Sirt6 cooperates with Blimp1 to positively regulate osteoclast differentiation

Unraveling the intricacies of osteoclast differentiation and maturation: insight into novel therapeutic strategies for bone-destructive diseases

Osteoclasts are the principal cells that efficiently resorb bone. Numerous studies have attempted to reveal the molecular pathways leading to the differentiation and activation of osteoclasts to improve the treatment and prevention of osteoporosis and other bone-destructive diseases. While the cumulative knowledge of osteoclast regulatory molecules, such as receptor activator of nuclear factor-kB ligand (RANKL) and nuclear factor of activated T cells 1 (NFATc1), contributes to the understanding of the developmental progression of osteoclasts, little is known about how the discrete steps of osteoclastogenesis modify osteoclast status but not the absolute number of osteoclasts. The regulatory mechanisms involved in osteoclast maturation but not those involved in differentiation deserve special attention due to their potential use in establishing a more effective treatment strategy: targeting late-phase differentiation while preserving coupled bone formation. Recent studies have shed light on the molecules that govern late-phase osteoclast differentiation and maturation, as well as the metabolic changes needed to adapt to shifting metabolic demands. This review outlines the current understanding of the regulation of osteoclast differentiation, as well as osteoclast metabolic adaptation as a differentiation control mechanism. Additionally, this review introduces molecules that regulate the late-phase osteoclast differentiation and thus minimally impact coupled bone formation.

Britanin inhibits titanium wear particle‑induced osteolysis and osteoclastogenesis

Wear particle‑induced osteolysis is a serious complication that occurs in individuals with titanium (Ti)‑based implants following long‑term usage due to loosening of the implants. The control of excessive osteoclast differentiation and inflammation is essential for protecting against wear particle‑induced osteolysis. The present study evaluated the effect of britanin, a pseudoguaianolide sesquiterpene isolated from Inula japonica, on osteoclastogenesis in vitro and Ti particle‑induced osteolysis in vivo. The effect of britanin was examined in the osteoclastogenesis of mouse bone marrow‑derived macrophages (BMMs) using TRAP staining, RT‑PCR, western blotting and immunocytochemistry. The protective effect of britanin was examined in a mouse calvarial osteolysis model and evaluated using micro‑CT and histomorphometry. Britanin inhibited osteoclast differentiation and F‑actin ring formation in the presence of macrophage colony‑stimulating factor and receptor activator of nuclear factor kB ligand in BMMs. The expression of osteoclast‑specific marker genes, including tartrate‑resistant acid phosphatase, cathepsin K, dendritic cell‑specific transmembrane protein, matrix metallopeptidase 9 and nuclear factor of activated T‑cells cytoplasmic 1, in the BMMs was significantly reduced by britanin. In addition, britanin reduced the expression of B lymphocyte‑induced maturation protein‑1, which is a transcriptional repressor of negative osteoclastogenesis regulators, including interferon regulatory factor‑8 and B‑cell lymphoma 6. Conversely, britanin increased the expression levels of anti‑oxidative stress genes, namely nuclear factor erythroid‑2‑related factor 2, NAD(P)H quinone oxidoreductase 1 and heme oxygenase 1 in the BMMs. Furthermore, the administration of britanin significantly reduced osteolysis in a Ti particle‑induced calvarial osteolysis mouse model. Based on these findings, it is suggested that britanin may be a potential therapeutic agent for wear particle‑induced osteolysis and osteoclast‑associated disease.

SIRT6-PAI-1 axis is a promising therapeutic target in aging-related bone metabolic disruption

The mechanistic regulation of bone mass in aged animals is poorly understood. In this study, we examined the role of SIRT6, a longevity-associated factor, in osteocytes, using mice lacking Sirt6 in Dmp-1-expressing cells (cKO mice) and the MLO-Y4 osteocyte-like cell line. cKO mice exhibited increased osteocytic expression of Sost, Fgf23 and senescence inducing gene Pai-1 and the senescence markers p16 and Il-6, decreased serum phosphate levels, and low-turnover osteopenia. The cKO phenotype was reversed in mice that were a cross of PAI-1-null mice with cKO mice. Furthermore, senescence induction in MLO-Y4 cells increased the Fgf23 and Sost mRNA expression. Sirt6 knockout and senescence induction increased HIF-1α binding to the Fgf23 enhancer sequence. Bone mass and serum phosphate levels were higher in PAI-1-null aged mice than in wild-type mice. Therefore, SIRT6 agonists or PAI-1 inhibitors may be promising therapeutic options for aging-related bone metabolism disruptions.

