Activation of AMP-activated protein kinase by compound 991 protects osteoblasts from dexamethasone

Dexamethasone-induced mitochondrial ROS-mediated inhibition of AMPK activity facilitates osteoblast necroptosis

Long-term or high-dose glucocorticoid use can lead to serious orthopedic complications, including femoral head necrosis. Both basic and clinical studies have shown that high doses dexamethasone (Dex) can directly induce osteoblasts death. This study investigated the mechanism underlying Dex induced osteoblast death. In this study, we showed that Dex induces osteoblast necroptosis, rather than apoptosis, through the inhibition of AMP-activated protein kinase (AMPK) activity. We also demonstrated that inactivation of AMPK-mediated necroptosis is through receptor-interacting protein kinase 3 (RIP3), but not RIP1. Furthermore, we found that Dex-induced necroptosis is dependent on mitochondrial reactive oxygen species (ROS) following with directly activation of RIP1 and inactivation of AMPK. These findings provide new insights into the mechanism of Dex-induced osteoblast death and may have implications for the development of new therapies for osteoporosis and other bone-related diseases.

Nrf2 signaling activation by a small molecule activator compound 16 inhibits hydrogen peroxide-induced oxidative injury and death in osteoblasts

We explored the potential activity of compound 16 (Cpd16), a novel small molecule Nrf2 activator, in hydrogen peroxide (H₂O₂)-stimulated osteoblasts. In the primary murine/human osteoblasts and MC3T3-E1 murine osteoblastic cells, Cpd16 treatment at micro-molar concentrations caused disassociation of Keap1-Nrf2 and Nrf2 cascade activation. Cpd16 induced stabilization of Nrf2 protein and its nuclear translocation, thereby increasing the antioxidant response elements (ARE) reporter activity and Nrf2 response genes transcription in murine and human osteoblasts. Significantly, Cpd16 mitigated oxidative injury in H₂O₂-stimulited osteoblasts. H₂O₂-provoked apoptosis as well as programmed necrosis in osteoblasts were significantly alleviated by the novel Nrf2 activator. Cpd16-induced Nrf2 activation and osteoblasts protection were stronger than other known Nrf2 activators. Dexamethasone- and nicotine-caused oxidative stress and death in osteoblasts were attenuated by Cpd16 as well. Cpd16-induced osteoblast cytoprotection was abolished by Nrf2 short hairpin RNA or knockout, but was mimicked by Keap1 knockout. Keap1 Cys151S mutation abolished Cpd16-induced Nrf2 cascade activation and osteoblasts protection against H₂O₂. Importantly, weekly Cpd16 administration largely ameliorated trabecular bone loss in ovariectomy mice. Together, Cpd16 alleviates H₂O₂-induced oxidative stress and death in osteoblasts by activating Nrf2 cascade.

The Role of NRF2 in Bone Metabolism - Friend or Foe?

Bone metabolism is closely related to oxidative stress. As one of the core regulatory factors of oxidative stress, NRF2 itself and its regulation of oxidative stress are both involved in bone metabolism. NRF2 plays an important and controversial role in the regulation of bone homeostasis in osteoblasts, osteoclasts and other bone cells. The role of NRF2 in bone is complex and affected by several factors, such as its expression levels, age, sex, the presence of various physiological and pathological conditions, as well as its interaction with certains transcription factors that maintain the normal physiological function of the bone tissue. The properties of NRF2 agonists have protective effects on the survival of osteogenic cells, including osteoblasts, osteocytes and stem cells. Activation of NRF2 directly inhibits osteoclast differentiation by resisting oxidative stress. The effects of NRF2 inhibition and hyperactivation on animal skeleton are still controversial, the majority of the studies suggest that the presence of NRF2 is indispensable for the acquisition and maintenance of bone mass, as well as the protection of bone mass under various stress conditions. More studies show that hyperactivation of NRF2 may cause damage to bone formation, while moderate activation of NRF2 promotes increased bone mass. In addition, the effects of NRF2 on the bone phenotype are characterized by sexual dimorphism. The efficacy of NRF2-activated drugs for bone protection and maintenance has been verified in a large number of in vivo and in vitro studies. Additional research on the role of NRF2 in bone metabolism will provide novel targets for the etiology and treatment of osteoporosis.

