Activating transcription factor 4, an ER stress mediator, is required for, but excessive ER stress suppresses osteoblastogenesis by bortezomib

Concomitant Detection of Serum Activating Transcription Factor 4 Levels and Mean Amplitude of Glycemic Excursion Is Helpful to Predict Osteoporosis in Adults With Type 2 Diabetes Mellitus

OBJECTIVES

Activating transcription factor 4 (ATF4) and mean amplitude of glycemic excursion (MAGE) have been implicated in the pathogenesis of osteoporosis (OP) and type 2 diabetes mellitus (T2DM). In this retrospective cohort study we assessed the predictive value of serum ATF4 and MAGE for occurrence of OP in individuals with T2DM.

METHODS

A total of 162 individuals with T2DM were assigned to T2DM or T2DM+OP groups. All participants underwent serum ATF4 level detection and 72-hour blood glucose monitoring (MAGE measurement). The correlations of ATF4 and MAGE with glucose and bone metabolism indicators and bone mineral density (BMD) were analyzed. A multivariate logistic regression model was developed to evaluate the correlations of ATF4 and MAGE with T2DM-associated OP. The diagnostic performance of concomitant detection of ATF4 and MAGE was assessed based on area under the receiver operating characteristic (AUC) curve.

RESULTS

Individuals with T2DM with OP had higher serum ATF4 levels and MAGE than those with T2DM only. ATF4 and MAGE correlated positively with fasting insulin, glycated hemoglobin, homeostatic model for insulin resistance assessment, beta-C-terminal crosslinking telopeptide of type I collagen, and tartrate-resistant acid phosphatase-5b, but negatively with bone alkaline phosphatase, serum procollagen type I N-propeptide, procollagen type I carboxy-terminal propeptide, and BMD. Elevated levels of ATF4 and MAGE were independent risk factors, but increased BMD at the hip, femoral neck, and lumbar spine was a protective factor for individuals with T2DM with OP. More importantly, the AUC of concomitant ATF4 and MAGE was considerably higher than that of ATF4 or MAGE alone.

CONCLUSION

Concomitant detection of ATF4 and MAGE may aid in predicting the occurrence of OP in individuals with T2DM.

Advances in the roles of ATF4 in osteoporosis

Osteoporosis (OP) is characterized by reduced bone mass, decreased strength, and enhanced bone fragility fracture risk. Activating transcription factor 4 (ATF4) plays a role in cell differentiation, proliferation, apoptosis, redox balance, amino acid uptake, and glycolipid metabolism. ATF4 induces the differentiation of bone marrow mesenchymal stem cells (BM-MSCs) into osteoblasts, increases osteoblast activity, and inhibits osteoclast formation, promoting bone formation and remodeling. In addition, ATF4 mediates the energy metabolism in osteoblasts and promotes angiogenesis. ATF4 is also involved in the mediation of adipogenesis. ATF4 can selectively accumulate in osteoblasts. ATF4 can directly interact with RUNT-related transcription factor 2 (RUNX2) and up-regulate the expression of osteocalcin (OCN) and osterix (Osx). Several upstream factors, such as Wnt/β-catenin and BMP2/Smad signaling pathways, have been involved in ATF4-mediated osteoblast differentiation. ATF4 promotes osteoclastogenesis by mediating the receptor activator of nuclear factor κ-B (NF-κB) ligand (RANKL) signaling. Several agents, such as parathyroid (PTH), melatonin, and natural compounds, have been reported to regulate ATF4 expression and mediate bone metabolism. In this review, we comprehensively discuss the biological activities of ATF4 in maintaining bone homeostasis and inhibiting OP development. ATF4 has become a therapeutic target for OP treatment.

Advances in the interaction between endoplasmic reticulum stress and osteoporosis

The endoplasmic reticulum (ER) is the main site for protein synthesis, folding, and secretion, and accumulation of the unfolded/misfolded proteins in the ER may induce ER stress. ER stress is an important participant in various intracellular signaling pathways. Prolonged- or high-intensity ER stress may induce cell apoptosis. Osteoporosis, characterized by imbalanced bone remodeling, is a global disease caused by many factors, such as ER stress. ER stress stimulates osteoblast apoptosis, increases bone loss, and promotes osteoporosis development. Many factors, such as the drug's adverse effects, metabolic disorders, calcium ion imbalance, bad habits, and aging, have been reported to activate ER stress, resulting in the pathological development of osteoporosis. Increasing evidence shows that ER stress regulates osteogenic differentiation, osteoblast activity, and osteoclast formation and function. Various therapeutic agents have been developed to counteract ER stress and thereby suppress osteoporosis development. Thus, inhibition of ER stress has become a potential target for the therapeutic management of osteoporosis. However, the in-depth understanding of ER stress in the pathogenesis of osteoporosis still needs more effort.

