Activation of AMP-activated protein kinase decreases receptor activator of NF-κB ligand expression and increases sclerostin expression by inhibiting the mevalonate pathway in osteocytic MLO-Y4 cells

All-trans Retinoic Acid and Beta-Carotene Increase Sclerostin Production in C2C12 Myotubes

Sclerostin is a protein secreted by osteocytes whose encoding gene SOST is regulated by mechanical stimuli, cytokines, and all-trans retinoic acid (ATRA) and mediates antianabolic effects on bone formation as an inhibitor of the canonical Wnt/β-catenin pathway. Interestingly, skeletal muscle has recently been identified as another source of sclerostin, suggesting that the musculature may play an important role in maintaining bone mass. However, regulators of muscular SOST expression are virtually unknown. This study investigates the influence of ATRA and the provitamin A derivative beta-carotene (β-C) on sclerostin synthesis in muscle cells. The impact of ATRA, its synthetic analog TTNPB, and β-C on Sost transcription was analyzed by qRT-PCR in C2C12 myotubes and the secreted sclerostin protein by ELISA. ATRA strongly increases the sclerostin synthesis in C2C12 myotubes in a dose-dependent manner. The stimulating effect of ATRA and TTNPB on Sost is largely reduced in the presence of the retinoic acid receptor inhibitor AGN193109. β-C also increases the Sost expression, but this effect vanishes when β-C is coincubated with beta-carotene 15,15'-monooxygenase 1 (BCMO1)-specific siRNA. Thus, ATRA is a potent stimulator of sclerostin release in muscle cells. β-C can also increase Sost mRNA abundance, but this effect depends on the conversion to a retinoid.

Resveratrol protects osteocytes against oxidative stress in ovariectomized rats through AMPK/JNK1-dependent pathway leading to promotion of autophagy and inhibition of apoptosis

A large number of studies in recent years indicate that osteocytes are the orchestrators of bone remodeling by regulating both osteoblast and osteoclast activities. Oxidative stress-induced osteocyte apoptosis plays critical roles in the pathological processes of postmenopausal osteoporosis. Resveratrol is a natural polyphenolic compound that ameliorates postmenopausal osteoporosis. However, whether resveratrol regulates osteocyte apoptosis via autophagy remains largely unknown. The effects of resveratrol on regulating osteocyte apoptosis and autophagy were analyzed both in vivo and in vitro. In vitro, cultured MLO-Y4 cells were exposed to H₂O₂ with or without resveratrol. In vivo, an ovariectomy-induced osteoporosis model was constructed in rats with or without daily intraperitoneal injection of 10 mg/kg body weight resveratrol. It was found that resveratrol attenuated H₂O₂-induced apoptosis through activating autophagy in cultured MLO-Y4 cells, which was mediated by the dissociation of Beclin-1/Bcl-2 complex in AMPK/JNK1-dependent pathway, ultimately regulating osteocytes function. Furthermore, it was shown that resveratrol treatment reduced osteocytes oxidative stress, inhibited osteocytes apoptosis and promoted autophagy in ovariectomized rats. Our study suggests that resveratrol protects against oxidative stress by restoring osteocytes autophagy and alleviating apoptosis via AMPK/JNK1 activation, therefore dissociating Bcl-2 from Beclin-1.

The Mechanosensory Role of Osteocytes and Implications for Bone Health and Disease States

Bone homeostasis is a dynamic equilibrium between bone-forming osteoblasts and bone-resorbing osteoclasts. This process is primarily controlled by the most abundant and mechanosensitive bone cells, osteocytes, that reside individually, within chambers of porous hydroxyapatite bone matrix. Recent studies have unveiled additional functional roles for osteocytes in directly contributing to local matrix regulation as well as systemic roles through endocrine functions by communicating with distant organs such as the kidney. Osteocyte function is governed largely by both biochemical signaling and the mechanical stimuli exerted on bone. Mechanical stimulation is required to maintain bone health whilst aging and reduced level of loading are known to result in bone loss. To date, both in vivo and in vitro approaches have been established to answer important questions such as the effect of mechanical stimuli, the mechanosensors involved, and the mechanosensitive signaling pathways in osteocytes. However, our understanding of osteocyte mechanotransduction has been limited due to the technical challenges of working with these cells since they are individually embedded within the hard hydroxyapatite bone matrix. This review highlights the current knowledge of the osteocyte functional role in maintaining bone health and the key regulatory pathways of these mechanosensitive cells. Finally, we elaborate on the current therapeutic opportunities offered by existing treatments and the potential for targeting osteocyte-directed signaling.