DCAF1-targeting microRNA-3175 activates Nrf2 signaling and inhibits dexamethasone-induced oxidative injury in human osteoblasts

Activation of nuclear-factor-E2-related factor 2 (Nrf2) signaling can protect human osteoblasts from dexamethasone-induced oxidative injury. DDB1 and CUL4 associated factor 1 (DCAF1) is a novel ubiquitin E3 ligase for Nrf2 protein degradation. We identified a novel DCAF1-targeting miRNA, miR-3175. RNA pull-down, Argonaute 2 RNA-immunoprecipitation, and RNA fluorescent in situ hybridization results confirmed a direct binding between miR-3175 and DCAF1 mRNA in primary human osteoblasts. DCAF1 3′-untranslated region luciferase activity and its expression were significantly decreased after miR-3175 overexpression but were augmented with miR-3175 inhibition in human osteoblasts and hFOB1.19 osteoblastic cells. miR-3175 overexpression activated Nrf2 signaling, causing Nrf2 protein stabilization, antioxidant response (ARE) activity increase, and transcription activation of Nrf2-dependent genes in human osteoblasts and hFOB1.19 cells. Furthermore, dexamethasone-induced oxidative injury and apoptosis were largely attenuated by miR-3175 overexpression in human osteoblasts and hFOB1.19 cells. Importantly, shRNA-induced silencing or CRISPR/Cas9-mediated Nrf2 knockout abolished miR-3175 overexpression-induced osteoblast cytoprotection against dexamethasone. Conversely, DFAC1 knockout, by the CRISPR/Cas9 method, activated the Nrf2 cascade and inhibited dexamethasone-induced cytotoxicity in hFOB1.19 cells. Importantly, miR-3175 expression was decreased in necrotic femoral head tissues of dexamethasone-taking patients, where DCAF1 mRNA was upregulated. Together, silencing DCAF1 by miR-3175 activated Nrf2 signaling to inhibit dexamethasone-induced oxidative injury and apoptosis in human osteoblasts.

Phosphoglycerate kinase 1 silencing by a novel microRNA microRNA-4523 protects human osteoblasts from dexamethasone through activation of Nrf2 signaling cascade

Nuclear-factor-E2-related factor 2 (Nrf2) cascade activation can ameliorate dexamethasone (DEX)-induced oxidative injury and death in human osteoblasts. Phosphoglycerate kinase 1 (PGK1) depletion is shown to efficiently activate Nrf2 signaling by inducing methylglyoxal modification of Kelch-like ECH-associated protein 1 (Keap1). We here identified a novel PGK1-targeting microRNA: microRNA-4523 (miR-4523). RNA fluorescent in situ hybridization, RNA pull-down, and Argonaute-2 RNA immunoprecipitation results confirmed a direct binding between miR-4523 and PGK1 mRNA in primary human osteoblasts and hFOB1.19 osteoblastic cells. Forced overexpression of miR-4523, using a lentiviral construct, robustly decreased PGK1 3′-UTR (untranslated region) luciferase activity and downregulated its expression in human osteoblasts and hFOB1.19 cells. Furthermore, miR-4523 overexpression activated the Nrf2 signaling cascade, causing Keap1–Nrf2 disassociation, Nrf2 protein stabilization, and its nuclear translocation as well as transcription activation of Nrf2-dependent genes (NQO1, GCLC, and HO1) in human osteoblasts. By expressing a UTR-null PGK1 construct, miR-4523 overexpression-induced Nrf2 cascade activation was however largely inhibited. Importantly, DEX-induced reactive oxygen species production, oxidative injury, and cell apoptosis were significantly attenuated by miR-4523 overexpression in human osteoblasts and hFOB1.19 cells. Such actions by miR-4523 were abolished by Nrf2 shRNA or knockout, but mimicked by PGK1 knockout (using CRISPR/Cas9 method). In PGK1 knockout human osteoblasts, miR-4523 overexpression failed to further increase Nrf2 cascade activation and offer osteoblast cytoprotection against DEX. Significantly, miR-4523 is downregulated in human necrotic femoral head tissues of DEX-taking patients. Together, PGK1 silencing by miR-4523 protected human osteoblasts from DEX through activation of the Nrf2 signaling cascade.

