The Link Between Bone Osteocalcin and Energy Metabolism in a Group of Postmenopausal Women

There is a dual relationship between bone and tissues involved in energy metabolism (fat tissue and beta-pancreatic cells). Thus, bone remodeling is an energy consuming process, but osteocalcin, the main on-collagenic protein, synthesized by osteoblas during bone formation exerts a number of biological effects on beta-pancreatic and adipose cells. With this data, we wanted to see if the presence of a chronic metabolic disorder such as type 2 diabetes mellitus (T2DM) influence this complex dual relationship. For this, we conducted a cross-sectional study to evaluate the relation between osteocalcin and energetic metabolism in a group of 146 postmenopausal womens with and without T2DM at CI Parhon National Institute of Endocrinology, Bucharest. Clinical, metabolic and hormonal parameters were evaluated. For statistical analysis we used Student t-test and the Spearman correlation (statistical significance: p <0.05). Results: 63 patients with T2DM (63.88±8.56 years) and 83 women in the control group (60.21±8.77 years) were included. Diabetic women showed a lower level of serum total osteocalcin (p<0.05) HDL-cholesterol (p=0.02), and 25-hydroxyvitamin D (25(OH)D). The body mass index (BMI), glycemic metabolism parameters and triglyceride levels (p<0.05) were higher in this group. We found correlations between osteocalcin and metabolic elements: negative with BMI (r=-0.329, p<0.05), glycated hemoglobin (HbA1c) (r=-0.398, p<0.05), and serum triglycerides (r=-0.329, p<0.05) respectively positive with HDL-cholesterol (r=0.279, p=0.001) for the entire group of patients. Conclusions: Our study indicated the presence of significant correlations between serum osteocalcin and glycemic and lipid metabolism parameters, independent of the presence of diabetes.

IL-6 exhibits both cis- and trans-signaling in osteocytes and osteoblasts, but only trans-signaling promotes bone formation and osteoclastogenesis

Interleukin 6 (IL-6) supports development of bone-resorbing osteoclasts by acting early in the osteoblast lineage via membrane-bound (cis) or soluble (trans) receptors. Here, we investigated how IL-6 signals and modifies gene expression in differentiated osteoblasts and osteocytes and determined whether these activities can promote bone formation or support osteoclastogenesis. Moreover, we used a genetically altered mouse with circulating levels of the pharmacological IL-6 trans-signaling inhibitor sgp130-Fc to determine whether IL-6 trans-signaling is required for normal bone growth and remodeling. We found that IL-6 increases suppressor of cytokine signaling 3 (Socs3) and CCAAT enhancer-binding protein δ (Cebpd) mRNA levels and promotes signal transducer and activator of transcription 3 (STAT3) phosphorylation by both cis- and trans-signaling in cultured osteocytes. In contrast, RANKL (Tnfsf11) mRNA levels were elevated only by trans-signaling. Furthermore, we observed soluble IL-6 receptor release and ADAM metallopeptidase domain 17 (ADAM17) sheddase expression by osteocytes. Despite the observation that IL-6 cis-signaling occurs, IL-6 stimulated bone formation in vivo only via trans-signaling. Although IL-6 stimulated RANKL (Tnfsf11) mRNA in osteocytes, these cells did not support osteoclast formation in response to IL-6 alone; binucleated TRAP+ cells formed, and only in response to trans-signaling. Finally, pharmacological, sgp130-Fc-mediated inhibition of IL-6 trans-signaling did not impair bone growth or remodeling unless mice had circulating sgp130-Fc levels > 10 μg/ml. At those levels, osteopenia and impaired bone growth occurred, reducing bone strength. We conclude that high sgp130-Fc levels may have detrimental off-target effects on the skeleton.

Increased glycolysis mediates Wnt7b-induced bone formation

Wingless/integrated (Wnt) signaling has emerged as a major mechanism for promoting bone formation and a target pathway for developing bone anabolic agents against osteoporosis. However, the downstream events mediating the potential therapeutic effect of Wnt proteins are not fully understood. Previous studies have indicated that increased glycolysis is associated with osteoblast differentiation in response to Wnt signaling, but direct genetic evidence for the importance of glucose metabolism in Wnt-induced bone formation is lacking. Here, we have generated compound transgenic mice to overexpress Wnt family member 7B (Wnt7b) transiently in the osteoblast lineage of postnatal mice, with or without concurrent deletion of the glucose transporter 1 (Glut1), also known as solute carrier family 2, facilitated glucose transporter member 1. Overexpression of Wnt7b in 1-mo-old mice for 1 wk markedly stimulated bone formation, but the effect was essentially abolished without Glut1, even though transient deletion of Glut1 itself did not affect normal bone accrual. Consistent with the in vivo results, Wnt7b increased Glut1 expression and glucose consumption in the primary culture of osteoblast lineage cells, and deletion of Glut1 diminished osteoblast differentiation in vitro. Thus, Wnt7b promotes bone formation in part through stimulating glucose metabolism in osteoblast lineage cells.-Chen, H., Ji, X., Lee, W.-C., Shi, Y., Li, B., Abel, E. D., Jiang, D., Huang, W., Long, F. Increased glycolysis mediates Wnt7b-induced bone formation.

