumab et.) or anabolic (Teriparatide) agents when osteoporosis persists.

Dietary flavonoid kaempferol inhibits glucocorticoid-induced bone loss by promoting osteoblast survival

OBJECTIVE

Kaempferol, a dietary flavonoid found in fruits and vegetables, has been reported to reverse osteopenic condition in ovariectomized rats. Because kaempferol is endowed with osteogenic activity, the aim of this study was to determine whether it has a beneficial effect on glucocorticoid (GC)-induced bone loss.

METHODS

Adult female rats were divided into four groups as control (vehicle; distilled water), methylprednisolone (MP; 5 mg•kg•d, subcutaneously), MP + kaempferol (5 mg•kg•d, oral), and MP + human parathyroid 1-34 (30 µg/kg, 5 times/wk, subcutaneously) and treated for 4 wk. To study the antagonizing effect of kaempferol on GC-induced inhibition of fracture healing, drill-hole injury was performed on control and GC-treated rats. An oral dose of kaempferol was given for 14 d to observe the effect on callus formation at the site of injury. After treatment, bones were collected for further analysis.

RESULTS

GC was associated with a decreased bone mineral density and impaired bone microarchitecture parameters. Consumption of kaempferol induced bone-sparing effects in GC-induced osteopenic condition. Additionally, improved callus formation at site of drill injury in femur diaphysis was observed with kaempferol consumption in animals on GC. Consistent with the in vivo data, kaempferol elicited a higher expression of osteogenic markers in vitro and antagonized the apoptotic effect of dexamethasone on calvarial osteoblasts.

CONCLUSION

These results suggested that kaempferol reduced GC-induced bone loss and enhanced bone regeneration at fractured site, thus emphasizing the positive role of flavonoids on bone health.

Proteasome-mediated degradation of collagen III by cortisol in amnion fibroblasts

Rupture of fetal membranes (ROM) can initiate parturition at both term and preterm. Collagen III in the compact layer of the amnion contributes to the tensile strength of fetal membranes. However, the upstream signals triggering collagen III degradation remain mostly elusive. In this study, we investigated the role of cortisol regenerated by 11β-hydroxysteroid dehydrogenase 1 (11β-HSD1) in collagen III degradation in human amnion fibroblasts with an aim to seek novel targets for the prevention of preterm premature ROM (pPROM)-elicited preterm birth. Human amnion tissue and cultured amnion tissue explants and amnion fibroblasts were used to study the regulation of collagen III, which is composed of three identical 3α 1 chains (COL3A1), by cortisol. Cortisol decreased COL3A1 protein but not mRNA abundance in a concentration-dependent manner. Cortisone also decreased COL3A1 protein, which was blocked by 11β-HSD1 inhibition. The reduction in COL3A1 protein by cortisol was not affected by a transcription inhibitor but was further enhanced by a translation inhibitor. Autophagic pathway inhibitor chloroquine or siRNA-mediated knock-down of ATG7, an essential protein for autophagy, failed to block cortisol-induced reduction in COL3A1 protein abundance, whereas proteasome pathway inhibitors MG132 and bortezomib significantly attenuated cortisol-induced reduction in COL3A1 protein abundance. Moreover, cortisol increased COL3A1 ubiquitination and the reduction of COL3A1 protein by cortisol was blocked by PYR-41, a ubiquitin-activating enzyme inhibitor. Conclusively, cortisol regenerated in amnion fibroblasts may be associated with ROM at parturition by reducing collagen III protein abundance through a ubiquitin-proteasome pathway.