Aucubin prevents steroid-induced osteoblast apoptosis by enhancing autophagy via AMPK activation

Steroid‐induced osteoblast apoptosis is a crucial pathological process in steroid‐induced osteonecrosis of the femoral head (SONFH). Autophagy can resist apoptosis and AMPK plays an important role in autophagy regulation. Aucubin from the small tree Eucommia ulmoides Oliv., which has a long history of use in orthopaedics and traumatology in Asian medicine, can promote bone formation, but whether it can slow or prevent steroid‐osteoblast apoptosis is unclear. Therefore, we investigated the pathogenesis of SONFH and how the osteoblast responds to aucubin under the dexamethasone stimulation. In human femoral head osteonecrosis specimens, we found that the autophage and apoptosis level were increased, and the AMPK signalling was crucial to autophagy. We observed that aucubin could prevent dexamethasone‐induced apoptosis in osteoblasts by enhancing the level of autophagy. Further, we confirmed that the regulatory effect of aucubin on autophagy and apoptosis was achieved by activating AMPK signalling. We have demonstrated a mechanism of disease progression and shown that aucubin could enhance autophagy through AMPK signalling to prevent osteoblast apoptosis. These findings provide a basis for the further investigation of the potential therapeutic role of aucubin in the SONFH.

Resveratrol Protects Osteoblasts Against Dexamethasone-Induced Cytotoxicity Through Activation of AMP-Activated Protein Kinase

Purpose

Glucocorticoids are used for the treatment of inflammatory diseases, but glucocorticoid treatment is associated with bone damage. Resveratrol is a phytoalexin found in many plants, and we investigated its protective role on dexamethasone-induced dysfunction in MC3T3-E1 cells and primary osteoblasts.

Materials and Methods

MC3T3-E1 cells and primary osteoblasts were treated with dexamethasone in the presence/absence of different doses of resveratrol for 24 or 48 h. Then, 3-(4,5-dimethyl-2-thiazolyl)-2,5-diphenyltetrazolium (MTT) and lactate dehydrogenase (LDH) assays were used to evaluate cell viability. Apoptosis was analyzed by a flow cytometry. An alkaline phosphatase (ALP) activity assay and Alizarin Red S staining were used to study osteoblast differentiation. Expression of osteoblast-related genes was measured by real-time reverse transcription-quantitative polymerase chain reaction (RT-qPCR). The AMP-activated protein kinase (AMPK) signaling pathway and mitochondrial expression of superoxide dismutase were evaluated by Western blotting. Intracellular reactive oxygen species (ROS), adenosine triphosphate (ATP) content, mitochondrial-complex activity, and mitochondrial DNA content were measured to evaluate mitochondrial function.

Results

Resveratrol induced the proliferation and inhibited apoptosis of osteoblasts in the presence of dexamethasone. Resveratrol increased the ALP activity and mineralization of osteoblasts. Resveratrol also attenuated dexamethasone-induced inhibition of mRNA expression of osteogenesis maker genes, including bone morphogenetic protein-2, osteoprotegerin, runt-related transcription factor-2, and bone Gla protein. Resveratrol alleviated dexamethasone-induced mitochondrial dysfunction. Resveratrol strongly stimulated expression of peroxisome proliferator–activated receptor-γ coactivator 1α and sirtuin-3 genes, as well as their downstream target gene superoxide dismutase-2. Resveratrol induced phosphorylation of AMPK and acetyl-CoA carboxylase (ACC). Blockade of AMPK signaling using compound C reversed the protective effects of resveratrol against dexamethasone.

Conclusion

Resveratrol showed protective effects against dexamethasone-induced dysfunction of osteoblasts by activating AMPK signaling.

FGF23 protects osteoblasts from dexamethasone-induced oxidative injury

Dexamethasone (DEX) can exert a cytotoxic effect on cultured osteoblasts. The current study explored the potential osteoblast cytoprotective effect of fibroblast growth factor 23 (FGF23). In OB-6 human osteoblastic cells and primary murine osteoblasts, FGF23 induced phosphorylation of the receptor FGFR1 and activated the downstream Akt-S6K1 signaling. FGF23-induced FGFR1-Akt-S6K phosphorylation was largely inhibited by FGFR1 shRNA, but augmented with ectopic FGFR1 expression in OB-6 cells. FGF23 attenuated DEX-induced death and apoptosis in OB-6 cells and murine osteoblasts. Its cytoprotective effects were abolished by FGFR1 shRNA, Akt inhibition or Akt1 knockout. Conversely, forced activation of Akt inhibited DEX-induced cytotoxicity in OB-6 cells. Furthermore, FGF23 activated Akt downstream nuclear-factor-E2-related factor 2 (Nrf2) signaling to alleviate DEX-induced oxidative injury. On the contrary, Nrf2 shRNA or knockout almost reversed FGF23-induced osteoblast cytoprotection against DEX. Collectively, FGF23 activates FGFR1-Akt and Nrf2 signaling cascades to protect osteoblasts from DEX-induced oxidative injury and cell death.