Hydrogen sulfide attenuates homocysteine-induced osteoblast dysfunction by inhibiting mitochondrial toxicity

Homocysteine (Hcy) is detrimental to bone health in a mouse model of diet-induced hyperhomocysteinemia (HHcy). However, little is known about Hcy-mediated osteoblast dysfunction via mitochondrial oxidative damage. Hydrogen sulfide (H₂ S) has potent antioxidant, anti-inflammatory, and antiapoptotic effects. In this study, we hypothesized that the H₂ S mediated recovery of osteoblast dysfunction by maintaining mitochondrial biogenesis in Hcy-treated osteoblast cultures in vitro. MC3T3-E1 osteoblastic cells were exposed to Hcy treatment in the presence or absence of an H₂ S donor (NaHS). Cell viability, osteogenic differentiation, reactive oxygen species (ROS) production were determined. Mitochondrial DNA copy number, adenosine triphosphate (ATP) production, and oxygen consumption were also measured. Our results demonstrated that administration of Hcy increases the intracellular Hcy level and decreases intracellular H₂ S level and expression of the cystathionine β-synthase/Cystathionine γ-lyase system, thereby inhibiting osteogenic differentiation. Pretreatment with NaHS attenuated Hcy-induced mitochondrial toxicity (production of total ROS and mito-ROS, ratio of mitochondrial fission (DRP-1)/fusion (Mfn-2)) and restored ATP production and mitochondrial DNA copy numbers as well as oxygen consumption in the osteoblast as compared with the control, indicating its protective effects against Hcy-induced mitochondrial toxicity. In addition, NaHS also decreased the release of cytochrome c from the mitochondria to the cytosol, which induces cell apoptosis. Finally, flow cytometry confirmed that NaHS can rescue cells from apoptosis induced by Hcy. Our studies strongly suggest that NaHS has beneficial effects on mitochondrial toxicity, and could be developed as a potential therapeutic agent against HHcy-induced mitochondrial dysfunction in cultured osteoblasts in vitro.

The Molecular Mechanism of Vitamin E as a Bone-Protecting Agent: A Review on Current Evidence

Bone remodelling is a tightly-coordinated and lifelong process of replacing old damaged bone with newly-synthesized healthy bone. In the bone remodelling cycle, bone resorption is coupled with bone formation to maintain the bone volume and microarchitecture. This process is a result of communication between bone cells (osteoclasts, osteoblasts, and osteocytes) with paracrine and endocrine regulators, such as cytokines, reactive oxygen species, growth factors, and hormones. The essential signalling pathways responsible for osteoclastic bone resorption and osteoblastic bone formation include the receptor activator of nuclear factor kappa-B (RANK)/receptor activator of nuclear factor kappa-B ligand (RANKL)/osteoprotegerin (OPG), Wnt/β-catenin, and oxidative stress signalling. The imbalance between bone formation and degradation, in favour of resorption, leads to the occurrence of osteoporosis. Intriguingly, vitamin E has been extensively reported for its anti-osteoporotic properties using various male and female animal models. Thus, understanding the underlying cellular and molecular mechanisms contributing to the skeletal action of vitamin E is vital to promote its use as a potential bone-protecting agent. This review aims to summarize the current evidence elucidating the molecular actions of vitamin E in regulating the bone remodelling cycle.

A Dual Molecular Biointerface Combining RGD and KRSR Sequences Improves Osteoblastic Functions by Synergizing Integrin and Cell-Membrane Proteoglycan Binding

