Inhibitory effects of osteoprotegerin on osteoclast formation and function under serum-free conditions

Biomarkers That Seem to Have the Greatest Impact on Promoting the Formation of Atherosclerotic Plaque in Current Scientific Research

Atherosclerosis is a chronic inflammatory disease that causes degenerative and productive changes in the arteries. The resulting atherosclerotic plaques restrict the vessel lumen, causing blood flow disturbances. Plaques are formed mainly in large- and medium-sized arteries, usually at bends and forks where there is turbulence in blood flow. Depending on their location, they can lead to various disease states such as myocardial infarction, stroke, renal failure, peripheral vascular diseases, or sudden cardiac death. In this work, we reviewed the literature on the early detection of atherosclerosis markers in the application of photodynamic therapy to atherosclerosis-related diseases. Herein, we described the roles of C-reactive protein, insulin, osteopontin, osteoprotegerin, copeptin, the TGF-β cytokine family, and the amino acid homocysteine. Also, we discuss the role of microelements such as iron, copper, zinc, and Vitamin D in promoting the formation of atherosclerotic plaque. Dysregulation of the administered compounds is associated with an increased risk of atherosclerosis. Additionally, taking into account the pathophysiology of atherosclerotic plaque formation, we believe that maintaining homeostasis in the range of biomarkers mentioned in this article is crucial for slowing down the process of atherosclerotic plaque development and the stability of plaque that is already formed.

Beclin 1 positively regulates osteoprotegerin-induced inhibition of osteoclastogenesis by increasing autophagy in vitro

Osteoclastogenesis is induced by receptor activator of nuclear factor-κB ligand (RANKL) and macrophage colony-stimulating factor (M-CSF), and can be suppressed by osteoprotegerin (OPG). Beclin1 has a dual role in osteoclastogenesis. However, the role of Beclin1-mediated autophagy during OPG-induced inhibition of osteoclastogenesis remains unclear. Here, we found that Beclin1 and matrix metalloproteinase 9 (MMP-9) expression were increased during osteoclastogenesis. OPG (20, 40, and 80 ng/mL) decreased Src and MMP-9 expression, but augmented Beclin1 expression and fluorescence intensity. Similarly, treatment with the autophagy activator rapamycin increased Beclin1 expression during OPG-induced inhibition of osteoclastogenesis. Further, Beclin1 knockdown restored osteoclast numbers by reducing autophagy during OPG-induced inhibition of osteoclastogenesis. These results indicate that Beclin1 has a positive role during OPG-induced inhibition of osteoclastogenesis by regulating autophagy, which might provide a potential basis for osteoclastogenesis.

The Role of Autophagy and Mitophagy in Bone Metabolic Disorders

Bone metabolic disorders include osteolysis, osteoporosis, osteoarthritis and rheumatoid arthritis. Osteoblasts and osteoclasts are two major types of cells in bone constituting homeostasis. The imbalance between bone formation by osteoblasts and bone resorption by osteoclasts has been shown to have a direct contribution to the onset of these diseases. Recent evidence indicates that autophagy and mitophagy, the selective autophagy of mitochondria, may play a vital role in regulating the proliferation, differentiation and function of osteoblasts and osteoclasts. Several signaling pathways, including PINK1/Parkin, SIRT1, MAPK8/FOXO3, Beclin-1/BECN1, p62/SQSTM1, and mTOR pathways, have been implied in the regulation of autophagy and mitophagy in these cells. Here we review the current progress about the regulation of autophagy and mitophagy in osteoblasts and osteoclasts in these bone metabolic disorders, as well as the molecular signaling activated or deactivated during this process. Together, we hope to draw attention to the role of autophagy and mitophagy in bone metabolic disorders, and their potential as a new target for the treatment of bone metabolic diseases and the requirements of further mechanism studies.

p53 positively regulates osteoprotegerin-mediated inhibition of osteoclastogenesis by downregulating TSC2-induced autophagy in vitro

Osteoclasts are terminally multinucleated cells that are regulated by nuclear factor-activated T cells c1 (NFATc1), and are responsible for bone resorption while the tartrate resistant acid phosphatase (TRAP) enzymes releases into bone resorption lacunae. Furthermore, tumor suppressor p53 is a negative regulator during osteoclastogenesis. Osteoprotegerin (OPG) inhibits osteoclastogenesis and bone resorption by activating autophagy, however, whether p53 is involved in OPG-mediated inhibition of osteoclastogenesis remains unclear. In the current study, OPG could enhance the expression of p53 and tuberin sclerosis complex 2 (TSC2). Moreover, the expression of p53 is regulated by autophagy during OPG-mediated inhibition of osteoclastogenesis. Inhibition of p53 by treated with pifithrin-α (PFTα) causing augments of osteoclastogenesis and bone resorption, also reversed OPG-mediated inhibition of osteoclastogenesis by reducing the expression of TSC2. In addition, knockdown of TSC2 using siRNA could rescue OPG-mediated inhibition of osteoclastogenesis by reducing autophagy, which is manifested by the decrease of the expression of Beclin1 and the phosphorylation of mammalian target of rapamycin (mTOR) and ribosomal protein S6 kinase beta 1 (S6K1, also known as p70S6K). Collectively, p53 plays a critical role during OPG-mediated inhibition of osteoclastogenesis via regulating the TSC2-induced autophagy in vitro.