Mechanisms of preferential bone formation in myeloma bone lesions by proteasome inhibitors

Proteasome inhibitors (PIs) can preferentially restore bone in bone-defective lesions of patients with multiple myeloma (MM) who respond favorably to these drugs. Most prior in vitro studies on PIs used continuous exposure to low PI concentrations, although pharmacokinetic analysis in patients has shown that serum concentrations of PIs change in a pulsatile manner. In the present study, we explored the effects of pulsatile treatment with PIs on bone metabolism to simulate in vivo PI pharmacokinetics. Pulsatile treatment with bortezomib, carfilzomib, or ixazomib induced MM cell death but only marginally affected the viability of osteoclasts (OCs) with F-actin ring formation. Pulsatile PI treatment suppressed osteoclastogenesis in OC precursors and bone resorption by mature OCs. OCs robustly enhanced osteoblastogenesis in cocultures with OCs and MC3T3-E1 pre-osteoblastic cells, indicating OC-mediated coupling to osteoblastogenesis. Importantly, pulsatile PI treatment did not impair robust OC-mediated osteoblastogenesis. These results suggest that PIs might sufficiently reduce MM cell-derived osteoblastogenesis inhibitors to permit OC-driven bone formation coupling while suppressing OC differentiation and activity in good responders to PIs. OC-mediated coupling to osteoblastogenesis appears to be a predominant mechanism for preferential occurrence of bone regeneration at sites of osteoclastic bone destruction in good responders.

Myeloma bone disease: pathogenesis and management in the era of new anti-myeloma agents

INTRODUCTION

Multiple myeloma (MM) is a malignancy of plasma cells with characteristic bone disease. Despite recent great strides achieved in MM treatment owing to the implementation of new anti-MM agents, MM is still incurable and bone destruction remains a serious unmet issue in patients with MM.

APPROACH

In this review, we will summarize and discuss the mechanisms of the formation of bone disease in MM and the available preclinical and clinical evidence on the treatment for MM bone disease.

CONCLUSIONS

MM cells produce a variety of cytokines to stimulate receptor activator of nuclear factor-κB ligand-mediated osteoclastogenesis and suppress osteoblastic differentiation from bone marrow stromal cells, leading to extensive bone destruction with rapid loss of bone. MM cells alter the microenvironment through bone destruction where they colonize, which in turn favors tumor growth and survival, thereby forming a vicious cycle between tumor progression and bone destruction. Denosumab or zoledronic acid is currently recommended to be administered at the start of treatment in newly diagnosed patients with MM with bone disease. Proteasome inhibitors and the anti-CD38 monoclonal antibody daratumumab have been demonstrated to exert bone-modifying activity in responders. Besides their anti-tumor activity, the effects of new anti-MM agents on bone metabolism should be more precisely analyzed in patients with MM. Because prognosis in patients with MM has been significantly improved owing to the implementation of new agents, the therapeutic impact of bone-modifying agents should be re-estimated in the era of these new agents.

Treatment of Gout with TCM Using Turmeric and Corn Silk: A Concise Review Article and Pharmacology Network Analysis

OBJECTIVE

This work aimed to study the compounds, targets, and pathways of turmeric and corn silk for gout and to explore the mechanism of "the same disease with different treatments" based on network pharmacology and molecular docking.

METHODS

We used the TCMSP, PubChem, and SEA databases to screen the compounds and targets of turmeric and corn silk, gout-related proteins through TTD, Drugbank, DisGeNET, GeneCards, OMIM, and PharmGkb, and used Cytoscape to construct a "compound-target-disease" network. Then, we constructed a protein-protein interaction network (PPI) and used Metascape to perform GO and KEGG analysis. Finally, molecular docking (SYBYL) was used to verify the degree of binding between key targets and compounds.

RESULTS

We found bisacumol, campesterol, and stigmasterol to be the main turmeric compounds that exerted a marked effect on gout treatment by targeting protein processing in the endoplasmic reticulum through the HSPA1B, HSP90AB1, and STUB1 proteins. The main corn silk compound, Mandenol, treated gout by targeting the Hippo signaling pathway through the CTNNB1, YWHAG, and YWHAZ proteins.