Unkeito Suppresses RANKL-Mediated Osteoclastogenesis via the Blimp1-Bcl6 and NF-κB Signaling Pathways and Enhancing Osteoclast Apoptosis

Osteoporosis is a common bone disease, particularly in menopausal women. Herein, we screened four Kampo medicines (Unkeito (UKT), Kamishoyosan (KSS), Kamikihito (KKT), and Ninjinyoeito (NYT)), frequently used to treat menopausal syndromes, for their effects on receptor activator of nuclear factor-kappaB ligand (RANKL)-induced osteoclast differentiation in RAW 264 cells. Considering that UKT exhibited the most potent effect, we examined its effect on RANKL-induced osteoclastogenesis, the induction of osteoclast apoptosis, and the mechanisms underlying its effects. UKT inhibits RANKL-induced osteoclast differentiation in the early stage and decreases osteoclast-related genes, including tartrate-resistant acid phosphatase (Trap), dendritic cell-specific transmembrane protein (Dcstamp), matrix metalloproteinase-9 (Mmp9), and cathepsin K (Ctsk). Specifically, UKT inhibits the nuclear factor of activated T cells 1 (NFATc1), which is essential for osteoclastogenesis. UKT increases Bcl6, which antagonizes NFATc1 and Dc-stamp, thereby blocking the progression of osteoclasts to maturation. UKT also decreased nuclear translocation by downregulating the activity of p65/NF-κB. In addition, UKT enhances mononuclear osteoclast apoptosis via activation of caspase-3. Herein, we demonstrate that UKT suppresses RANKL-mediated osteoclastogenesis via the Blimp1–Bcl6 and NF-κB signaling pathways and enhances mononuclear osteoclast apoptosis. Furthermore, UKT prevents bone loss in OVX mice. Thus, UKT might be a potential therapeutic agent for postmenopausal osteoporosis.

Role of sirtuins in bone biology: Potential implications for novel therapeutic strategies for osteoporosis

The decline in bone mass and bone strength and musculoskeletal problems associated with aging constitute a major challenge for affected individuals and the healthcare system globally. Sirtuins 1-7 (SIRT1-SIRT7) are a family of nicotinamide adenine dinucleotide-dependent deacetylases with remarkable abilities to promote longevity and counteract age-related diseases. Sirtuin knockout and transgenic models have provided novel insights into the function and signaling of these proteins in bone homeostasis. Studies have revealed that sirtuins play a critical role in normal skeletal development and homeostasis through their direct action on bone cells and that their dysregulation might contribute to different bone diseases. Preclinical studies have demonstrated that mice treated with sirtuin agonists show protection against age-related, postmenopausal, and immobilization-induced osteoporosis. These findings suggest that sirtuins could be potential targets for the modulation of the imbalance in bone remodeling and treatment of osteoporosis and other bone disorders. The aim of this review was to provide a comprehensive updated review of the current knowledge on sirtuin biology, focusing specifically on their roles in bone homeostasis and osteoporosis, and potential pharmacological interventions targeting sirtuins for the treatment of osteoporosis.

Inhibitory Effect of Sirtuin6 (SIRT6) on Osteogenic Differentiation of Bone Marrow Mesenchymal Stem Cells

BACKGROUND The imbalance between bone resorption and formation is the basic mechanism underlying osteoporosis in the elderly. Osteogenesis is the differentiation of human mesenchymal stem cells (hMSCs) into osteoblasts. Sirtuin6 (SIRT6) regulates various biological functions, including differentiation. Transient receptor potential cation channel subfamily V member 1 (TRPV1) is a non-selective cation channel that can be activated by physical and chemical stimulation. However, experimental data supporting the role of SIRT6 in osteogenic differentiation (OD) of hMSCs are lacking. MATERIAL AND METHODS Differentiation of hMSCs was induced. The expressions of SIRT6, TRPV1, and CGRP were detected by Q-PCR, Western blot, and ELISA, respectively. SIRT6 was overexpressed in hMSCs by transfection. ALP activity and Alizarin Red staining were utilized to detect the effect of SIRT6 on hMSC OD. Then, capsaicin and capsazepine, the TRPV1 agonist and antagonist, respectively, were administrated to assess the role of TRPV1. RESULTS SIRT6 expression was downregulated during hMSC differentiation. SIRT6 overexpression was accompanied by reduced expression of specific genes and alkaline phosphatase (ALP) activity in osteoblasts. Furthermore, TRPV1 channel was also reduced by SIRT6 overexpression via ubiquitinating TRPV1. Capsaicin was utilized in SIRT6-overexpressed cells. Capsaicin therapy counteracted the effect of SIRT6 overexpression on OD, and markedly decreased OD. CONCLUSIONS The SIRT6-TRPV1-CGRP signal axis is the key to regulating OD in hMSCs, which could be a potential therapeutic target for osteoporosis and bone loss-related diseases.