miR-107 inhibition upregulates CAB39 and activates AMPK-Nrf2 signaling to protect osteoblasts from dexamethasone-induced oxidative injury and cytotoxicity

To human osteoblasts dexamethasone (DEX) treatment induces significant oxidative injury and cytotoxicity. Inhibition of CAB39 (calcium binding protein 39)-targeting microRNA can induce CAB39 upregulation, activating AMP-activated protein kinase (AMPK) signaling and offering osteoblast cytoprotection. Here we identified a novel CAB39-targeting miRNA: the microRNA-107 (miR-107). RNA-Pull down assay results demonstrated that the biotinylated-miR-107 directly binds to CAB39 mRNA in OB-6 human osteoblastic cells. Forced overexpression of miR-107, by infection of pre-miR-107 lentivirus or transfection of wild-type miR-107 mimic, largely inhibited CAB39 expression in OB-6 cells and primary human osteoblasts. Contrarily, miR-107 inhibition, by antagomiR-107, increased its expression, resulting in AMPK cascade activation. AntagomiR-107 largely attenuated DEX-induced cell death and apoptosis in OB-6 cells and human osteoblasts. Importantly, osteoblast cytoprotection by antagomiR-107 was abolished with AMPK in-activation by AMPKα1 dominant negative mutation, silencing or knockout. Further studies demonstrated that antagomiR-107 activated AMPK downstream Nrf2 cascade to inhibit DEX-induced oxidative injury. Conversely, Nrf2 knockout almost abolished antagomiR-107-induced osteoblast cytoprotection against DEX. Collectively, miR-107 inhibition induced CAB39 upregulation and activated AMPK-Nrf2 signaling to protect osteoblasts from DEX-induced oxidative injury and cytotoxicity.

PGK1 depletion activates Nrf2 signaling to protect human osteoblasts from dexamethasone

Activation of nuclear-factor-E2-related factor 2 (Nrf2) cascade can alleviate dexamethasone (DEX)-induced oxidative injury and death of human osteoblasts. A recent study has shown that phosphoglycerate kinase 1 (PGK1) inhibition/depletion will lead to Kelch-like ECH-associated protein 1 (Keap1) methylglyoxal modification, thereby activating Nrf2 signaling cascade. Here, in OB-6 osteoblastic cells and primary human osteoblasts, PGK1 silencing, by targeted shRNA, induced Nrf2 signaling cascade activation, causing Nrf2 protein stabilization and nuclear translocation, as well as increased expression of ARE-dependent genes (HO1, NQO1, and GCLC). Functional studies demonstrated that PGK1 shRNA largely attenuated DEX-induced oxidative injury and following death of OB-6 cells and primary osteoblasts. Furthermore, PGK1 knockout, by the CRISPR/Cas9 method, similarly induced Nrf2 signaling activation and protected osteoblasts from DEX. Importantly, PGK1 depletion-induced osteoblast cytoprotection against DEX was almost abolished by Nrf2 shRNA. In addition, Keap1 shRNA mimicked and nullified PGK1 shRNA-induced anti-DEX osteoblast cytoprotection. At last we show that PGK1 expression is downregulated in human necrotic femoral head tissues of DEX-taking patients, correlating with HO1 depletion. Collectively, these results show that PGK1 depletion protects human osteoblasts from DEX via activation of Keap1-Nrf2 signaling cascade.

Corticosterone induces neurotoxicity in PC12 cells via disrupting autophagy flux mediated by AMPK/mTOR signaling

Aims

Our previous study indicated that chronic stress caused autophagy impairment and subsequent neuron apoptosis in hippocampus. However, the mechanism underlying the stress‐induced damage to neurons is unclear. In present work, we investigated whether stress‐level glucocorticoids (GCs) GCs promoted PC12 cell damage via AMPK/mTOR signaling‐mediated autophagy.

Methods

Chronic stress‐induced PC12 cell injury model was built by treatment with high level corticosterone (CORT). Cell injury was evaluated by flow cytometry assay and transmission electron microscopy observation.

Results

Autophagy flux was measured based on the changes in LC3‐II and P62 protein expressions, and the color alteration of mCherry‐GFP‐LC3‐II transfection. Our results showed that CORT not only increased cell injury and apoptosis, but also dysregulated AMPK/mTOR signaling‐mediated autophagy flux, as indicated by the upregulated expression of LC3‐II and P62 proteins, and the lowered ration of autolysosomes to autophagosomes. Mechanistically, our results demonstrated that autophagy activation by AMPK activator metformin or mTOR inhibitor rapamycin obviously promotes cell survival and autophagy flux, improved mitochondrial ultrastructure, and reduced expression of Cyt‐C and caspase‐3 in CORT‐induced PC12 cells.