Saxagliptin enhances osteogenic differentiation in MC3T3-E1 cells, dependent on the activation of AMP-activated protein kinase α (AMPKα)/runt-related transcription factor-2 (Runx-2)

Osteoporosis is a metabolic bone disease commonly observed in the elderly, and its pathogenesis is associated with declined osteogenic differentiation. Osteogenic differentiation could be facilitated by the activation of the AMP-activated protein kinase (AMPK) pathway. Saxagliptin, an anti-diabetic agent with inhibitory effects against dipeptidyl peptidase 4 (DPP-4), has been recently reported to induce the activation of the AMPK pathway. The present study proposes to explore the function and mechanism of Saxagliptin in osteogenic differentiation. Osteogenic differentiation induction medium (ODIM) was utilized to induce osteogenic differentiation in MC3T3-E1 cells. Significantly increased mineral nodule formation, elevated alkaline phosphatase (ALP) activity, and upregulated expression of osteogenic marker genes activating transcription factor-4 (ATF-4), osteopontin (OPN), and type I collagen (Col1) were observed in ODIM-cultured MC3T3-E1 cells, all of which were further enhanced by the introduction of Saxagliptin. The elevated expression level of runt-related transcription factor-2 (Runx-2), an important transcriptional factor involved in the progression of osteogenic differentiation, in ODIM-cultured MC3T3-E1 cells was further promoted by Saxagliptin. The AMPK pathway in ODIM-cultured MC3T3-E1 cells was significantly activated by Saxagliptin, and the functions of Saxagliptin in promoting osteogenic differentiation were abolished by compound C, the inhibitor of the AMPK pathway. Conclusively, Saxagliptin enhanced osteogenic differentiation in MC3T3-E1 cells, dependent on the activation of AMPKα/RUNX-2.

L-carnitine and Co Q10 ameliorate potassium dichromate -induced acute brain injury in rats targeting AMPK/AKT/NF-κβ

Adenosine monophosphate-activated protein kinase (AMPK) has a crucial role in neuroprotection. It phosphorylates serine/threonine kinase (Akt) Substrate inhibiting the inflammatory responses induced by the nuclear factor-κB (NF-κB). Exposure to chromium VI dust among workers has been reported and induced brain injury, as the absorption of chromium through the nasal membrane has been found to deliver it directly to the brain. The study aimed to investigate the influence of administration of L-carnitine or/and Co Q10 as theraputic agents against potassium dichromate (PD)-induced brain injury via AMPK/AKT/NF-κβ signaling pathway. Brain injury was induced by PD intranasally as a single dose of 2 mg/kg, 24 h latter rats received L-carnitine (100 mg/kg; orally), Co Q10 (50 mg/kg; orally) and L-carnitine (50 mg/kg; orally) + Co Q10 (25 mg/kg; orally) respectively for 3 days. Locomotor activity was assessed before and at the end of the experiment, then, biochemical and histopathological investigations were assessed in brain homogenate. The exposure of rats to PD promoted oxidative stress and inflammation via an increase in MDA and a decrease in GSH serum contents with an increase in brain contents of TNF-α, IL-6, and NF-kβ and reduced AMPK and AKT brain contents as compared to the control group. Treatment with L-carnitine + Co Q10 ameliorated cognitive impairment and oxidative stress, decreased the brain contents of inflammatory mediators; TNF-α, IL-6, and NF-κβ elevated AMPK and AKT, as compared to each drug. Also, L-carnitine + Co Q10 administration restored morphological changes as degenerated neurons and necrosis. L-carnitine + Co Q10 play important role in AMPK/AKT/NF-κβ pathway that responsible for their antioxidant and anti-inflammatory effects against PD-induced brain injury in rats.

Effects of mild hyperbaric oxygen on osteoporosis induced by hindlimb unloading in rats

INTRODUCTION

Disuse-induced bone loss is caused by a suppression of osteoblastic bone formation and an increase in osteoclastic bone resorption. There are few data available for the effects of environmental conditions, i.e., atmospheric pressure and/or oxygen concentration, on osteoporosis. This study examined the effects of mild hyperbaric oxygen at 1317 hPa with 40% oxygen on unloading-induced osteoporosis.

MATERIALS AND METHODS

Eighteen 8-week old male Wistar rats were randomly divided into three groups: the control for 21 days without unloading and mild hyperbaric oxygen (NOR, n = 6), the unloading for 21 days and recovery for 10 days without mild hyperbaric oxygen (HU + NOR, n = 6), and the unloading for 21 days and recovery for 10 days with mild hyperbaric oxygen (HU + MHO, n = 6).