Role of APD-Ribosylation in Bone Health and Disease

The transfer of adenosine diphosphate (ADP)-ribose unit(s) from nicotinamide adenine dinucleotide (NAD⁺) to acceptor proteins is known as ADP-ribosylation. This post-translational modification (PTM) unavoidably alters protein functions and signaling networks, thereby impacting cell behaviors and tissue outcomes. As a ubiquitous mechanism, ADP-ribosylation affects multiple tissues, including bones, as abnormal ADP-ribosylation compromises bone development and remodeling. In this review, we describe the effects of ADP-ribosylation in bone development and maintenance, and highlight the underlying mechanisms.

RACK1 interaction with c-Src is essential for osteoclast function

The scaffolding protein receptor for activated C-kinase 1 (RACK1) mediates receptor activator of nuclear factor κΒ ligand (RANKL)-dependent activation of p38 MAPK in osteoclast precursors; however, the role of RACK1 in mature osteoclasts is unclear. The aim of our study was to identify the interaction between RACK1 and c-Src that is critical for osteoclast function. A RACK1 mutant protein (mutations of tyrosine 228 and 246 residues to phenylalanine; RACK1 Y228F/Y246F) did not interact with c-Src. The mutant retained its ability to differentiate into osteoclasts; however, the integrity of the RANKL-mediated cytoskeleton, bone resorption activity, and phosphorylation of c-Src was significantly decreased. Importantly, lysine 152 (K152) within the Src homology 2 (SH2) domain of c-Src is involved in RACK1 binding. The c-Src K152R mutant (mutation of lysine 152 into arginine) impaired the resorption of bone by osteoclasts. These findings not only clarify the role of the RACK1-c-Src axis as a key regulator of osteoclast function but will also help to develop new antiresorption therapies to prevent bone loss-related diseases.

Bone Remodeling: Histone Modifications as Fate Determinants of Bone Cell Differentiation

The bone tissue is a dynamic complex that constitutes of several interdependent systems and is continuously remodeled through the concerted actions of bone cells. Osteoblasts are mononucleated cells, derived from mesenchymal stem cells, responsible for bone formation. Osteoclasts are large multinucleated cells that differentiate from hematopoietic progenitors of the myeloid lineage and are responsible for bone resorption. The lineage-specific differentiation of bone cells requires an epigenetic regulation of gene expressions involving chromatin dynamics. The key step for understanding gene regulatory networks during bone cell development lies in characterizing the chromatin modifying enzymes responsible for reorganizing and potentiating particular chromatin structure. This review covers the histone-modifying enzymes involved in bone development, discusses the impact of enzymes on gene expression, and provides future directions and clinical significance in this area.

Electronic structure theory gives insights into the higher efficiency of the PTB electron-donor polymers for organic photovoltaics in comparison with prototypical P3HT

The electron donor poly-thienothiophene-benzodithiophene (PTB) polymer series displays remarkable properties that lead to more efficient bulk heterojunction (BHJ) organic solar cells. In this work, the ground and four excited states (bright S ₁ and dark S ₂-S ₄) of three different members of the PTBn (n = 1, 6, 7) series were studied and compared with the prototypical poly(3-hexylthiophene) (P3HT) donor polymer. Time-dependent density functional theory was employed to investigate oligomers of similar sizes (∼50 Å). Charge alternation electron accumulation and depletion regions of the four transitions are concentrated on the inner units, thereby favoring interaction with the electron acceptor in a BHJ. The bright S ₁ transition energies of PTBn are about 0.2 eV lower as compared to P3HT, thereby allowing a better match of their levels with the typical C₆₀-type acceptor moiety in a BHJ. Side chains play a minor role in the electronic spectrum (less than ∼0.1 eV). The most efficient PTB7 transfers more electronic charge from its electron-rich benzodithiophene subunit to its electron-deficient thieno[3,4-b] thiophene subunit as compared to PTB1 and PTB6. We show that the dipolar effect, a partial concentration of negative and positive charges on the different parts of the donor polymer that favors charge separation, is more pronounced in PTBn polymers and typically an order of magnitude larger as compared to P3HT. These effects are conspicuous for the most efficient polymer of the series, PTB7, with its fluorine substituent shown to play a crucial role.