Conclusion

These results indicate that high CORT triggers PC12 cell damage through disrupting AMPK/mTOR‐mediated autophagy flux. Targeting this signaling may be a promising approach to protect against high CORT and chronic stress‐induced neuronal impairment.

Silencing FOXO1 attenuates dexamethasone-induced apoptosis in osteoblastic MC3T3-E1 cells

Dexamethasone (DEX), a widely used glucocorticoid with strong anti-inflammatory and immunosuppressive activities, has been reported to induce apoptosis in osteoblasts, but the underlying mechanisms are still not comprehensively investigated. FOXO1 plays an important role in the regulation of cell proliferation and apoptosis. Our study aims to explore the role of FOXO1 in DEX-induced apoptosis of osteoblastic MC3T3-E1 cells through bioinformatics and experiments. We first employed bioinformatics to identify DEX-related genes and revealed their functions by GO enrichment analysis including FOXO1 associated biological processes. Expression level of FOXO1 was validated by GEO data. Then, experiments were performed to verify the hypothesis. CCK8 was used to detect cell viability and apoptosis was detected by flow cytometry. SiRNA was used to silence FOXO1 and western-blot was employed to detect protein expression. Results demonstrated DEX-related genes involved in cell proliferation, apoptosis and angiogenesis and FOXO1 was a regulator of apoptosis. DEX could up-regulate FOXO1 expression, inhibit cell viability, promote apoptosis. SiRNA-FOXO1 could attenuate DEX-induced apoptosis in MC3T3-E1. These findings suggested DEX could affect some vital biological processes of MC3T3-E1 and FOXO1 played an essential role in DEX-induced apoptosis in MC3T3-E1.

Aster glehni Extract Containing Caffeoylquinic Compounds Protects Human Keratinocytes through the TRPV4-PPARδ-AMPK Pathway

Aster glehni (AG) has been used in cooking and as a medicine to treat various diseases for over hundreds of years in Korea. To speculate the protective effects of AG on skin barrier, we estimated the protein levels of biomarkers related to skin barrier protection in human keratinocytes, HaCaT cells treated with sodium dodecyl sulfate (SDS), or 2,4-dinitrochlorobenzene (DNCB). The protein levels for keratin, involucrin, defensin, tumor necrosis factor alpha (TNFα), peroxisome proliferator-activated receptor delta (PPARδ), 5′ adenosine monophosphate-activated protein kinase (AMPK), serine palmitoyltransferase long chain base subunit 2 (SPTLC2), and transient receptor potential cation channel subfamily V member 4 (TRPV4) were evaluated using western blotting or immunocytochemistry in HaCaT cells. AG extract increased the protein levels of PPARδ, phosphorylated AMPK, SPTLC2, keratin, involucrin, and defensin compared to the SDS or DNCB control group. However, TNFα expression increased by SDS or DNCB was decreased with AG extract. The order of action of each regulatory biomarker in AG pathway was identified TRPV4→PPARδ→AMPK from antagonist and siRNA treatment studies. AG can ameliorate the injury of keratinocytes caused by SDS or DNCB through the sequential regulation of TRPV4→PPARδ→AMPK pathway.

PP2A inhibition by LB-100 protects retinal pigment epithelium cells from UV radiation via activation of AMPK signaling

AMP-activated protein kinase (AMPK) signaling activation can inhibit Ultra-violet (UV) radiation (UVR)-induced retinal pigment epithelium (RPE) cell injuries. LB-100 is a novel inhibitor of protein phosphatase 2A (PP2A), the AMPKα1 phosphatase. Here, our results demonstrated that LB-100 significantly inhibited UVR-induced viability reduction, cell death and apoptosis in established ARPE-19 cells and primary murine RPE cells. LB-100 activated AMPK, nicotinamide adenine dinucleotide phosphate (NADPH) and Nrf2 (NF-E2-related factor 2) signalings, inhibiting UVR-induced oxidative injuries and DNA damage in RPE cells. Conversely, AMPK inhibition, by AMPKα1-shRNA, -CRISPR/Cas9 knockout or -T172A mutation, almost blocked LB-100-induced RPE cytoprotection against UVR. Importantly, CRISPR/Cas9-mediated PP2A knockout mimicked and nullified LB-100-induced anti-UVR activity in RPE cells. Collectively, these results show that PP2A inhibition by LB-100 protects RPE cells from UVR via activation of AMPK signaling.