RESULTS

The cortical thickness and trabecular bone surface area were decreased in the HU + NOR group compared to the NOR group. There were no differences between the NOR and HU + MHO groups. Osteoclast surface area and Sclerostin (Sost) mRNA expression levels were decreased in the HU + MHO group compared to the HU + NOR group. These results suggested that the loss of the cortical and trabecular bone is inhibited by mild hyperbaric oxygen, because of an inhibition of osteoclasts and enhancement of bone formation with decreased Sost expression.

CONCLUSIONS

We conclude that exposure to mild hyperbaric oxygen partially protects from the osteoporosis induced by hindlimb unloading.

AMP-activated protein kinase (AMPK) regulates autophagy, inflammation and immunity and contributes to osteoclast differentiation and functionabs

Osteoclasts are multinucleated giant cells, responsible for bone resorption. Osteoclast differentiation and function requires a series of cytokines to remove the old bone, which coordinates with the induction of bone remodelling by osteoblast-mediated bone formation. Studies have demonstrated that AMP-activated protein kinase (AMPK) play a negative regulatory role in osteoclast differentiation and function. Research involving AMPK, a nutrient and energy sensor, has primarily focused on osteoclast differentiation and function; thus, its role in autophagy, inflammation and immunity remains poorly understood. Autophagy is a conservative homoeostatic mechanism of eukaryotic cells, and response to osteoclast differentiation and function; however, how it interacts with inflammation remains unclear. Additionally, based on the regulatory function of different AMPK subunits for osteoclast differentiation and function, its activation is regulated by upstream factors to perform bone metabolism. This review summarises the critical role of AMPK-mediated autophagy, inflammation and immunity by upstream and downstream signalling during receptor activator of nuclear factor kappa-B ligand-induced osteoclast differentiation and function. This pathway may provide therapeutic targets for bone-related diseases, as well as function as a biomarker for bone homoeostasis.

MicroRNA-429 inhibits bone metastasis in breast cancer by regulating CrkL and MMP-9

Bone metastasis is common in late-stage breast cancer patients and leads to skeletal-related events that affect the quality of life and decrease survival. Numerous miRNAs have been confirmed to be involved in metastatic breast cancer, such as the miR200 family. Our previous study identified microRNA-429 (miR-429) as a regulatory molecule in breast cancer bone metastasis. However, the effects of miR-429 and its regulatory axis in the metastatic breast cancer bone microenvironment have not been thoroughly investigated. We observed a positive correlation between miR-429 expression in clinical tissues and the bone metastasis-free interval and a negative correlation between miR-429 expression and the degree of bone metastasis. We cultured bone metastatic MDA-MB-231 cells and used conditioned medium (CM) to detect the effect of miR-429 on osteoblast and osteoclast cells in vitro. We constructed an orthotopic bone destruction model and a left ventricle implantation model to examine the effect of miR-429 on the metastatic bone environment in vivo. The transfection experiments showed that the expression levels of V-crk sarcoma virus CT10 oncogene homolog-like (CrkL) and MMP-9 were negatively regulated by miR-429. The in vitro coculture experiments showed that miR-429 promoted osteoblast differentiation and that CrkL promoted osteoclast differentiation. The two animal models showed that miR-429 diminished local bone destruction and distant bone metastasis but CrkL enhanced these effects. Furthermore, CrkL and MMP-9 expression decreased simultaneously in response to increased miR-429 expression. These findings further reveal the possible mechanism and effect of the miR-429/CrkL/MMP-9 regulatory axis in the bone microenvironment in breast cancer bone metastasis.

Mitochondrial dysfunction, AMPK activation and peroxisomal metabolism: A coherent scenario for non-canonical 3-methylglutaconic acidurias

The occurrence of 3-methylglutaconic aciduria (3-MGA) is a well understood phenomenon in leucine oxidation and ketogenesis disorders (primary 3-MGAs). In contrast, its genesis in non-canonical (secondary) 3-MGAs, a growing-up group of disorders encompassing more than a dozen of inherited metabolic diseases, is a mystery still remaining unresolved for three decades. To puzzle out this anthologic problem of metabolism, three clues were considered: (i) the variety of disorders suggests a common cellular target at the cross-road of metabolic and signaling pathways, (ii) the response to leucine loading test only discriminative for primary but not secondary 3-MGAs suggests these latter are disorders of extramitochondrial HMG-CoA metabolism as also attested by their failure to increase 3-hydroxyisovalerate, a mitochondrial metabolite accumulating only in primary 3-MGAs, (iii) the peroxisome is an extramitochondrial site possessing its own pool and displaying metabolism of HMG-CoA, suggesting its possible involvement in producing extramitochondrial 3-methylglutaconate (3-MG). Following these clues provides a unifying common basis to non-canonical 3-MGAs: constitutive mitochondrial dysfunction induces AMPK activation which, by inhibiting early steps in cholesterol and fatty acid syntheses, pipelines cytoplasmic acetyl-CoA to peroxisomes where a rise in HMG-CoA followed by local dehydration and hydrolysis may lead to 3-MGA yield. Additional contributors are considered, notably for 3-MGAs associated with hyperammonemia, and to a lesser extent in CLPB deficiency. Metabolic and signaling itineraries followed by the proposed scenario are essentially sketched, being provided with compelling evidence from the literature coming in their support.