CONCLUSION

Turmeric and corn silk can treat the same disease, gout, through different pathways and targets. The scientific connotation of "same disease with different treatments" can be preliminarily clarified by analyzing targets and pathways.

Folate ameliorates homocysteine-induced osteoblast dysfunction by reducing endoplasmic reticulum stress-activated PERK/ATF-4/CHOP pathway in MC3T3-E1 cells

INTRODUCTION

Homocysteine (Hcy) is considered a newly identified risk factor for osteoporosis. Nevertheless, the underlying mechanism of folate (FA), a key factor in the metabolism of Hcy, in protection against osteoblast dysfunction remains unclear. The purpose of this study was to investigate the mechanism by which FA attenuates Hcy-induced osteoblast damage.

MATERIALS AND METHODS

The Hcy-induced MC3T3-E1 cells were treated with different concentrations of FA. Cell morphology, cell density, cell proliferation ability, alkaline phosphatase (ALP) activity and mineralization capacity were observed and determined; the gene expression of B-cell lymphoma 2 (Bcl-2) and Bcl-2-associated X protein (BAX) and ERS-associated factors, including glucose-regulated protein 78 (GRP-78), activating transcription factor 4 (ATF-4) and growth arrest and DNA damage inducible gene 153 (CHOP/GADD153), were assessed by RT-PCR; and protein levels of GRP-78 and ATF-4 were analyzed by western blotting.

RESULTS

Hcy suppressed the proliferation, differentiation and mineralization ability of MC3T3-E1 cells in a concentration-dependent manner and activated the ERS signaling pathway. After intervention with different concentrations of FA, the cell viability and density, ALP activity, number of mineralized nodules, calcium content and Bcl-2 gene expression were all significantly increased, whereas the gene expression of GRP-78, CHOP/GADD153, ATF-4 and Bax was markedly downregulated, and protein levels of GRP-78 and ATF-4 were also markedly decreased.

CONCLUSION

The adverse effects of Hcy on osteoblast differentiation are dose dependent. FA not only protects against osteoblasts apoptosis but also has a direct osteogenic effect on Hcy-induced osteoblasts, which could be partially mediated by inhibition of the PERK-activated ERS pathway.

Lansoprazole-induced osteoporosis via the IP3R- and SOCE-mediated calcium signaling pathways

BACKGROUND

Many clinical studies have shown a correlation between proton pump inhibitors (PPIs) and osteoporosis or fractures. The purpose of this study was to establish a murine model of chronic oral PPI administration to verify whether PPIs caused bone metabolic impairment and investigate the relevant molecular mechanism underlying the effects of PPIs on MC3T3-E1 murine osteoblasts.

METHODS

A lansoprazole-induced bone loss model was used to investigate the damaging effects of PPIs. In vivo, immunohistochemistry, Hematoxylin-Eosin (HE) staining, micro-CT analysis, and blood biochemical analyses were used to evaluate the effect of lansoprazole on bone injury in mice. In vitro, the effects of lansoprazole and related signaling pathways in MC3T3-E1 cells were investigated by CCK-8 assays, EdU assays, flow cytometry, laser confocal microscopy, patch clamping, reverse transcription-quantitative polymerase chain reaction and Western blotting.

RESULTS

After 6 months of lansoprazole gavage in ICR mice, the micro-CT results showed that compared with that in the vehicle group, the bone mineral density (BMD) in the high-dose group was significantly decreased (P < 0.05), and the bone microarchitecture gradually degraded. Biochemical analysis of bone serum showed that blood calcium and phosphorus were both decreased (P < 0.01). We found that long-term administration of lansoprazole impaired skeletal function in mice. In vitro, we found that lansoprazole (LPZ) could cause calcium overload in MC3T3-E1 cells leading to apoptosis, and 2-APB, an inhibitor of IP3R calcium release channel and SOCE pathway, effectively blocked increase in calcium caused by LPZ, thus protecting cell viability.

CONCLUSIONS

Longterm administration of LPZ induced osteoporotic symptoms in mice, and LPZ triggered calcium increases in osteoblasts in a concentration-dependent manner. Intracellular calcium ([Ca2+]i) persisted at a high concentration, thereby causing endoplasmic reticulum stress (ERS) and inducing osteoblast apoptosis.