Effects of estrogen status in osteocyte autophagy and its relation to osteocyte viability in alveolar process of ovariectomized rats

Estrogen maintains osteocyte viability, whereas its deficiency induces osteocyte apoptosis. As autophagy is important for osteocyte viability, we hypothesized whether the anti-apoptotic effect of estrogen is related to autophagy in osteocytes. Thirty adult female rats were sham-operated (SHAM) or ovariectomized (OVX). After three weeks, twelve rats of SHAM and OVX groups were killed before treatment (basal period), whereas the remaining rats received estrogen (OVXE) or vehicle (OVX) for 45 days. Fragments of maxilla containing alveolar process of the first molars were embedded in paraffin or Araldite. Paraffin-sections were stained with hematoxylin/eosin for histomorphometry, or subjected to the silver impregnation method for morphological analysis of osteocyte cytoplasmic processes. Autophagy was analyzed by immunohistochemical detections of beclin-1, MAP-LC3α and p62, whereas apoptosis was evaluated by immunohistochemical detections of cleaved caspase-3 and BAX, TUNEL (Terminal deoxynucleotidyl transferase dUTP nick end labeling) method and by ultrastructural analysis. Araldite-semithin sections were subjected to the Sudan-black method for detection of lipids. OVX-basal group showed high frequency of caspase-3-, TUNEL- and p62-positive osteocytes accompanied with low frequency of beclin-1- and MAP-LC3α-positive osteocytes. At 45 days, OVXE group exhibited higher number of osteocytes, higher frequency of beclin-1- and MAP-LC3α-positive osteocytes, and lower frequency of caspase-3, BAX-, TUNEL- and p62-positive osteocytes than OVX group. Significant reduction in bone area was observed in the OVX compared to OVXE and SHAM groups. The highest frequency of Sudan-Black-positive osteocytes and osteocytes with scarce cytoplasmic processes, or showing apoptotic features were mainly observed in OVX groups. Our results indicate that estrogen deficiency decreases autophagy and increases apoptosis, whereas estrogen replacement enhances osteocyte viability by inhibiting apoptosis and maintaining autophagy in alveolar process osteocytes. These results suggest that the anti-apoptotic effect of estrogen may be, at least in part, related to autophagy regulation in osteocytes.

Glucocorticoid impairs cell-cell communication by autophagy-mediated degradation of connexin 43 in osteocytes

Osteocytes comprising over 90% of the bone cell population are highly susceptible to the adverse effects of glucocorticoids (GC) administration. Here we observed that Dexamethasone (Dex) induces a robust cytoskeleton rearrangement and decreases Cx43 protein expression in osteocyte-like MLO-Y4 cells. Using a Dmp1Cre-mT/mG osteocyte ex vivo culture system, we found significant shortening of dendritic processes in primary osteocytes following Dex administration. Loss of dendritic processes is a consequence of reduced Cx43 connectivity upon Dex induced autophagy in both RFP-GFP-LC3B transfected MLO-Y4 cells and primary calvarial osteocytes from LC3GFP transgenic mice. Upon the induction of autophagy by Dex, Cx43 was internalized into autophagosome/autolysosomes and degraded by autophagy. The degradation was attenuated following lysosomal inhibition using chloroquine (CLQ) and suppression of autophagy by Atg5 silencing. Inhibition Akt-mTORC1 signaling by Dex induces autophagy subsequently resulting in Cx43 degradation.Activation of Akt phosphorylation by IGF-1 attenuated Dex induced autophagy and degradation of Cx43. Together, we demonstrated that GC impair osteocyte cell-cell connectivity via autophagy mediated degradation of Cx43 through inhibition of the Akt-mTORC1 signaling. This may account for the deleterious effect of GC-induced bone loss.

Osteoporosis and autophagy: What is the relationship?

Autophagy is a survival pathway wherein non-functional proteins and organelles are degraded in lysosomes for recycling and energy production. Therefore, autophagy is fundamental for the maintenance of cell viability, acting as a quality control process that prevents the accumulation of unnecessary structures and oxidative stress. Increasing evidence has shown that autophagy dysfunction is related to several pathologies including neurodegenerative diseases and cancer. Moreover, recent studies have shown that autophagy plays an important role for the maintenance of bone homeostasis. For instance, in vitro and animal and human studies indicate that autophagy dysfunction in bone cells is associated with the onset of bone diseases such as osteoporosis. This review had the purpose of discussing the issue to confirm whether a relationship between autophagy dysfunction and osteoporosis exits.