Ameloblastin attenuates RANKL-mediated osteoclastogenesis by suppressing activation of nuclear factor of activated T-cell cytoplasmic 1 (NFATc1)

Ameloblastin (Ambn) is an extracellular matrix protein and member of the family of enamel-related gene products. Like amelogenin, Ambn is mainly associated with tooth development, especially biomineralization of enamel. Previous studies have shown reductions in the skeletal dimensions of Ambn-deficient mice, suggesting that the protein also has effects on the differentiation of osteoblasts and/or osteoclasts. However, the specific pathways used by Ambn to influence osteoclast differentiation have yet to be identified. In the present study, two cellular models, one based on bone marrow cells and another on RAW264.7 cells, were used to examine the effects of Ambn on receptor activator of nuclear factor kappa-B ligand (RANKL)-induced osteoclastogenesis. The results showed that Ambn suppresses osteoclast differentiation, cytoskeletal organization, and osteoclast function by the downregulation of the number of tartrate-resistant acid phosphatase (TRAP)-positive multinucleated osteoclasts, actin ring formation, and areas of pit resorption. The expression of the osteoclast-specific genes TRAP, MMP9, cathepsin K, and osteoclast stimulatory transmembrane protein (OC-STAMP) was abolished in the presence of Ambn, while that of nuclear factor of activated T cells cytoplasmic 1 (NFATc1), the master regulatory factor of osteoclastogenesis, was also attenuated by the downregulation of c-Fos expression. In Ambn-induced RAW264.7 cells, phosphorylation of cAMP-response element-binding protein (CREB), c-Jun N-terminal kinase (JNK), and p38 mitogen-activated protein kinase (p38 MAPK), but not extracellular signal-regulated kinase 1/2 (ERK1/2), was reduced. Calcium oscillation was also decreased in the presence of Ambn, suggesting its involvement in both RANKL-induced osteoclastogenesis and costimulatory signaling. B-lymphocyte-induced maturation protein-1 (Blimp1), a transcriptional repressor of negative regulators of osteoclastogenesis, was also downregulated by Ambn, resulting in the elevated expression of v-maf musculoaponeurotic fibrosarcoma oncogene family, protein B (MafB), B-cell lymphoma 6 (Bcl6), and interferon regulatory factor-8 (Irf8). Taken together, these findings suggest that Ambn suppresses RANKL-induced osteoclastogenesis by modulating the NFATc1 axis.

Evidence for excessive osteoclast activation in SIRT6 null mice

SIRT6 is a NAD-dependent histone 3 deacetylase. SIRT6 null mice have been reported suffering osteopenia. However, the role of SIRT6 in bone resorption is still not well understood. In this study, we focused on the role of SIRT6 in osteoclast. We performed histological analysis on the femur, spine, alveolar bone and even tail of mutant mice, and found the bone mass is sharply decreased while the osteoclast activity is significantly increased. These phenotypes were further demonstrated by the osteoclast differentiation in cell-cultures with TRAP staining and Pit Resorption Assay. We next found the proliferation activity of mutant osteoclast precursors was increased, which might account for the enhanced osteoclast formation. The concentration of tartrate-resistant acid phosphatase 5b, a marker of osteoclast differentiation, was significantly higher in the mutant mice than control. Besides, the osteoclastogenic and NF-κB signaling related genes were significantly up-regulated. Moreover, osteoblast/osteoclast co-culture demonstrated that SIRT6 regulated osteoclast mainly through osteoblast paracrine manner, rather than osteoclast-autonomous behavior. Together, the enhanced osteoclast activation in SIRT6 null mice might be regulated by the hyperactive NF-κB signaling and the enhanced proliferation activity of osteoclast precursors through osteoblast paracrine manner at the cellular level.

SIRT6 overexpression inhibits HIF1α expression and its impact on tumor angiogenesis in lung cancer

OBJECTIVE

To investigate the effect of silencing information regulator 6 (SIRT6) on HIF1α expression of cell line A549 in non-small cell lung cancer and on tumor angiogenesis in lung cancer.

