Ellagic acid protects ovariectomy-induced bone loss in mice by inhibiting osteoclast differentiation and bone resorption

Osteoporosis is a devastating disease that features reduced bone quantity and microstructure, which causes fragility fracture and increases mortality, especially in the aged population. Due to the long-term side-effects of current drugs for osteoporosis, it is of importance to find other safe and effective medications. Ellagic acid (EA) is a phenolic compound found in nut galls, plant extracts, and fruits, and exhibits antioxidant and antineoplastic effects. Here, we showed that EA attenuated the formation and function of osteoclast dose-dependently. The underlying mechanism was further discovered by western blot, immunofluorescence assay, and luciferase assay, which elucidated that EA suppressed osteoclastogenesis and bone resorption mainly through attenuating receptor activator of nuclear factor-κB (NF-κB) ligand-induced NF-κB activation and extracellular signal-regulated kinase signaling pathways, accompanied by decreased protein expression of nuclear factor of activated T-cells calcineurin-dependent 1 and c-Fos. Moreover, EA inhibits osteoclast marker genes expression including Dc-stamp, Ctsk, Atp6v0d2, and Acp5. Intriguingly, we also found that EA treatment could significantly protect ovariectomy-induced bone loss in vivo. Conclusively, this study suggested that EA might have the therapeutic potentiality for preventing or treating osteoporosis.     

Ligustilide suppresses RANKL-induced osteoclastogenesis and bone resorption via inhibition of RANK expression

Osteoclast (OC) is the only cell involved in bone resorption. Dysfunction of OCs leads to a variety of bone diseases. Ligustilide (LIG) is the main component of the volatile oil isolated and purified from Angelica sinensis. LIG exerts many pharmacological activities, but its effects on osteoclastogenesis and bone resorption are still unclear. Our study showed that LIG inhibited receptor activator of nuclear factor-κB (NF-κB) ligand-induced OC formation and activation in a dose-dependent manner. Additionally, LIG downregulated the messenger RNA (mRNA) expression of OC-specific genes, such as V-ATPase d2, tartrate-resistant acid phosphatase, a dendritic cell-specific transmembrane protein, cathepsin K, and nuclear factor of activated T cells cl. Furthermore, LIG blocked the activation of NF-κB/extracellular signal-regulated kinase/p38/immunoreceptor tyrosine-based activation motif signaling pathways. Crucially, the expression of tumor necrosis factor receptor-associated factor 6 proteins and the expression of receptor activator of NF-κB mRNA were inhibited by LIG. However, LIG did not affect the formation and mineralization of osteoblasts. Collectively, this observation suggests that LIG may serve as a promising agent for the prevention and treatment of diseases caused by abnormal bone resorption.     

Antiosteoclastic bone resorption activity of osteoprotegerin via enhanced AKT/mTOR/ULK1-mediated autophagic pathway

Autophagy plays a critical role in the maintenance of bone homeostasis. Osteoprotegerin (OPG) is an inhibitor of osteoclast-mediated bone resorption. However, whether autophagy is involved in the antiosteoclastogenic effects of OPG remains unclear. The present study aimed to investigate the potential mechanism of autophagy during OPG-induced bone resorption via inhibition of osteoclasts differentiated from bone marrow-derived macrophages in BALB/c mice. The results showed that after treatment with receptor activator of nuclear factor-κΒ ligand and macrophage colony-stimulating factor for 3 days, TRAP⁺ osteoclasts formed, representing the resting state of autophagy. These osteoclasts were treated with OPG and underwent autophagy, as demonstrated by LC3-II accumulation, acidic vesicular organelle formation, and the presence of autophagosomes. The levels of autophagy-related proteins, LC3-II increased and P62 decreased at 3 hr in OPG-treated osteoclasts. The viability, differentiation, and bone resorption activity of osteoclasts declined after OPG treatment. Treatment with OPG and chloroquine, an autophagy inhibitor, attenuated OPG-induced inhibition of osteoclastic bone resorption, whereas rapamycin (RAP), an autophagy inducer, enhanced OPG-induced inhibition of differentiation, survival, and bone resorption activity of osteoclasts. Furthermore, OPG reduced the amount of phosphorylated(p) protein kinase B (AKT) and pmTOR and increased the level of pULK, in a dose-dependant manner. LY294002, a phosphatidylinositol-4,5-bisphosphate 3-kinase (PI3K)/AKT pathway inhibitor, attenuated the decline in pAKT, but enhanced the decline in pmTOR and the increase in pULK1 following OPG treatment. RAP enhanced the OPG-induced increase in pULK1. The PI3K inhibitor 3-methyladenine partly blocked OPG-induced autophagy. Thus, the results revealed that OPG inhibits osteoclast bone resorption by inducing autophagy via the AKT/mTOR/ULK1 signaling pathway.     