GPX7 Facilitates BMSCs Osteoblastogenesis via ER Stress and mTOR Pathway

Emerging evidence indicates extensive oxidative stress is a consequence of obesity which impairs bone formation. Glutathione peroxidase 7 (GPX7) is a conserved endoplasmic reticulum (ER) retention protein, lacking of which causes accumulation of reactive oxygen species (ROS) and promotes adipogenesis. Since the imbalance between osteogenic and adipogenic differentiation of bone marrow mesenchymal stem cell (BMSC) leads to severe bone diseases such as osteoporosis, it is critical to investigate the potential protective role of Gpx7 in osteogenesis. Here, we provide evidence that deficiency of Gpx7 reduces osteogenesis, but increases adipogenesis in both human BMSCs (hBMSCs) and mouse mesenchymal stem cell line. Interestingly, further studies indicate this defect can be alleviated by the ER stress antagonist, but not the ROS inhibitor, unveiling an unexpected finding that, unlike adipogenesis, lacking of Gpx7 inhibits osteogenesis mediating by induced ER stress instead of enhanced ROS. Furthermore, the mTOR signalling pathway is found down‐regulation during osteogenic differentiation in Gpx7‐deficient condition, which can be rescued by relief of ER stress. Taken together, for the first time we identify a novel function of Gpx7 in BMSCs’ osteogenic differentiation and indicate that Gpx7 may protect against osteoporotic deficits in humans through ER stress and mTOR pathway interplay.

ER stress arm XBP1s plays a pivotal role in proteasome inhibition-induced bone formation

BACKGROUND

Bone destruction is a hallmark of multiple myeloma (MM). It has been reported that proteasome inhibitors (PIs) can reduce bone resorption and increase bone formation in MM patients, but the underlying mechanisms remain unclear.

METHODS

Mesenchymal stem cells (MSCs) were treated with various doses of PIs, and the effects of bortezomib or carfilzomib on endoplasmic reticulum (ER) stress signaling pathways were analyzed by western blotting and real-time PCR. Alizarin red S (ARS) and alkaline phosphatase (ALP) staining were used to determine the osteogenic differentiation in vitro. Specific inhibitors targeting different ER stress signaling and a Tet-on inducible overexpressing system were used to validate the roles of key ER stress components in regulating osteogenic differentiation of MSCs. Chromatin immunoprecipitation (ChIP) assay was used to evaluate transcription factor-promoter interaction. MicroCT was applied to measure the microarchitecture of bone in model mice in vivo.

RESULTS

We found that both PERK-ATF4 and IRE1α-XBP1s ER stress branches are activated during PI-induced osteogenic differentiation. Inhibition of ATF4 or XBP1s signaling can significantly impair PI-induced osteogenic differentiation. Furthermore, we demonstrated that XBP1s can transcriptionally upregulate ATF4 expression and overexpressing XBP1s can induce the expression of ATF4 and other osteogenic differentiation-related genes and therefore drive osteoblast differentiation. MicroCT analysis further demonstrated that inhibition of XBP1s can strikingly abolish bortezomib-induced bone formation in mouse.

CONCLUSIONS

These results demonstrated that XBP1s is a master regulator of PI-induced osteoblast differentiation. Activation of IRE1α-XBP1s ER stress signaling can promote osteogenesis, thus providing a novel strategy for the treatment of myeloma bone disease.

Role of endoplasmic reticulum stress in disuse osteoporosis

Osteoporosis is a major skeletal disease with low bone mineral density, which leads to an increased risk of bone fracture. Salubrinal is a synthetic chemical that inhibits dephosphorylation of eukaryotic translation initiation factor 2 alpha (eIF2α) in response to endoplasmic reticulum (ER) stress. To understand possible linkage of osteoporosis to ER stress, we employed an unloading mouse model and examined the effects of salubrinal in the pathogenesis of disuse osteoporosis. The results presented several lines of evidence that osteoclastogenesis in the development of osteoporosis was associated with ER stress, and salubrinal suppressed unloading-induced bone loss. Compared to the age-matched control, unloaded mice reduced the trabecular bone area/total area (B.Ar/T.Ar) as well as the number of osteoblasts, and they increased the osteoclasts number on the trabecular bone surface in a time-dependent way. Unloading-induced disuse osteoporosis significantly increased the expression of Bip, p-eIF2α and ATF4 in short-term within 6h of tail suspension, but time-dependent decreased in HU2d to HU14d. Furthermore, a significant correlation of ER stress with the differentiation of osteoblasts and osteoclasts was observed. Administration of salubrinal suppressed the unloading-induced decrease in bone mineral density, B.Ar/T.Ar and mature osteoclast formation. Salubrinal also increased the colony-forming unit-fibroblasts and colony-forming unit-osteoblasts. It reduced the formation of mature osteoclasts, suppressed their migration and adhesion, and increased the expression of Bip, p-eIF2α and ATF4. Electron microscopy showed that rough endoplasmic reticulum expansion and a decreased number of ribosomes on ER membrane were observed in osteoblast of unloading mice, and the abnormal ER expansion was significantly improved by salubrinal treatment. A TUNEL assay together with CCAAT/enhancer binding protein homologous protein (CHOP) expression indicated that ER stress-induced osteoblast apoptosis was rescued by salubrinal. Collectively, the results support the notion that ER stress plays a key role in the pathogenesis of disuse osteoporosis, and salubrinal attenuates unloading-induced bone loss by altering proliferation and differentiation of osteoblasts and osteoclasts via eIF2α signaling.