Screening of key candidate genes and pathways for osteocytes involved in the differential response to different types of mechanical stimulation using a bioinformatics analysis

This study aimed to predict the key genes and pathways that are activated when different types of mechanical loading are applied to osteocytes. mRNA expression datasets (series number of GSE62128 and GSE42874) were obtained from Gene Expression Omnibus database (GEO). High gravity-treated osteocytic MLO-Y4 cell-line samples from GSE62128 (Set1), and fluid flow-treated MLO-Y4 samples from GSE42874 (Set2) were employed. After identifying the differentially expressed genes (DEGs), functional enrichment was performed. The common DEGs between Set1 and Set2 were considered as key DEGs, then a protein-protein interaction (PPI) network was constructed using the minimal nodes from all of the DEGs in Set1 and Set2, which linked most of the key DEGs. Several open source software programs were employed to process and analyze the original data. The bioinformatic results and the biological meaning were validated by in vitro experiments. High gravity and fluid flow induced opposite expression trends in the key DEGs. The hypoxia-related biological process and signaling pathway were the common functional enrichment terms among the DEGs from Set1, Set2 and the PPI network. The expression of almost all the key DEGs (Pdk1, Ccng2, Eno2, Egln1, Higd1a, Slc5a3 and Mxi1) were mechano-sensitive. Eno2 was identified as the hub gene in the PPI network. Eno2 knockdown results in expression changes of some other key DEGs (Pdk1, Mxi1 and Higd1a). Our findings indicated that the hypoxia response might have an important role in the differential responses of osteocytes to the different types of mechanical force.

Safflower Yellow B Protects Brain against Cerebral Ischemia Reperfusion Injury through AMPK/NF-kB Pathway

Inflammation had showed its important role in the pathogenesis of cerebral ischemia and secondary damage. Safflower yellow B (SYB) had neuroprotective effects against oxidative stress-induced brain injuries, but the mechanisms were still largely unknown to us. In this study, we tried to investigate the anti-inflammation effects of SYB and the possible roles of AMPK/NF-κB signaling pathway on these protective effects. In vivo, brain ischemia/reperfusion (I/R) was induced by transient middle cerebral artery occlusion for 2 h and reperfusion for 20 h. Neurofunctional evaluation, infarction area, and brain water contents were measured. Brain injury markers and inflammatory cytokines levels were measured by ELISA kits. In vitro, cell viability, apoptosis, and LDH leakage were measured after I/R in PC12 cells. The expression and phosphorylation levels of AMPK, NF-κB p65, and P-IκB-α in cytoplasm and nuclear were measured by Western blotting. SiRNA experiment was performed to certify the role of AMPK. The results showed SYB reduced infarct size, improved neurological outcomes, and inhibited brain injury after I/R. In vitro test, SYB treatment alleviated PC12 cells injury and apoptosis and inhibited the inflammatory cytokines (IL-1, IL-6, TNF-α, and COX-2) in a dose-dependent manner. SYB treatment induced AMPK phosphorylation and inhibited NF-κB p65 nuclear translocation both in brain and in PC12 cells. Further studies also showed that the inhibition of NF-κB activity of SYB was through AMPK. In conclusion, SYB protected brain I/R injury through reducing expression of inflammatory cytokines and this effect might be partly due to the inhibition of NF-κB mediated by AMPK.