Synergizing integrin and cell-membrane heparan sulfate proteoglycan signaling on biomaterials through peptidic sequences is known to have beneficial effects in the attachment and behavior of osteoblasts; however, controlling the exact amount and ratio of peptides tethered on a surface is challenging. Here, we present a dual molecular-based biointerface combining integrin (RGD) and heparin (KRSR)-binding peptides in a chemically controlled fashion. To this end, a tailor-made synthetic platform (PLATF) was designed and synthesized by solid-phase methodologies. The PLATF and the control linear peptides (RGD or KRSR) were covalently bound to titanium via silanization. Physicochemical characterization by means of contact angle, Raman spectroscopy and XPS proved the successful and stable grafting of the molecules. The biological potential of the biointerfaces was measured with osteoblastic (Saos-2) cells both at short and long incubation periods. Biomolecule grafting (either the PLATF, RGD or KRSR) statistically improved (p < 0.05) cell attachment, spreading, proliferation and mineralization, compared to control titanium. Moreover, the molecular PLATF biointerface synergistically enhanced mineralization (p < 0.05) of Saos-2 cells compared to RGD or KRSR alone. These results indicate that dual-function coatings may serve to improve the bioactivity of medical implants by mimicking synergistic receptor binding.

Stimulatory effects of platycodin D on osteoblast differentiation

Previous studies have suggested that platycodin D is implicated in bone biology and ameliorates osteoporosis development. Platycodin D repressed the osteoclast activity and enhanced bone mineral density in the mouse model. However, the effects of platycodin D on osteoblast differentiation have not been elucidated yet. In C3H10T1/2 cells, platycodin D upregulated osteogenic markers including alkaline phosphatase (ALP), bone sialoprotein, and collagen type 1 alpha 1, and transcription factors, such as Runx2 and osterix, subsequently enhancing the bone mineralization. In a molecular mechanism study, platycodin D induced β-catenin nuclear accumulation by upregulating GSK3β phosphorylation. Furthermore, platycodin D upregulated the ALP activity and enhanced the mineralization process in osteoblast cells via the sirtuin 1/β-catenin pathways. Taken together, these results suggested that platycodin D could be an effective therapeutic compound against osteoporosis because of its regulatory effects during the osteoblast differentiation.

Effects of Silicon Compounds on Biomineralization, Osteogenesis, and Hard Tissue Formation

Bioinspired stem cell-based hard tissue engineering includes numerous aspects: The synthesis and fabrication of appropriate scaffold materials, their analytical characterization, and guided osteogenesis using the sustained release of osteoinducing and/or osteoconducting drugs for mesenchymal stem cell differentiation, growth, and proliferation. Here, the effect of silicon- and silicate-containing materials on osteogenesis at the molecular level has been a particular focus within the last decade. This review summarizes recently published scientific results, including material developments and analysis, with a special focus on silicon hybrid bone composites. First, the sources, bioavailability, and functions of silicon on various tissues are discussed. The second focus is on the effects of calcium-silicate biomineralization and corresponding analytical methods in investigating osteogenesis and bone formation. Finally, recent developments in the manufacturing of Si-containing scaffolds are discussed, including in vitro and in vivo studies, as well as recently filed patents that focus on the influence of silicon on hard tissue formation.

A Novel Small Molecule Which Increases Osteoprotegerin Expression and Protects Against Ovariectomy-Related Bone Loss in Rats

The ratio of osteoprotegerin (OPG) to the receptor activator of NF-κB ligand (RANKL) is a key determinant in the regulation of bone metabolism. The study was performed to screen novel anti-osteoporotic drugs regulating OPG/RANKL ratio and evaluate their effect on bone metabolism. According to the screening results and in vitro results, we found a small molecule, E09241, significantly increased the ratio of OPG/RANKL by mainly increasing OPG expression. Our in vitro studies showed that E09241 increased the alkaline phosphatase (ALP) activity of mouse osteoblasts, promoted mineralization, and increased the expression of osteogenic differentiation-related genes. In addition, we observed that E09241 inhibited RANKL-induced osteoclast differentiation and reduced the expression of osteoclast differentiation-related proteins nuclear factor of activated T cells c1 (NFATc1) and matrix metalloproteinase 9 (MMP-9). More importantly, E09241 exerted therapeutic protection against bone loss in ovariectomized rats in vivo. This protective effect was confirmed to be achieved by inhibiting bone resorption and promoting bone formation in vivo. Mechanistically, E09241 regulates OPG expression through canonical Wnt/β-catenin signaling. Our findings suggest that E09241 is a promising small-molecule compound for treating osteoporosis with a dual effect on osteoblasts and osteoclasts.