METHODS

Cell line A549 in the logarithmic growth phase was transfected with Ad-SIRT6 and Ad-null respectively. According to the study design, the cells were divided into control group, Ad-null group and Ad-SIRT6 group. The HIF1α and HIF2α mRNA expression in each group were detected by real-time quantitative PCR (qPCR). The level of prolyl hydroxylase (PHD) 1-3 after 48 h of Ad-SIRT6-transfected cell line A549 and the levels of VEGF-C, VEGF-D, VEGFR-2 and VEGFR-3 in the supernatants were determined by ELISA. The nude mice were injected subcutaneously with Ad-null or Ad-SIRT6 transfected cell line A549. The tumor volume was observed at 6, 12, 18, 24 and 30 d after inoculation, and the tumor mass was weighed at 30 d. Also, microvessel density (MVD) and the number of positive HIF1α and VEGF cells were detected by immunohistochemistry. The VEGF and HIF1α levels in tumor tissue were detected by ELISA and qPCR respectively.

RESULTS

qPCR showed that the levels of HIF-1α mRNA, VEGF-C, VEGF-D, VEGFR-2 and VEGFR-3 in the supernatant were decreased, the level of PHD2 was increased (P<0.05), and the levels of HIF-2α mRNA, PHD1 and PHD3 did not change much (P>0.05) in the Ad-SIRT6 group as compared with those in the control group and Ad-null group. The tumor growth rate was decreased, and the tumor volume at 12-30 d after inoculation was less in the Ad-SIRT6 group than in the control group and Ad-null group (P<0.05); the tumor mass was also lower than that of control and Ad-null groups (P<0.05). Immunohistochemistry showed that MVD and the number of HIF-1α and VEGF positive cells were less in the Ad-SIRT6 group than in control and Ad-null groups (P<0.05); and HIF-1α and VEGF levels in tumor tissue were decreased in the Ad-SIRT6 group compared to the control and Ad-null groups (P<0.05). There were no significant differences in the above measurements between the control group and Ad-null group (P>0.05).

CONCLUSION

SIRT6 overexpression can inhibit HIF1α and VEGF expression, promoting PHD2 expression, which can inhibit angiogenesis and xenograft growth and may play a role in reducing HIF1α and VEGF expression.

STAC2 negatively regulates osteoclast formation by targeting the RANK signaling complex

The receptor activator of nuclear factor-κB (RANK) protein activates various protein kinase signaling cascades, including those involving NF-κB, mitogen-activated protein kinase (MAPK), and Bruton tyrosine kinase (Btk)/tyrosine-protein kinase Tec. However, the mechanism underlying the negative regulation of RANK by downstream signaling molecules remains unclear. Here, we report that Src homology 3 domain and cysteine-rich domain-containing protein 2 (STAC2) is a novel RANK ligand-inducible protein that negatively regulates RANK-mediated osteoclast formation. STAC2 physically interacts with RANK and inhibits the formation of the RANK signaling complex, which contains Grb-2-associated binder 2 (Gab2) and phospholipase Cγ2 (PLCγ2), thus leading to the suppression of RANK-mediated NF-κB and MAPK activation. Furthermore, STAC2 overexpression limits Btk/Tec-mediated PLCγ2 phosphorylation via the interaction between STAC2 and Btk/Tec. Taken together, our results reveal a novel mechanism whereby RANK signaling is restricted by its physical interaction with STAC2.

Current Understanding of RANK Signaling in Osteoclast Differentiation and Maturation

Osteoclasts are bone-resorbing cells that are derived from hematopoietic precursor cells and require macrophage-colony stimulating factor and receptor activator of nuclear factor-κB ligand (RANKL) for their survival, proliferation, differentiation, and activation. The binding of RANKL to its receptor RANK triggers osteoclast precursors to differentiate into osteoclasts. This process depends on RANKL-RANK signaling, which is temporally regulated by various adaptor proteins and kinases. Here we summarize the current understanding of the mechanisms that regulate RANK signaling during osteoclastogenesis. In the early stage, RANK signaling is mediated by recruiting adaptor molecules such as tumor necrosis factor receptor-associated factor 6 (TRAF6), which leads to the activation of mitogen-activated protein kinases (MAPKs), and the transcription factors nuclear factor-κB (NF-κB) and activator protein-1 (AP-1). Activated NF-κB induces the nuclear factor of activated T-cells cytoplasmic 1 (NFATc1), which is the key osteoclastogenesis regulator. In the intermediate stage of signaling, the co-stimulatory signal induces Ca2+ oscillation via activated phospholipase Cγ2 (PLCγ2) together with c-Fos/AP-1, wherein Ca2+ signaling facilitates the robust production of NFATc1. In the late stage of osteoclastogenesis, NFATc1 translocates into the nucleus where it induces numerous osteoclast-specific target genes that are responsible for cell fusion and function.