Corticosteroid treatment exacerbates bone osteopenia in mice with gonadal hormone deficiency-induced osteoporosis

Gonadic deficiency and corticotherapy are important risk factors in the pathogenesis of osteoporosis. This study was outlined to assess the effects of combined orchidectomy (ORX) and corticosteroid (cortisol; CS) administration on bone remodeling and metabolism. Twenty-week-old male Swiss mice were randomized into four groups: either sham operated (sham), ORX, CS injected (CS), or ORX and CS injected (ORX+CS). After 28days, mice were euthanized. Both ORX and CS resulted in reduced trabecular volume, and mineral apposition rate and increased osteoclast number and activity. TRAcP levels were increased in ORX and CS mice, but reached highest values in ORX+CS. Bone and serum mineral content (calcium and phosphorus) were disrupted in ORX and CS groups when compared to Sham, and were more affected in ORX+CS group. Urinary calcium measures were increased in ORX, CS, and ORX+CS during the time course of the study. Increases were more prominent in ORX+CS. The differences between groups were generally more accentuated at ORX+CS group. Biochemical data showed a parallel extent to the histologic and histomorphometric changes. This study provides a valid pre-clinical model for severe and rapid osteopenia by ORX associated corticotherapy in which bone loss was significantly higher than either ORX or CS alones.

Glucocorticoids Suppress Antimicrobial Autophagy and Nitric Oxide Production and Facilitate Mycobacterial Survival in Macrophages

Chronic administration of glucocorticoids has been shown to render individuals highly susceptible to mycobacterial infection and lead to reactivation of latent bacilli. However, the effect of glucocorticoids on innate anti-mycobacterial defense, especially in macrophages remains largely unknown. Here, we found that glucocorticoids inhibited the innate immune response, antimicrobial nitric oxide production and autophagy in mycobacteria-challenged macrophages. Meanwhile, maturation and acidification of mycobacterial phagosomes were attenuated in RAW264.7 cells after glucocorticoids treatment. Consequently, we observed a glucocorticoid-induced increase in the survival of intracellular mycobacteria in both primary macrophages and cell lines. Glucocorticoids treatment decreased the activation of TBK1 kinase, which promotes the maturation of autophagosomes. Inhibition of TBK1 also decreased the production of nitric oxide. Furthermore, several autophagy-related genes were down-regulated, while activation of the Akt/mTOR signaling pathway was increased after glucocorticoids treatment, which may account for autophagy inhibition during mycobacterial infection. Restoration of autophagy with the agonist rapamycin abolished glucocorticoid-mediated enhancement of mycobacterial survival, suggesting that glucocorticoids blocked anti-mycobacterial defense via autophagy inhibition. Collectively, this study demonstrates that glucocorticoids impair innate antimicrobial autophagy and promote mycobacterial survival in macrophages, which is a novel mechanism for glucocorticoid-mediated immunosuppression. Our findings may provide important clues for tuberculosis prevention.

Autophagic Degradation of Collagen 1A1 by Cortisol in Human Amnion Fibroblasts

Rupture of fetal membranes can initiate parturition at both term and preterm. Collagen is the crucial factor determining the tensile strength of the membranes. Toward the end of gestation, a feed-forward regeneration of cortisol via 11β-hydroxysteroid dehydrogenase 1 exists in fetal membranes. It remains undetermined whether cortisol contributes to collagen reduction in fetal membranes. In this study, we have examined whether cortisol accumulation is a causative factor for collagen reduction in human amnion fibroblasts, the major source of collagens in the membranes. Cortisol had no effect on collagen 1A1 (COL1A1) and 1A2 (COL1A2) messenger RNA (mRNA) abundance but decreased their protein abundance. The latter effect was affected by neither mRNA transcription inhibitor nor protein translation inhibitor. Mechanistic studies revealed that the reduction in COL1A1 but not COL1A2 protein by cortisol was blocked by lysosome inhibitor chloroquine or small interfering RNA (siRNA)-mediated knockdown of autophagy-related protein 7, an essential protein for autophagy, whereas the proteasome inhibitors MG132 and bortezomib were ineffective. Further analysis showed that cortisol dose dependently increased the ratio of LC3II/LC3I, a marker of lysosome activation, an effect blocked by the glucocorticoid receptor (GR) antagonist RU486 and siRNA-mediated knockdown of GR. Consistently, cortisol decreased COL1A1 and COL1A2 protein abundance in amnion tissue explants, and decreased COL1A1 and COL1A2 protein abundance was observed at parturition in the amnion tissue. Conclusively, cortisol regeneration in fetal membranes may contribute to rupture of fetal membranes at parturition by reducing collagen protein abundance. Lysosome-mediated autophagy accounts for the reduction in COL1A1 by cortisol, but the mechanism underlying the reduction in COL1A2 awaits further investigation.