MACF1 Overexpression by Transfecting the 21 kbp Large Plasmid PEGFP-C1A-ACF7 Promotes Osteoblast Differentiation and Bone Formation

Microtubule actin crosslinking factor 1 (MACF1) is a large spectraplakin protein known to have crucial roles in regulating cytoskeletal dynamics, cell migration, growth, and differentiation. However, its role and action mechanism in bone remain unclear. The present study investigated optimal conditions for effective transfection of the large plasmid PEGFP-C1A-ACF7 (∼21 kbp) containing full-length human MACF1 cDNA, as well as the potential role of MACF1 in bone formation. To enhance MACF1 expression, the plasmid was transfected into osteogenic cells by electroporation in vitro and into mouse calvaria with nanoparticles. Then, transfection efficiency, osteogenic marker expression, calvarial thickness, and bone formation were analyzed. Notably, MACF1 overexpression triggered a drastic increase in osteogenic gene expression, alkaline phosphatase activity, and matrix mineralization in vitro. Mouse calvarial thickness, mineral apposition rate, and osteogenic marker protein expression were significantly enhanced by local transfection. In addition, MACF1 overexpression promoted β-catenin expression and signaling. In conclusion, MACF1 overexpression by transfecting the large plasmid containing full-length MACF1 cDNA promotes osteoblast differentiation and bone formation via β-catenin signaling. Current data will provide useful experimental parameters for the transfection of large plasmids and a novel strategy based on promoting bone formation for prevention and therapy of bone disorders.

microRNA-203 promotes proliferation, differentiation, and migration of osteoblasts by upregulation of Msh homeobox 2

Despite the improvements in fracture healing, about 10% of patients undergo abnormal healing. As a tumor suppressor, upregulation of microRNA (miR)-203 has been observed in osteogenic differentiation. Herein, we aimed to explore the functional role of miR-203 in osteoblasts as well as the underlying mechanisms. The expression of miR-203 in MC3T3-E1 cells that underwent osteogenic differentiation was determined by quantitative reverse transcription PCR (qRT-PCR). The effects of aberrantly expressed miR-203 on cell viability, migration, and expressions of proteins associated with proliferation, migration, and osteogenic differentiation were measured by using a Cell Counting Kit-8 assay, Transwell cell migration assay, and western blot/qRT-PCR, respectively. The possible downstream factor of miR-203 was subsequently studied. Finally, involvements of the mitogen-activated protein kinase (MAPK)/activator of transcription (STAT) pathways were assessed by western blot. We found that the miR-203 level was increased in osteogenic differentiation of MC3T3-E1 cells with increasing duration time (28th day, p < 0.001). After cell transfection, we interestingly found that miR-203 overexpression could increase cell viability (p < 0.05), promote proliferation, migration (p < 0.05), and osteogenic differentiation, and upregulate Msh homeobox 2 (Msx2) expression. Furthermore, Msx2 knockdown was proved to abrogate the effects of miR-203 overexpression on MC3T3-E1 cells. Finally, phosphorylated levels of key kinases in the MAPK/STAT pathways were increased by miR-203 overexpression via upregulating Msx2 expression. In conclusion, miR-203 overexpression promoted proliferation, migration, and osteogenic differentiation of MC3T3-E1 cells through upregulating Msx2 along with activation of the MAPK/STAT pathways.

Lrp4 expression by adipocytes and osteoblasts differentially impacts sclerostin's endocrine effects on body composition and glucose metabolism

Sclerostin exerts profound local control over bone acquisition and also mediates endocrine communication between fat and bone. In bone, sclerostin's anti-osteoanabolic activity is enhanced by low-density lipoprotein receptor-related protein 4 (Lrp4), which facilitates its interaction with the Lrp5 and Lrp6 Wnt co-receptors. To determine whether Lrp4 similarly affects sclerostin's endocrine function, we examined body composition as well as glucose and fatty acid metabolism in mice rendered deficient of Lrp4 in the adipocyte (AdΔLrp4) or the osteoblast (ObΔLrp4). AdΔLrp4 mice exhibit a reduction in adipocyte hypertrophy and improved glucose and lipid homeostasis, marked by increased glucose and insulin tolerance and reduced serum fatty acids, and mirror the effect of sclerostin deficiency on whole-body metabolism. Indeed, epistasis studies place adipocyte-expressed Lrp4 and sclerostin in the same genetic cascade that regulates adipocyte function. Intriguingly, ObΔLrp4 mice, which exhibit dramatic increases in serum sclerostin, accumulate body fat and develop impairments in glucose tolerance and insulin sensitivity despite development of a high bone mass phenotype. These data indicate that expression of Lrp4 by both the adipocyte and osteoblast is required for normal sclerostin endocrine function and that the impact of sclerostin deficiency on adipocyte physiology is distinct from the effect on osteoblast function.

