Inhibition of the estrogen receptor alpha signaling delays bone regeneration and alters osteoblast maturation, energy metabolism, and angiogenesis

Using osteogenic medium in the in vitro evaluation of bone biomaterials: Artefacts due to a synergistic effect

In vitro tests using bone cells to evaluate the osteogenic potential of biomaterials usually employ the osteogenic medium (OM). The lack of correlation frequently reported between in vitro and in vivo studies in bone biomaterials, makes necessary the evaluation of the impact of osteogenic supplements on these results. This study analysed the proteomic profiles of human osteoblasts (HOb) cultured in the media with and without osteogenic agents (ascorbic acid and β-glycerol phosphate). The cells were incubated for 1 and 7 days, on their own or in contact with Ti. The comparative Perseus analysis identified 2544 proteins whose expression was affected by osteogenic agents. We observed that the OM strongly alters protein expression profiles with a complex impact on multiple pathways associated with adhesion, immunity, oxidative stress, coagulation, angiogenesis and osteogenesis. OM-triggered changes in the HOb intracellular energy production mechanisms, with key roles in osteoblast maturation. HOb cultured with and without Ti showed enrichment in the skeletal system development function due to the OM. However, differentially expressed proteins with key regenerative functions were associated with a synergistic effect of OM and Ti. This synergy, caused by the Ti-OM interaction, could complicate the interpretation of in vitro results, highlighting the need to analyse this phenomenon in biomaterial testing.

Deletion of osteopontin or bone sialoprotein induces opposite bone responses to mechanical stimulation in mice

Osteopontin (OPN) and Bone Sialoprotein (BSP) are co-expressed in bone and display overlapping and complementary physiological properties. Both genes show a rapid expression response to mechanical stimulation. We used mice with single and double deletions (DKO) of BSP and OPN to assess the specificity of their roles in skeletal adaptation to loading. Two-month-old Wild-Type (WT), BSP knockout (BSP^−/−), OPN^−/− and DKO male mice were submitted to two mechanical stimulation regimen (n = 10 mice/group) respectively impacting trabecular bone (Hypergravity, HG) and cortical bone (Whole Body Vibration, WBV). HG increased trabecular bone volume (BV/TV) in WT femur through reduced resorption, and in BSP^−/− mice femur and vertebra through increased bone formation. In contrast, HG increased the turnover of OPN^−/− bone, resulting in reduced femur and vertebra BV/TV. HG did not affect DKO bones. Similarly, WBV increased cortical thickness in BSP^−/− mice and decreased it in OPN^−/−, without affecting structurally WT and DKO bone. Vibrated BSP^−/− mice displayed increased endocortical bone formation with a drop in Sclerostin expression, and reduced periosteal osteoclasts with lower Rankl and Cathepsin K expression. In contrast, vibrated OPN^−/− endocortical bone displayed decreased formation and increased osteoclast coverage. Therefore, under two regimen (HG and WBV) targeting distinct bone compartments, absence of OPN resulted in bone loss while lack of BSP induced bone gain, reflecting divergent structural adaptations. Strikingly, absence of both proteins led to a relative insensitivity to either mechanical challenge. Interplay between OPN and BSP thus appears as a key element of skeletal response to mechanical stimulation.

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Osteopontin gene knockout induces bone loss under mechanical stimulation

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Bone Sialoprotein gene knockout potentiates bone gain under mechanical stimulation

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Knockout of both genes leads to bone insensitivity

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Their interplay is crucial for bone response to mechanical challenges

Progranulin, a moderator of estrogen/estrogen receptor α binding, regulates bone homeostasis through PERK/p-eIF2 signaling pathway

Under normal conditions, the human body employs the synergistic action of osteoblasts and osteoclasts to maintain a dynamic balance between bone formation and resorption. Bone homeostasis plays a very important role in the process of bone formation. Various bone diseases can occur if bone homeostasis is disrupted. In this study, the serum estrogen levels were significantly increased in the granulin (GRN)-deficient mice and PGRN regulates the binding of estrogen and estrogen receptor α (ERα) and then affects estrogen's ability to regulate bone formation and resorption. In addition, this study also explored the role that PGRN plays in regulating bone homeostasis by affecting the binding of estrogen and estrogen receptors through the protein kinase R-like endoplasmic reticulum kinase/phosphorylation of the eukaryotic initiation factor 2 signaling pathway. In summary, we confirmed the important role of PGRN in regulating the estrogen (E2)/ERα signal in maintaining bone homeostasis. Our findings may provide a new strategy for the treatment of osteoporosis and maintaining bone homeostasis. KEY MESSAGES: PGRN is a molecular regulator of the binding of E2 and ERα signal in maintaining bone homeostasis. PGRN plays in regulating bone homeostasis through the PERK/p-eIF2α signaling pathway. The best therapeutic effect of PGRN in osteoporosis is associated with different concentration of E2.