Anti-osteoclastogenesis of Mineral Trioxide Aggregate through Inhibition of the Autophagic Pathway

INTRODUCTION

Mineral trioxide aggregate (MTA) regulates bone remodeling, particularly osteoclast differentiation. However, intracellular mechanisms underlying the anti-osteoclastogenesis of MTA remain unclear. This study aimed to evaluate the potential alterations of autophagic pathway during anti-osteoclastogenic effects by MTA in vitro and investigate their underlying mechanisms.

METHODS

Osteoclast precursors were treated with MTA extracts containing the receptor activator of nuclear factor-kappa B ligand (RANKL). Rapamycin was used to activate autophagy. RANKL-induced osteoclast differentiation was stained with tartrate-resistant acid phosphatase. Several specific autophagy features in osteoclast precursors were measured by using immunofluorescence, monodansylcadaverine, and transmission electron microscope. Autophagy-related proteins were investigated via Western blot analysis. The mRNA expression involved in autophagic and osteoclastic activities was detected with quantitative real-time polymerase chain reaction.

RESULTS

MTA extracts inhibited osteoclast differentiation via preventing the fusion of osteoclast precursors without cytotoxicity. MTA extracts interrupted RANKL-induced acidic vesicular organelle formation and autophagic vacuole appearance in osteoclast precursors. Moreover, autophagic genes and proteins stimulated with RANKL diminished with MTA extracts. Notably, autophagy activation through rapamycin promoted multinucleated osteoclasts formation and increased osteoclastic genes expression, which almost reversed MTA-mediated anti-osteoclastogenic effects.

CONCLUSIONS

MTA inhibited osteoclastogenesis for bone repair through attenuating the autophagic pathway.

Reciprocal Crosstalk Between Autophagic and Endocrine Signaling in Metabolic Homeostasis

Autophagy is a cellular quality control and energy-providing process that is under strict control by intra- and extracellular stimuli. Recently, there has been an exponential increase in autophagy research and its implications for mammalian physiology. Autophagy deregulation is now being implicated in many human diseases, and its modulation has shown promising results in several preclinical studies. However, despite the initial discovery of autophagy as a hormone-regulated process by De Duve in the early 1960s, endocrine regulation of autophagy still remains poorly understood. In this review, we provide a critical summary of our present understanding of the basic mechanism of autophagy, its regulation by endocrine hormones, and its contribution to endocrine and metabolic homeostasis under physiological and pathological settings. Understanding the cross-regulation of hormones and autophagy on endocrine cell signaling and function will provide new insight into mammalian physiology as well as promote the development of new therapeutic strategies involving modulation of autophagy in endocrine and metabolic disorders.