Xenograft of Human Umbilical Mesenchymal Stem Cells from Wharton's Jelly Differentiating into Osteocytes and Reducing Osteoclast Activity Reverses Osteoporosis in Ovariectomized Rats

We examined the effects of human umbilical cord-derived mesenchymal stem cells (HUMSCs) in Wharton's jelly on ovariectomy (OVX)-induced osteoporosis by using in vitro and in vivo experiments. Two months after OVX, the rats gained weight and had a decreased serum estradiol level . Both micro-computed tomography (micro-CT) and histochemical analyses revealed a marked decrease in the bone volume (BV) and collagen content within the head, neck, and distal condyle of the femur, indicating that the osteoporosis animal model was successfully established 2 mo after bilateral OVX. Subsequently, 2.5 × 10⁶ HUMSCs were injected into the bone marrow cavity of the left femurs 2 mo after OVX. The rats were divided into the following groups: normal + phosphate-buffered saline (PBS), normal + HUMSCs, OVX + PBS, and OVX + HUMSCs. Two months after transplantation, both micro-CT imaging and histochemical staining revealed that the normal + HUMSCs group had higher BV and collagen content in the epiphysis and metaphysis than did the normal + PBS group. In the OVX + HUMSCs group, a substantial increase in the rod-shaped trabecular bone and the abundant accumulation of collagen were observed around the site of HUMSC transplantation. Plenty of transplanted HUMSCs remained viable and differentiated into osteoblasts. In addition, HUMSC transplantation reduced the number of osteoclasts. Compared with HUMSCs cultured alone, HUMSCs cocultured with osteoblasts showed that the percentage of cells differentiating into osteoblasts significantly increased. Furthermore, osteoclasts cocultured with HUMSCs had significantly decreased cellular activity and differentiation capability. HUMSC transplantation into the distal femur of OVX rats could locally stimulate osteocalcin synthesis, increase the trabecular bone, and inhibit osteoclast activity.

Cytological mechanism of astragaloside IV in promoting repair of bone defects

To explore the possible cytological mechanism underlying the role of Astragaloside IV in promoting the repair of bone defects, osteoblasts were cultured in vitro and identified using inverted phase contrast microscopy, alkaline phosphatase (ALP) staining and alizarin red staining.

Lysophosphatidic acid enhanced the osteogenic and angiogenic capability of osteoblasts via LPA1/3 receptor

Lysophosphatidic acid is a serum-derived growth factor that is involved in wound healing. Although in its infancy, a growing body of evidence has demonstrated that lysophosphatidic acid exerts a potentially significant role in regulating bone cell biology. However, previous studies mainly focused on the osteoinductive potential of lysophosphatidic acid, its effects on bone tissue vascularization, another essential element during bone regeneration, remains ill-defined so far. Here in this study, we examined the effects of lysophosphatidic acid on osteogenic differentiation as well as the angiogenesis-inducing capacity of pre-osteoblasts, a cell population that coordinates osteogenic and angiogenic processes in bone regenerating niche. Our results showed that treatment of MC3T3-E1 pre-osteoblastic cells with lysophosphatidic acid enhanced alkaline phosphatase activity and matrix mineralization, demonstrating in vitro osteoblastic differentiation. Of particular importance was the finding that vascular endothelial growth factor secretion also increased after lysophosphatidic acid treatment. Lysophosphatidic acid conditioned media of MC3T3-E1 cells was capable of promoting angiogenic behavior of endothelial cells, as evidenced by stimulating proliferation, migration, and tube formation. Besides, inhibition of LPA1/3 receptor abolished lysophosphatidic acid-induced elevation of the osteogenic and angiogenic capability of pre-osteoblasts. Our research demonstrated the important role of lysophosphatidic acid in coupling osteogenesis and angiogenesis during bone remodeling through orchestrating pre-osteoblast behavior, and implications therein for novel and effective treatment strategies for bone regeneration success.

Pulsed electromagnetic fields promote bone formation by activating the sAC-cAMP-PKA-CREB signaling pathway

The application of pulsed electromagnetic fields (PEMFs) in the prevention and treatment of osteoporosis has long been an area of interest. However, the clinical application of PEMFs remains limited because of the poor understanding of the PEMF action mechanism. Here, we report that PEMFs promote bone formation by activating soluble adenylyl cyclase (sAC), cyclic adenosine monophosphate (cAMP), protein kinase A (PKA), and cAMP response element-binding protein (CREB) signaling pathways. First, it was found that 50 Hz 0.6 millitesla (mT) PEMFs promoted osteogenic differentiation of rat calvarial osteoblasts (ROBs), and that PEMFs activated cAMP-PKA-CREB signaling by increasing intracellular cAMP levels, facilitating phosphorylation of PKA and CREB, and inducing nuclear translocation of phosphorylated (p)-CREB. Blocking the signaling by adenylate cyclase (AC) and PKA inhibitors both abolished the osteogenic effect of PEMFs. Second, expression of sAC isoform was found to be increased significantly by PEMF treatment. Blocking sAC using sAC-specific inhibitor KH7 dramatically inhibited the osteogenic differentiation of ROBs. Finally, the peak bone mass of growing rats was significantly increased after 2 months of PEMF treatment with 90 min/day. The serum cAMP content, p-PKA, and p-CREB as well as the sAC protein expression levels were all increased significantly in femurs of treated rats. The current study indicated that PEMFs promote bone formation in vitro and in vivo by activating sAC-cAMP-PKA-CREB signaling pathway of osteoblasts directly or indirectly.