Two novel SNPs in genes involved in immune response and their association with mandibular residual ridge resorption

“Residual ridge resorption” (RRR) is a multifactorial condition involving bone resorption of the residual ridge. We investigated 10 single nucleotide polymorphisms (SNPs) in seven genes with the aim of identifying the genetic factors associated with RRR susceptibility. The study group included 96 RRR patients and 96 controls. Age at first edentulism, duration of edentulism, and bone height were recorded. Saliva was collected from the subjects for DNA extraction. Genotype analysis was performed on the ‘SequenomMassARRAYiPLEX’. The genotype and allele frequencies calculated in patients and controls were compared. We found that rs1800896 in the IL10 gene and rs5743289 in NOD2 gene showed significant association with RRR. Within the RRR group, genotypes for each SNP were separated, and we observed that the age at first edentulism and bone height showed variations in the different genotypes of the ten studied SNPs. This study showed an association between SNPs in IL10 and NOD2 genes. It also revealed that the genotypes of the different SNPs influence bone resorption and health. Additionally, age at first edentulism and bone height were much lower in some genotypes. This study demonstrates the need for larger multicenter trials to confirm these findings. Finally, we suggest that the results of this study may be utilized for developing novel genetic diagnostic tests and for identifying Saudi individuals who may be more susceptible to RRR development following dental extraction.

The effect of P2X7R-mediated Ca2+ signaling in OPG-induced osteoclasts adhesive structure damage

Osteoclast adhesion is important for bone resorption. Osteoprotegerin inhibits osteoclast differentiation and bone resorption via Ca²⁺ signaling. Purinergic receptor P2X7 (P2X7R) affects osteoclastogenesis by activating transcription factor nuclear factor of activated T cells 1 (NFATc1). However, the detailed mechanism of osteoprotegerin-mediated P2X7R modulation of osteoclast adhesion is unclear. This study aimed to determine the effect of P2X7R on osteoprotegerin-induced damage to osteoclast adhesion. Osteoprotegerin reduced the expression of P2X7R, and protein tyrosine kinase 2 (PYK2) and SRC phosphorylation, and reduced calcium concentration, significantly decreasing Ca²⁺-NFATc1 signaling. 1,2-Bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid tetrakis (acetoxymethyl ester) (BAPTA-AM)/N-(6-Aminohexyl)-5-chloro-1-naphthalenesulfonamide hydrochloride (W-7) partly or absolutely recovered osteoprotegerin-induced osteoclasts adhesion structure damage, including increased the PYK2 and SRC phosphorylation, changed the distribution of PYK2/SRC and integrinαvβ3, and inhibited retraction of lamellipodia and filopodia and recovered osteoclast bone resorption activity. In addition, BAPTA-AM/W-7 also increased osteoprotegerin-induced activation of Ca²⁺-NFATc1 signaling, and restored normal P2X7R levels. P2X7R knockdown significantly inhibited osteoclast differentiation, and the formation of lamellipodia and filopodia, reduced the PYK2 and SRC phosphorylation, and inhibited Ca²⁺-related protein activation. However, P2X7R knockdown aggravated osteoprotegerin-induced osteoclast adhesion damage via Ca²⁺ signaling. In conclusion, the P2X7R-Ca²⁺ NFATc1 signaling pathway has a key functional role in osteoprotegerin-induced osteoclast adhesion structure damage.

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.

Osteoprotegerin disrupts peripheral adhesive structures of osteoclasts by modulating Pyk2 and Src activities