Tetramethylpyrazine Protects Against Glucocorticoid-Induced Apoptosis by Promoting Autophagy in Mesenchymal Stem Cells and Improves Bone Mass in Glucocorticoid-Induced Osteoporosis Rats

Glucocorticoid-induced osteoporosis (GIOP) is a widespread clinical complication due to the common use of glucocorticoids. Excess glucocorticoids induce apoptosis of bone marrow-derived mesenchymal stem cells (BMSCs), which have been shown to play an increasingly important role in the pathogenesis and therapy of osteoporosis. Tetramethylpyrazine (TMP), an extract from one of the most recognized herbs in traditional Chinese medicine (Chuanxiong), has been reported to have antiapoptotic properties. In this study, we tested whether TMP protects rat BMSCs following exposure to glucocorticoids in vitro and in vivo. We treated BMSCs with different concentrations of TMP (50, 100, or 200 μM) and exposed them to 10^-6 M dexamethasone (Dex) for 48 h in vitro. Our data showed that TMP inhibited Dex-induced cytotoxicity and protected BMSCs from apoptosis. Interestingly, further results demonstrated that TMP prevented apoptosis in BMSCs by promoting autophagy in an AMP-activated protein kinase (AMPK) and mammalian target of rapamycin (mTOR) pathway-dependent manner. In addition, calcein fluorescence double labeling and microcomputed tomography scanning indicated that 12 weeks of TMP administration augmented bone formation and protected trabecular bone mass in GIOP rats. We also discovered that first-passage BMSCs isolated from the TMP treatment group had a lower rate of apoptosis and a higher light chain 3 (LC3)-II/LC3-I ratio than the GIOP group. Our findings demonstrate for the first time that TMP can protect BMSCs from exposure to excess glucocorticoids by promoting autophagy through AMPK/mTOR pathway and might be an effective agent for the prevention and treatment of GIOP.

The anti-metastatic micro-environment of the bone: Importance of osteocyte Cx43 hemichannels

Bone metastases of tumor cells are a common and life-threatening feature of a variety of late-stage cancers, including breast cancers. However, until now, much less has been known about the intrinsic anti-metastatic properties of the bones and how these could be exploited to prevent or treat bone metastases. Very recently, native Cx43 hemichannels present in osteocytes have been identified as important anti-metastatic signaling complexes by establishing high local extracellular ATP levels. Moreover, bisphosphonate drugs, applied as adjuvant therapies in the treatment of breast cancer patients and bone diseases, are known to display anti-metastatic properties. Now, it became clear that these compounds exert their effects through osteocyte Cx43 hemichannels, thereby triggering their opening and promoting ATP release in the extracellular micro-environment. Hence, endogenous osteocyte Cx43 hemichannels emerge as important and promising therapeutic targets for the prevention of bone metastases and/or clinical treatment of bone-metastasized breast cancers.

Molecular Actions of Glucocorticoids in Cartilage and Bone During Health, Disease, and Steroid Therapy

Cartilage and bone are severely affected by glucocorticoids (GCs), steroid hormones that are frequently used to treat inflammatory diseases. Major complications associated with long-term steroid therapy include impairment of cartilaginous bone growth and GC-induced osteoporosis. Particularly in arthritis, GC application can increase joint and bone damage. Contrarily, endogenous GC release supports cartilage and bone integrity. In the last decade, substantial progress in the understanding of the molecular mechanisms of GC action has been gained through genome-wide binding studies of the GC receptor. These genomic approaches have revolutionized our understanding of gene regulation by ligand-induced transcription factors in general. Furthermore, specific inactivation of GC signaling and the GC receptor in bone and cartilage cells of rodent models has enabled the cell-specific effects of GCs in normal tissue homeostasis, inflammatory bone diseases, and GC-induced osteoporosis to be dissected. In this review, we summarize the current view of GC action in cartilage and bone. We further discuss future research directions in the context of new concepts for optimized steroid therapies with less detrimental effects on bone.