Osteoprotegerin inhibit osteoclast differentiation and bone resorption by enhancing autophagy via AMPK/mTOR/p70S6K signaling pathway in vitro

Osteoclasts are highly differentiated terminal cells formed by fusion of hematopoietic stem cells. Previously, osteoprotegerin (OPG) inhibit osteoclast differentiation and bone resorption by blocking receptor activator of nuclear factor-κB ligand (RANKL) binding to RANK indirect mechanism. Furthermore, autophagy plays an important role during osteoclast differentiation and function. However, whether autophagy is involved in OPG-inhibited osteoclast formation and bone resorption is not known. To elucidate the role of autophagy in OPG-inhibited osteoclast differentiation and bone resorption, we used primary osteoclast derived from mice bone marrow monocytes/macrophages (BMM) by induced M-CSF and RANKL. The results showed that autophagy-related proteins expression were upregulated; tartrate-resistant acid phosphatase-positive osteoclast number and bone resorption activity were decreased; LC3 puncta and autophagosomes number were increased and activated AMPK/mTOR/p70S6K signaling pathway. In addition, chloroquine (as the autophagy/lysosome inhibitor, CQ) or rapamycin (as the autophagy/lysosome inhibitor, Rap) attenuated osteoclast differentiation and bone resorption activity by OPG treatment via AMPK/mTOR/p70S6K signaling pathway. Our data demonstrated that autophagy plays a critical role in OPG inhibiting osteoclast differentiation and bone resorption via AMPK/mTOR/p70S6K signaling pathway in vitro.     

Icariin protects against iron overload-induced bone loss via suppressing oxidative stress

Iron overload is common in patients with diseases such as hemoglobinopathies, hereditary hemochromatosis or elderly men and postmenopausal women. This disorder is frequently associated with bone loss and recently has been considered as an independent risk factor for osteoporosis. By excess reactive oxygen species (ROS) production through Fenton reaction, iron could induce osteoblast apoptosis, inhibit osteoblast osteogenic differentiation. Moreover, Iron could also promote osteoclasts differentiation and bone absorption. The goal of the study is to investigate whether icariin could reverse iron overload-induced bone loss in vitro and in vivo. Icariin is the major active ingredient of Herba Epimedii and has antioxidant, antiosteoporosis functions. In the current study, we demonstrated that oral administration of icariin significantly prevented bone loss in iron overloaded mice. Icariin could protect against iron overload-induced mitochondrial membrane potential dysfunction and ROS production, promote osteoblast survival and reverse the reduction of Runx2, alkaline phosphatase, and osteopontin expression induced by iron overload. Icariin also inhibited osteoclasts differentiation and function. Moreover, we also found that icariin remarkably reduced iron accumulation in bone marrow, suggesting that icariin has the ability to regulate systemic iron metabolism in vivo. These results indicated that icariin could be a potential natural resource for developing medicines to prevent or treat iron overload-induced osteoporosis.     

Corilagin suppresses RANKL‐induced osteoclastogenesis and inhibits oestrogen deficiency‐induced bone loss via the NF‐κB and PI3K/AKT signalling pathways

Over‐activated osteoclastogenesis, which is initiated by inflammation, has been implicated in osteoporosis. Corilagin, a natural compound extracted from various medicinal herbaceous plants, such as Cinnamomum cassia, has antioxidant and anti‐inflammatory activities. We found that Corilagin suppressed osteoclast differentiation in a dose‐dependent manner, significantly decreased osteoclast‐related gene expression and impaired bone resorption by osteoclasts. Moreover, phosphorylation of members of the nuclear factor‐kappaB (NF‐κB) and PI3K/AKT signalling pathways was reduced by Corilagin. In a murine model of osteoporosis, Corilagin inhibited osteoclast functions in vivo and restored oestrogen deficiency‐induced bone loss. In conclusion, our findings suggested that Corilagin inhibited osteoclastogenesis by down‐regulating the NF‐κB and PI3K/AKT signalling pathways, thus showing its potential possibility for the treatment of osteoporosis.     

Bone regeneration via skeletal cell lineage plasticity: All hands mobilized for emergencies

An emerging concept is that quiescent mature skeletal cells provide an important cellular source for bone regeneration. It has long been considered that a small number of resident skeletal stem cells are solely responsible for the remarkable regenerative capacity of adult bones. However, recent in vivo lineage‐tracing studies suggest that all stages of skeletal lineage cells, including dormant pre‐adipocyte‐like stromal cells in the marrow, osteoblast precursor cells on the bone surface and other stem and progenitor cells, are concomitantly recruited to the injury site and collectively participate in regeneration of the damaged skeletal structure. Lineage plasticity appears to play an important role in this process, by which mature skeletal cells can transform their identities into skeletal stem cell‐like cells in response to injury. These highly malleable, long‐living mature skeletal cells, readily available throughout postnatal life, might represent an ideal cellular resource that can be exploited for regenerative medicine.     

Retinoid metabolism and its effects on the vasculature

Retinoids, the metabolically-active structural derivatives of vitamin A, are critical signaling molecules in many fundamental biological processes including cell survival, proliferation and differentiation. Emerging evidence, both clinical and molecular, implicates retinoids in atherosclerosis and other vasculoproliferative disorders such as restenosis. Although the data from clinical trials examining effect of vitamin A and vitamin precursors on cardiac events have been contradictory, this data does suggest that retinoids do influence fundamental processes relevant to atherosclerosis. Preclinical animal model and cellular studies support these concepts. Retinoids exhibit complex effects on proliferation, growth, differentiation and migration of vascular smooth muscle cells (VSMC), including responses to injury and atherosclerosis. Retinoids also appear to exert important inhibitory effects on thrombosis and inflammatory responses relevant to atherogenesis. Recent studies suggest retinoids may also be involved in vascular calcification and endothelial function, for example, by modulating nitric oxide pathways. In addition, established retinoid effects on lipid metabolism and adipogenesis may indirectly influence inflammation and atherosclerosis. Collectively, these observations underscore the scope and complexity of retinoid effects relevant to vascular disease. Additional studies are needed to elucidate how context and metabolite-specific retinoid effects affect atherosclerosis. This article is part of a Special Issue entitled: Retinoid and Lipid Metabolism.