Osteoblast AMP-Activated Protein Kinase Regulates Postnatal Skeletal Development in Male Mice

Studies have shown that AMP-activated protein kinase (AMPK), a crucial regulator of energy homeostasis, plays important roles in osteoblast differentiation and mineralization. However, little is known about in vivo roles of osteoblastic AMPK in bone development. Thus, to investigate in vivo roles of osteoblast AMPK, we conditionally inactivated Ampk in osterix (Osx)-expressing cells by crossing Osx-Cre mice with floxed AMPKα1 to generate mice lacking AMPKα1 in osteoblasts (Ampk-/- mice). Compared with wild-type and Ampk+/- mice, Ampk-/- mice displayed retardation of postnatal bone development, although bone deformity was not observed at birth. Microcomputed tomography showed significant reductions in trabecular bone volume, cortical bone length, and density, as well as increased cortical porosity in femur as well as development defects of skull in 8-week-old Ampk-/- mice. Surprisingly, histomorphometric analysis demonstrated that the number of osteoclasts was significantly increased, although bone formation rate was not altered. Loss of trabecular network connections and mass, as well as shortened growth plates and reduced thickness of cartilage adjacent to the growth plate, was observed in Ampk-/- mice. In primary cultured osteoblasts from calvaria, the expressions of alkaline phosphatase, type 1 collagen, osteocalcin, bone morphogenetic protein 2, Runx2, and osterix were significantly inhibited in Ampk-/- osteoblasts, whereas the expression of receptor activator of nuclear κB ligand (RANKL) and the RANKL/osteoprotegerin ratio were significantly increased. These findings indicate that osteoblastic AMPK plays important roles in bone development in vivo and that deletion of AMPK in osteoblasts decreases osteoblastic differentiation and enhances bone turnover by increasing RANKL expression.

Glucose uptake inhibition decreases expressions of receptor activator of nuclear factor-kappa B ligand (RANKL) and osteocalcin in osteocytic MLO-Y4-A2 cells

Bone and glucose metabolism are closely associated with each other. Both osteoblast and osteoclast functions are important for the action of osteocalcin, which plays pivotal roles as an endocrine hormone regulating glucose metabolism. However, it is unknown whether osteocytes are involved in the interaction between bone and glucose metabolism. We used MLO-Y4-A2, a murine long bone-derived osteocytic cell line, to investigate effects of glucose uptake inhibition on expressions of osteocalcin and bone-remodeling modulators in osteocytes. We found that glucose transporter 1 (GLUT1) is expressed in MLO-Y4-A2 cells and that treatment with phloretin, a GLUT inhibitor, significantly inhibited glucose uptake. Real-time PCR and Western blot showed that phloretin significantly and dose-dependently decreased the expressions of RANKL and osteocalcin, whereas osteoprotegerin or sclerostin was not affected. Moreover, phloretin activated AMP-activated protein kinase (AMPK), an intracellular energy sensor. Coincubation of ara-A, an AMPK inhibitor, with phloretin canceled the phloretin-induced decrease in osteocalcin expression, but not RANKL. In contrast, phloretin suppressed phosphorylation of ERK1/2, JNK, and p38 MAPK, and treatments with the p38 inhibitor SB203580 and the MEK inhibitor PD98059, but not the JNK inhibitor SP600125, significantly decreased expressions of RANKL and osteocalcin. These results indicate that glucose uptake by GLUT1 is required for RANKL and osteocalcin expressions in osteocytes, and that inhibition of glucose uptake decreases their expressions through AMPK, ERK1/2, and p38 MAPK pathways. These findings suggest that lowering glucose uptake into osteocytes may contribute to maintain blood glucose levels by decreasing osteocalcin expression and RANKL-induced bone resorption.

Interaction between bone and glucose metabolism [Review]

Accumulating evidence has shown that bone and glucose metabolism are closely associated with each other. Since the risk of osteoporotic fractures is increased in patients with diabetes mellitus (DM), osteoporosis is recently recognized as one of diabetic complications, called DM-induced bone fragility. Previous studies showed that collagen cross-links of advanced glycation end products (AGEs) and dysfunctions of osteoblast and osteocyte are involved in DM-induced bone fragility. Circulating levels of AGEs and homocysteine are increased in patients with DM, and they directly impair the functions of osteoblast and osteocyte, resulting in decreased bone formation and bone remodeling. On the other hand, bone is recently recognized as an endocrine organ. Previous studies based on in vitro and animal studies showed that osteocalcin, which is specifically expressed in osteoblasts and secreted into the circulation, may regulate glucose homeostasis. Although several clinical studies reported the relationship between osteocalcin and glucose metabolism, further large-scale and intervention studies are necessary to confirm the beneficial effects of osteocalcin on glucose metabolism in human. It has been shown that adenosine monophosphate-activated protein kinase (AMPK), an intracellular energy sensor, is involved in bone metabolism. Adiponectin and metformin stimulate osteocalcin expression and the differentiation of osteoblasts via AMPK activation. Also, AMPK activation protects against oxidative stress-induced apoptosis of osteocytes. These findings suggest that AMPK in osteoblasts and osteocytes may be a therapeutic target for DM-induced bone fragility.