The Proteasome Inhibitor Bortezomib Maintains Osteocyte Viability in Multiple Myeloma Patients by Reducing Both Apoptosis and Autophagy: A New Function for Proteasome Inhibitors

Multiple myeloma (MM) is characterized by severely imbalanced bone remodeling. In this study, we investigated the potential effect of proteasome inhibitors (PIs), a class of drugs known to stimulate bone formation, on the mechanisms involved in osteocyte death induced by MM cells. First, we performed a histological analysis of osteocyte viability on bone biopsies on a cohort of 37 MM patients with symptomatic disease. A significantly higher number of viable osteocytes was detected in patients treated with a bortezomib (BOR)-based regimen compared with those treated without BOR. Interestingly, both osteocyte autophagy and apoptosis were affected in vivo by BOR treatment. Thereafter, we checked the in vitro effect of BOR to understand the mechanisms whereby BOR maintains osteocyte viability in bone from MM patients. We found that osteocyte and preosteocyte autophagic death was triggered during coculturing with MM cells. Our evaluation was conducted by analyzing either autophagy markers microtubule-associated protein light chain 3 beta (LC3B) and SQSTM1/sequestome 1 (p62) levels, or the cell ultrastructure by transmission electron microscopy. PIs were found to increase the basal levels of LC3 expression in the osteocytes while blunting the myeloma-induced osteocyte death. PIs also reduced the autophagic death of osteocytes induced by high-dose dexamethasone (DEX) and potentiated the anabolic effect of PTH(1-34). Our data identify osteocyte autophagy as a new potential target in MM bone disease and support the use of PIs to maintain osteocyte viability and improve bone integrity in MM patients.

Multiscale alterations in bone matrix quality increased fragility in steroid induced osteoporosis

A serious adverse clinical effect of glucocorticoid steroid treatment is secondary osteoporosis, enhancing fracture risk in bone. This rapid increase in bone fracture risk is largely independent of bone loss (quantity), and must therefore arise from degradation of the quality of the bone matrix at the micro- and nanoscale. However, we lack an understanding of both the specific alterations in bone quality n steroid-induced osteoporosis as well as the mechanistic effects of these changes. Here we demonstrate alterations in the nanostructural parameters of the mineralized fibrillar collagen matrix, which affect bone quality, and develop a model linking these to increased fracture risk in glucocorticoid induced osteoporosis. Using a mouse model with an N-ethyl-N-nitrosourea (ENU)-induced corticotrophin releasing hormone promoter mutation (Crh(-120/+)) that developed hypercorticosteronaemia and osteoporosis, we utilized in situ mechanical testing with small angle X-ray diffraction, synchrotron micro-computed tomography and quantitative backscattered electron imaging to link altered nano- and microscale deformation mechanisms in the bone matrix to abnormal macroscopic mechanics. We measure the deformation of the mineralized collagen fibrils, and the nano-mechanical parameters including effective fibril modulus and fibril to tissue strain ratio. A significant reduction (51%) of fibril modulus was found in Crh(-120/+) mice. We also find a much larger fibril strain/tissue strain ratio in Crh(-120/+) mice (~1.5) compared to the wild-type mice (~0.5), indicative of a lowered mechanical competence at the nanoscale. Synchrotron microCT show a disruption of intracortical architecture, possibly linked to osteocytic osteolysis. These findings provide a clear quantitative demonstration of how bone quality changes increase macroscopic fragility in secondary osteoporosis.

Sclerostin-antibody treatment of glucocorticoid-induced osteoporosis maintained bone mass and strength

This study was to determine if antibody against sclerostin (Scl-Ab) could prevent glucocorticoid (GC)-induced osteoporosis in mice. We found that Scl-Ab prevented GC-induced reduction in bone mass and bone strength and that the anabolic effects of Scl-Ab might be partially achieved through the preservation of osteoblast activity through autophagy.

INTRODUCTION

Glucocorticoids (GCs) inhibit bone formation by altering osteoblast and osteocyte cell activity and lifespan. A monoclonal antibody against sclerostin, Scl-Ab, increased bone mass in both preclinical animal and clinical studies in subjects with low bone mass. The objectives of this study were to determine if treatment with the Scl-Ab could prevent loss of bone mass and strength in a mouse model of GC excess and to elucidate if Scl-Ab modulated bone cell activity through autophagy.

METHODS

We generated reporter mice that globally expressed dsRed fused to LC3, a protein marker for autophagosomes, and evaluated the dose-dependent effects of GCs (0, 0.8, 2.8, and 4 mg/kg/day) and Scl-Ab on autophagic osteoblasts, bone mass, and bone strength.