Multiple myeloma mesenchymal stromal cells: Contribution to myeloma bone disease and therapeutics

Multiple myeloma is a hematological malignancy in which clonal plasma cells proliferate and accumulate within the bone marrow. The presence of osteolytic lesions due to increased osteoclast (OC) activity and suppressed osteoblast (OB) function is characteristic of the disease. The bone marrow mesenchymal stromal cells (MSCs) play a critical role in multiple myeloma pathophysiology, greatly promoting the growth, survival, drug resistance and migration of myeloma cells. Here, we specifically discuss on the relative contribution of MSCs to the pathophysiology of osteolytic lesions in light of the current knowledge of the biology of myeloma bone disease (MBD), together with the reported genomic, functional and gene expression differences between MSCs derived from myeloma patients (pMSCs) and their healthy counterparts (dMSCs). Being MSCs the progenitors of OBs, pMSCs primarily contribute to the pathogenesis of MBD because of their reduced osteogenic potential consequence of multiple OB inhibitory factors and direct interactions with myeloma cells in the bone marrow. Importantly, pMSCs also readily contribute to MBD by promoting OC formation and activity at various levels (i.e., increasing RANKL to OPG expression, augmenting secretion of activin A, uncoupling ephrinB2-EphB4 signaling, and through augmented production of Wnt5a), thus further contributing to OB/OC uncoupling in osteolytic lesions. In this review, we also look over main signaling pathways involved in the osteogenic differentiation of MSCs and/or OB activity, highlighting amenable therapeutic targets; in parallel, the reported activity of bone-anabolic agents (at preclinical or clinical stage) targeting those signaling pathways is commented.

Effects of the proteasome inhibitor, bortezomib, on cytodifferentiation and mineralization of periodontal ligament cells

BACKGROUND AND OBJECTIVE

The proteasome inhibitor, bortezomib, is known to induce osteoblastic differentiation in a number of cell lines, such as mesenchymal stem cells and osteoblastic precursor cells. As periodontal ligament (PDL) cells are multipotent, we examined whether bortezomib may induce the differentiation of PDL cells into hard-tissue-forming cells.

MATERIAL AND METHODS

A mouse PDL clone cell line, MPDL22 cells, was cultured in mineralization medium in the presence or absence of bortezomib. Expression of calcification-related genes and calcified-nodule formation were evaluated by real-time PCR and Alizarin Red staining, respectively.

RESULTS

Bortezomib increased the expression of calcification-related mRNAs, such as tissue nonspecific alkaline phosphatase isoenzyme (ALPase), bone sialoprotein (Bsp), runt-related transcription factor 2 (Runx2) and osteopontin, and calcified-nodule formation in MPDL22 cells. These effects were induced, in part, by increasing the cytosolic accumulation and nuclear translocation of β-catenin, leading to an increase in expression of bone morphogenetic protein (Bmp)-2, -4 and -6 mRNAs. In addition, bortezomib enhanced BMP-2-induced expression of Bsp and osteopontin mRNAs and increased calcified-nodule formation in MPDL22 cells.

CONCLUSION

Bortezomib induced cytodifferentiation and mineralization of PDL cells by enhancing the accumulation of β-catenin within the cytosol and the nucleus and increasing the expression of Bmp-2, -4 and -6 mRNAs. Moreover, bortezomib enhanced the BMP-2-induced cytodifferentiation and mineralization of PDL cells, suggesting that bortezomib may be efficacious for use in periodontal regeneration therapy.