Effects of dietary fatty acid content on humeral cartilage and bone structure in a mouse model of diet-induced obesity

Obesity is a primary risk factor for osteoarthritis (OA), and previous studies have shown that dietary content may play an important role in the pathogenesis of cartilage and bone in knee OA. Several previous studies have shown that the ratio of ω-3 polyunsaturated fatty acids (PUFAs), ω-6 PUFAs, and saturated fatty acids can significantly influence bone structure and OA progression. However, the influence of obesity or dietary fatty acid content on shoulder OA is not well understood. The goal of this study was to investigate the role of dietary fatty acid content on bone and cartilage structure in the mouse shoulder in a model of diet-induced obesity. For 24 weeks, mice were fed control or high-fat diets supplemented with ω-3 PUFAs, ω-6 PUFAs, or saturated fatty acids. The humeral heads were analyzed for bone morphometry and mineral density by microCT. Cartilage structure and joint synovitis were determined by histological grading, and microscale mechanical properties of the cartilage extracellular and pericellular matrices were quantified using atomic force microscopy. Diet-induced obesity significantly altered bone morphology and mineral density in a manner that was dependent on dietary free fatty acid content. In general, high-fat diet groups showed decreased bone quality, with the ω-3 diet being partially protective. Cartilage mechanical properties and OA scores showed no changes with obesity or diet. These findings are consistent with clinical literature showing little if any relationship between obesity and shoulder OA (unlike knee OA), but suggest that diet-induced obesity may influence other joint tissues. © 2019 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res.

Melatonin: Another avenue for treating osteoporosis?

Melatonin is a signal molecule that modulates the biological circadian rhythms of vertebrates. Melatonin deficiency is thought to be associated with several disorders, including insomnia, cancer, and cardiovascular and neurodegenerative diseases. Accumulating evidence has also indicated that melatonin may be involved in the homeostasis of bone metabolism. Age-related reductions in melatonin are considered to be critical factors in bone loss and osteoporosis with aging. Thus, serum melatonin levels might serve as a biomarker for the early detection and prevention of osteoporosis. Compared to conventional antiosteoporosis medicines, which primarily inhibit bone loss, melatonin both suppresses bone loss and promotes new bone formation. Mechanistically, by activating melatonin receptor 2 (MT2), melatonin upregulates the gene expression of alkaline phosphatase (ALP), bone morphogenetic protein 2 (BMP2), BMP6, osteocalcin, and osteoprotegerin to promote osteogenesis while inhibiting the receptor activator of NF-kB ligand (RANKL) pathway to suppress osteolysis. In view of the distinct actions of melatonin on bone metabolism, we hypothesize that melatonin may be a novel remedy for the prevention and clinical treatment of osteoporosis.

Fluid shear stress promotes osteoblast proliferation through the NFATc1-ERK5 pathway

PURPOSE

Extracellular-regulated kinase 5 (ERK5) is thought to regulate osteoblast proliferation. To further understand how ERK5 signaling regulates osteoblast proliferation induced by fluid shear stress (FSS), we examined some potential signaling targets associated with ERK5 in MC3T3-E1 cells.

METHODS

MC3T3-E1 cells were treated with XMD8-92 (an ERK5 inhibitor) or Cyclosporin A (CsA, a nuclear factor of activated T cells (NFAT) c1 inhibitor) and/or exposed to 12 dyn/cm² FSS. Phosphorylated-ERK5 (p-ERK5) and expression levels of NFATc1, ERK5, E2F2, and cyclin E1 were analyzed by western blot. The mRNA levels of genes associated with cell proliferation were analyzed by Polymerase Chain Reaction (PCR) array. Subcellular localization of p-ERK5 and NFATc1 were determined by immunofluorescence. Cell proliferation was evaluated by MTT assay.