RESULTS

GC treatment at 2.8 and 4 mg/kg/day of methylprednisolone significantly lowered trabecular bone volume (Tb-BV/TV) at the lumbar vertebrae and distal femurs, cortical bone mass at the mid-shaft femur (FS), and cortical bone strength compared to placebo (PL). In mice treated with GC and Scl-Ab, Tb-BV/TV increased by 60-125 %, apparent bone strength of the lumbar vertebrae by 30-70 %, FS-BV by 10-18 %, and FS-apparent strength by 13-15 %, as compared to GC vehicle-treated mice. GC treatment at 4 mg/kg/day reduced the number of autophagic osteoblasts by 70 % on the vertebral trabecular bone surface compared to the placebo group (PL, GC 0 mg), and GC + Scl-Ab treatment.

CONCLUSIONS

Treatment with Scl-Ab prevented GC-induced reduction in both trabecular and cortical bone mass and strength and appeared to maintain osteoblast activity through autophagy.

BMP-2 Enhances Lgr4 Gene Expression in Osteoblastic Cells

Osteoporosis is one of the most prevalent diseases and the number of patients suffering from this disease is soaring due to the increase in the aged population in the world. The severity of bone loss in osteoporosis is based on the levels of impairment in the balance between bone formation and bone resorption, two arms of the bone metabolism, and bone remodeling. However, determination of bone formation levels is under many layers of control that are as yet fully defined. Bone morphogenetic protein (BMP) plays a key role in regulation of bone formation while its downstream targets are still incompletely understood. Lgr4 gene encodes an orphan receptor and has been identified as a genetic determinant for bone mass in osteoporotic patients. Here, we examine the effects of BMP on the expression of Lgr4 in osteoblastic cells. Lgr4 gene is expressed in an osteoblastic cell line, MC3T3E1 in a time dependent manner during the culture. BMP treatment enhances Lgr4 mRNA expression at least in part via transcriptional event. When Lgr4 mRNA is knocked down, the levels of BMP-induced increase in alkaline phosphatase (Alp) activity and Alp mRNA are suppressed. BMP enhancement of Lgr4 gene expression is suppressed by FGF and reversed by dexamethasone. BMP also enhances Lgr4 expression in primary cultures of calvarial osteoblasts. These data indicate that Lgr4 gene is regulated by BMP and is required for BMP effects on osteoblastic differentiation. J. Cell. Physiol. 231: 887-895, 2016. © 2015 Wiley Periodicals, Inc.

Chronic obstructive pulmonary disease and allied conditions is a strong independent risk factor for osteoporosis and pathologic fractures: a population-based cohort study

BACKGROUND

Chronic obstructive pulmonary disease and allied conditions (COPD) is frequently associated with various comorbidities. This study examined the association between osteoporosis and pathologic fractures in a sample of patients with COPD.

METHODS

In this cohort study, claims data from the National Health Insurance Research Database of Taiwan were used to evaluate the risk between COPD and osteoporosis. Using data from the Longitudinal Health Insurance Database 2000, we conducted a retrospective cohort study by investigating patients aged 20 years and older who were newly diagnosed with COPD and comparing them with controls without COPD during 2000-2010. In addition, we used univariable and multivariable Cox proportional hazards regression models to measure the association between COPD and the risk of osteoporosis.

RESULTS

Our results revealed that COPD was significantly associated with a high risk of osteoporosis, regardless of whether the patients with COPD were corticosteroid users and irrespective of age and sex. After adjustment for covariates, the COPD patients exhibited a 1.54-fold higher risk of developing osteoporosis (hazard ratio 1.54, 95% confidence interval 1.44-1.64). COPD was a stronger risk factor for osteoporosis in men. Moreover, patients with severe COPD had a higher risk of osteoporosis or pathologic fractures.

CONCLUSION

This study revealed that COPD, which shares the characteristics of inflammatory diseases, is associated with a higher risk of osteoporosis after adjustment for comorbidities.