Osteoprotegerin has previously been shown to modulate bone mass by blocking osteoclast maturation and function. The detailed mechanisms of osteoprotegerin-induced disassembly of podosomes, disruption of adhesive structures and modulation of adhesion-related proteins in osteoclasts, however, are not well characterized. In this study, tartrate-resistant acidic phosphatase staining demonstrated that osteoprotegerin inhibited differentiation of osteoclasts. The use of scanning electron microscopy, real-time cell monitoring and confocal microscopy indicated that osteoclasts responded in a time and dose-dependent manner to osteoprotegerin treatments with retraction of peripheral adhesive structures and detachment from the extracellular substrate. Combined imaging and Western blot studies showed that osteoprotegerin induced dephosphorylation of Tyr 402 in Pyk2 and decreased its labeling in peripheral adhesion regions. osteoprotegerin induced increased intracellular labeling of Tyr 402 in Pyk2, Tyr 416 in Src, increased dephosphorylation of Tyr 527 in Src, and increased Pyk2/Src association in the central region of osteoclasts. This evidence suggests that Src may function as an adaptor protein that competes for Pyk2 and relocates it from the peripheral adhesive zone to the central region of osteoclasts in response to osteoprotegerin treatment. Osteoprotegerin may induce podosome reassembly and peripheral adhesive structure detachment by modulating phosphorylation of Pyk2 and Src and their intracellular distribution in osteoclasts.

Osteoprotegerin exposure at different stages of osteoclastogenesis differentially affects osteoclast formation and function

This study aimed to investigate the effects of osteoprotegerin (OPG), a decoy receptor for receptor activator for nuclear factor κB ligand (RANKL), during the various stages of osteoclast differentiation, and additionally investigate its effects on osteoclast adhesion and activity. RAW264.7 murine monocytic cells were incubated with macrophage colony-stimulating factor and RANKL for 1, 3, 5, or 7 days, followed by an additional 24-h incubation in the presence or absence of OPG (80 ng/mL). We examined osteoclast differentiation and adhesion capacity using the tartrate-resistant acid phosphatase (TRAP) assay and immunofluorescence microscopy, and additionally examined cell growth in real time using the xCELLigence system. Furthermore, the expression levels of TRAP, RANK, integrin β3, matrix metalloproteinase 9, cathepsin K, carbonic anhydrase II, and vesicular-type H(+)-ATPase A1 were examined using western blotting. OPG exposure on day 1 enhanced the osteoclast growth curve as well as adhesion, and increased RANK and integrin β3 expression. In contrast, exposure to OPG at later time points (days 3-7) inhibited osteoclast differentiation, adhesion structure formation, and protease expression. In conclusion, the biological effects of OPG exposure at the various stages of osteoclast differentiation were varied, and included the enhanced adhesion and survival of preosteoclasts, the block of differentiation from the early to the terminal stages of osteoclastogenesis, and suppression of mature osteoclast activation following OPG exposure during the terminal differentiation stage, suggesting that the effects of OPG exposure differ based on the stage of differentiation.

Osteoprotegerin induces podosome disassembly in osteoclasts through calcium, ERK, and p38 MAPK signaling pathways

Osteoclasts are critical for bone resorption and use podosomes to attach to bone matrix. Osteoprotegerin (OPG) is a negative regulator of osteoclast function that can affect the formation and function of podosomes. However, the signaling pathways that link OPG to podosome function have not been well characterized. Therefore, this study examined the roles of intracellular calcium and MAPKs in OPG-induced podosome disassembly in osteoclasts. We assessed the effects of the intracellular calcium chelator Bapta-AM, ERK inhibitor U0126, and p38 inhibitor SB202190 on OPG-treated osteoclast differentiation, adhesion structures, intracellular free Ca(2+) concentration and the phosphorylation state of podosome associated proteins (Pyk2 and Src). Mouse monocytic RAW 264.7 cells were differentiated to osteoclasts using RANKL (30ng/mL) and M-CSF (25ng/mL). The cells were pretreated with Bapta-AM (5μM), U0126 (5μM), or SB202190 (10μM) for 30min, followed by 40ng/mL OPG for 3h. Osteoclastogenesis, adhesion structure, viability and morphology, intracellular free Ca(2+) concentration and the phosphorylation state of Pyk2 and Src were measured by TRAP staining, scanning electron microscopy, real-time cell analyzer, flow cytometry and western blotting, respectively. OPG significantly inhibited osteoclastogenesis, the formation of adhesion structures, and reduced the amount of phosphorylated Pyk2 and Src-pY527, but increased phosphorylation of Src-pY416. Bapta-AM, U0126, and SB202190 partially restored osteoclast differentiation and adhesion structures. Both Bapta-AM and U0126, but not SB202190, restored the levels of intracellular free Ca(2+) concentration, phosphorylated Pyk2 and Src-pY527. All three inhibitors blocked OPG-induced phosphorylation at Src-pY416. These results suggest OPG disrupts the attachment structures of osteoclasts and activates Src as an adaptor protein that competes for the reduced amount of phosphorylated Pyk2 through calcium- and ERK-dependent signaling pathways. p38 MAPK signaling may have a different role in OPG-induced osteoclast retraction. Our findings potentially offer novel insights into the signaling mechanisms downstream of OPG that affect osteoclast attachment to the extracellular matrix.