RESULTS

NFATc1 expression was up-regulated by FSS. XMD8-92 only blocked ERK5 activation; however, CsA decreased NFATc1 and p-ERK5 levels, including after FSS stimulation. Exposure to NFATc1 inhibitor or ERK5 inhibitor resulted in decreased E2F2 and cyclin E1 expression and proliferation by proliferative MC3T3-E1 cells. Furthermore, immunofluorescence results illustrated that NFATc1 induced ERK5 phosphorylation, resulting in p-ERK5 translocation to the nucleus.

CONCLUSIONS

Our results reveal that NFATc1 acts as an intermediate to promote the phosphorylation of ERK5 induced by FSS. Moreover, activated NFATc1-ERK5 signaling up-regulates the expression of E2F2 and cyclin E1, which promote osteoblast proliferation.

Autophagy plays an essential role in bone homeostasis

Autophagy is very critical for multiple cellular processes. Autophagy plays a critical role in bone cell differentiation and function.

Participation of integrin β3 in osteoblast differentiation induced by titanium with nano or microtopography

The major role of integrins is to mediate cell adhesion but some of them are involved in the osteoblasts-titanium (Ti) interactions. In this study, we investigated the participation of integrins in osteoblast differentiation induced by Ti with nanotopography (Ti-Nano) and with microtopography (Ti-Micro). By using a PCR array, we observed that, compared with Ti-Micro, Ti-Nano upregulated the expression of five integrins in mesenchymal stem cells, including integrin β3, which increases osteoblast differentiation. Silencing integrin β3, using CRISPR-Cas9, in MC3T3-E1 cells significantly reduced the osteoblast differentiation induced by Ti-Nano in contrast to the effect on T-Micro. Concomitantly, integrin β3 silencing downregulated the expression of integrin αv, the parent chain that combines with other integrins and several components of the Wnt/β-catenin and BMP/Smad signaling pathways, all involved in osteoblast differentiation, only in cells cultured on Ti-Nano. Taken together, our results showed the key role of integrin β3 in the osteogenic potential of Ti-Nano but not of Ti-Micro. Additionally, we propose a novel mechanism to explain the higher osteoblast differentiation induced by Ti-Nano that involves an intricate regulatory network triggered by integrin β3 upregulation, which activates the Wnt and BMP signal transductions. © 2019 Wiley Periodicals, Inc. J Biomed Mater Res Part A: 107A: 1303-1313, 2019.

Tankyrase (PARP5) Inhibition Induces Bone Loss through Accumulation of Its Substrate SH3BP2

There is considerable interest in tankyrase because of its potential use in cancer therapy. Tankyrase catalyzes the ADP-ribosylation of a variety of target proteins and regulates various cellular processes. The anti-cancer effects of tankyrase inhibitors are mainly due to their suppression of Wnt signaling and inhibition of telomerase activity, which are mediated by AXIN and TRF1 stabilization, respectively. In this review, we describe the underappreciated effects of another substrate, SH3 domain-binding protein 2 (SH3BP2). Specifically, SH3BP2 is an adaptor protein that regulates intracellular signaling pathways. Additionally, in the human genetic disorder cherubism, the gain-of-function mutations in SH3BP2 enhance osteoclastogenesis. The pharmacological inhibition of tankyrase in mice induces bone loss through the accumulation of SH3BP2 and the subsequent increase in osteoclast formation. These findings reveal the novel functions of tankyrase influencing bone homeostasis, and imply that tankyrase inhibitor treatments in a clinical setting may be associated with adverse effects on bone mass.

Programmed conversion of hypertrophic chondrocytes into osteoblasts and marrow adipocytes within zebrafish bones

Much of the vertebrate skeleton develops from cartilage templates that are progressively remodeled into bone. Lineage tracing studies in mouse suggest that chondrocytes within these templates persist and become osteoblasts, yet the underlying mechanisms of this process and whether chondrocytes can generate other derivatives remain unclear. We find that zebrafish cartilages undergo extensive remodeling and vascularization during juvenile stages to generate fat-filled bones. Growth plate chondrocytes marked by sox10 and col2a1a contribute to osteoblasts, marrow adipocytes, and mesenchymal cells within adult bones. At the edge of the hypertrophic zone, chondrocytes re-enter the cell cycle and express leptin receptor (lepr), suggesting conversion into progenitors. Further, mutation of matrix metalloproteinase 9 (mmp9) results in delayed growth plate remodeling and fewer marrow adipocytes. Our data support Mmp9-dependent growth plate remodeling and conversion of chondrocytes into osteoblasts and marrow adipocytes as conserved features of bony vertebrates.

Our adult bones are made of a fatty tissue, called marrow, wrapped inside a hard outer layer produced by bone cells. They may appear stiff and unyielding, but our bones are actually dynamic structures. Early in life, most bones start as small ‘templates’ made of another, flexible tissue called cartilage. As the templates grow into adult bones, the cartilage is gradually replaced by bone and fat, but this process is still poorly understood. For example, it is not clear whether cartilage cells simply die and make way for new cells, or instead if they turn into bone and fat cells. To investigate this question, Giovannone, Paul et al. set out to follow the fate of early cartilage cells in zebrafish, and to compare this with what happens in mammals. Zebrafish were chosen because their skeleton and ours develop in similar ways; yet, these animals are much easier to study, in particular because their embryos are transparent.

Young cartilage cells were ‘tagged’ with a long-lasting fluorescent protein in genetically engineered zebrafish embryos, and then followed over time. As the embryos started to form bones, the cartilage cells gave rise to bone cells, fat cells, and also potentially adult stem cells within the marrow, which can become other types of cells. This process required a protein called Mmp9, which also helps shape bone development in other organisms, including humans.

Defects in how early cartilage templates morph into bone and fat may contribute to dwarfism and other severe conditions. Fully grasping the molecular mechanisms that preside over this complex transition may one day help design drugs to treat skeletal disorders.

miR-181c-5p mediates simulated microgravity-induced impaired osteoblast proliferation by promoting cell cycle arrested in the G2 phase

Impaired osteoblast proliferation plays fundamental roles in microgravity‐induced bone loss, and cell cycle imbalance may result in abnormal osteoblast proliferation. However, whether microgravity exerts an influence on the cell cycle in osteoblasts or what mechanisms may underlie such an effect remains to be fully elucidated. Herein, we confirmed that simulated microgravity inhibits osteoblast proliferation. Then, we investigated the effect of mechanical unloading on the osteoblast cell cycle and found that simulated microgravity arrested the osteoblast cell cycle in the G₂ phase. In addition, our data showed that cell cycle arrest in osteoblasts from simulated microgravity was mainly because of decreased cyclin B1 expression. Furthermore, miR‐181c‐5p directly inhibited cyclin B1 protein translation by binding to a target site in the 3′UTR. Lastly, we demonstrated that inhibition of miR‐181c‐5p partially counteracted cell cycle arrest and decreased the osteoblast proliferation induced by simulated microgravity. In conclusion, our study demonstrates that simulated microgravity inhibits cell proliferation and induces cell cycle arrest in the G₂ phase in primary mouse osteoblasts partially through the miR‐181c‐5p/cyclin B1 pathway. This work may provide a novel mechanism of microgravity‐induced detrimental effects on osteoblasts and offer a new avenue to further investigate bone loss induced by mechanical unloading.

Zoledronic Acid Modulation of TRPV1 Channel Currents in Osteoblast Cell Line and Native Rat and Mouse Bone Marrow-Derived Osteoblasts: Cell Proliferation and Mineralization Effect

Bisphosphonates (BPs) reduce bone pain and fractures by balancing the osteoblast/osteoclast ratio. The behavior of ion channels in the presence of BPs is not known. To investigate this, the effect of zoledronic acid BP (ZOL) (3 × 10⁻⁸ to 5 × 10⁻⁴ M) treatment, on ion channels, cell proliferation, and mineralization, has been investigated on preosteoclast-like cells, RAW264.7, preosteoblast-like cells MC3T3-E1, and rat/mouse native bone marrow-derived osteoblasts. In whole-cell patch clamp on cell line- and bone marrow-derived osteoblasts, ZOL potentiated outward currents. On RAW264.7, ZOL (10⁻⁴ M)-evoked current was reduced by the Kv channel blocker tetraethylammonium hydrochloride (TEA), but not by the selective TRPV1-channel antagonist capsazepine. On MC3T3-E1 cells and bone marrow-derived osteoblasts, ZOL-evoked current (5 × 10⁻⁸ to 10⁻⁴ M) was reduced by capsazepine, whereas the selective TRPV1-channel agonist capsaicin potentiated the control current. In the cell proliferation assay, 72 h incubation of RAW264.7 and MC3T3-E1 cells with ZOL reduced proliferation, with IC₅₀ values of 2.62 × 10⁻⁷ M and 2.02 × 10⁻⁵ M, respectively. Mineralization of MC3T3-E1 cells and bone marrow-derived osteoblasts was observed in the presence of capsaicin and ZOL (5 × 10⁻⁸–10⁻⁷ M); ZOL effects were antagonized by capsazepine. In summary, the ZOL-induced activation of TRPV1 channel mediates the mineralization of osteoblasts and counterbalances the antiproliferative effects, increasing the IC₅₀. This mechanism is not operative in osteoclasts lacking the TRPV1 channel.