The Interaction Between Intracellular Energy Metabolism and Signaling Pathways During Osteogenesis

Osteoblasts primarily mediate bone formation, maintain bone structure, and regulate bone mineralization, which plays an important role in bone remodeling. In the past decades, the roles of cytokines, signaling proteins, and transcription factors in osteoblasts have been widely studied. However, whether the energy metabolism of cells can be regulated by these factors to affect the differentiation and functioning of osteoblasts has not been explored in depth. In addition, the signaling and energy metabolism pathways are not independent but closely connected. Although energy metabolism is mediated by signaling pathways, some intermediates of energy metabolism can participate in protein post-translational modification. The content of intermediates, such as acetyl coenzyme A (acetyl CoA) and uridine diphosphate N-acetylglucosamine (UDP-N-acetylglucosamine), determines the degree of acetylation and glycosylation in terms of the availability of energy-producing substrates. The utilization of intracellular metabolic resources and cell survival, proliferation, and differentiation are all related to the integration of metabolic and signaling pathways. In this paper, the interaction between the energy metabolism pathway and osteogenic signaling pathway in osteoblasts and bone marrow mesenchymal stem cells (BMSCs) will be discussed.

Naringin Improves Osteoblast Mineralization and Bone Healing and Strength through Regulating Estrogen Receptor Alpha-Dependent Alkaline Phosphatase Gene Expression

Phytoestrogens are strongly recommended for treating osteoporosis. Our previous study showed that naringin, a citrus flavonoid, can enhance the bone mass in ovariectomized rats. In this study, we further elucidated the mechanisms of naringin-induced osteoblast maturation and bone healing. Treatment of human osteoblasts with naringin increased cell viability and proliferation. In parallel, exposure to naringin enhanced translocation of estrogen receptor alpha (ERα) to nuclei and its transactivation activity. Sequentially, naringin induced alkaline phosphatase (ALP) mRNA and protein expression and its enzyme activity. Pretreatment with methylpiperidinopyrazole (MPP), a specific inhibitor of ERα, attenuated naringin-induced augmentations in ERα transactivation activity, ALP gene expression, and cell mineralization. The beneficial effects of naringin were also confirmed in mouse MC3T3-E1 cells. Moreover, administration of mice with a bone defect with naringin increased levels of ERα and ALP in damaged sites and simultaneously enhanced the healing rate and bone strength. Nevertheless, treatment with MPP weakened naringin-triggered expression of ERα and ALP and improved bone healing and mass. Therefore, naringin could improve osteoblast mineralization and bone healing via regulating ERα-dependent ALP gene expression. Naringin can be clinically applied for treatment of osteoporosis-related bone diseases.

Induction of Bone Remodeling by Raloxifene-Doped Iron Oxide Functionalized with Hydroxyapatite to Accelerate Fracture Healing

Repairing fractures in the presence of infection is a major challenge that is currently declining using nanotechnology. By producing iron oxide nanoparticles (NPs) containing hydroxyapatite and Raloxifene (R-IONPs-HA), this study tries to target drug delivery, control infection and promotion of the cells proliferation/differentiation to repair damaged tissue. After the production of R-IONPs-HA through co-precipitation, the physicochemical features of the NPs were considered by SEM, TEM, DLS and XRD methods, and the possibility of drug release. The effect of R-IONPs-HA on MC3T3-E1 cell proliferation/differentiation was determined by CCK-assay and microscopic observations. Also, Gram-negative and -positive bacteria were applied to evaluate the antibacterial activity. Finally, cell differentiation biomarkers like an ALP, OCN, and RUNX-2 genes were examined by real time (RT)-PCR. The results showed that R-IONPs-HA was spherical with dimensions of 98.1 ± 1.17 nm. In addition, the results of Zeta and XRD confirmed the loading HA and R on IONPs. Also, the release rate of R and HA in 64 h with pH 6 reached 61.4 and 30.4%, respectively. The anti-bacterial activity of R-IONPs-HA on Staphylococcus aureus, Escherichia coli, and Pseudomonas aeruginosa bacteria showed a significant reduction in infection. Also, MC3T3-E1 cells showed greater proliferation and differentiation by R-IONPs-HA compared to other groups. Increased expression of ossification genes such as OCN, and RUNX-2 confirmed this claim. Finally, R-IONPs-HA with good biocompatibility, antibacterial activity and ossification induction has great potential to repair bone fractures and prevent infection.

A Review of Recent Developments in the Molecular Mechanisms of Bone Healing

Between 5 and 10 percent of fractures do not heal, a condition known as nonunion. In clinical practice, stable fracture fixation associated with autologous iliac crest bone graft placement is the gold standard for treatment. However, some recalcitrant nonunions do not resolve satisfactorily with this technique. For these cases, biological alternatives are sought based on the molecular mechanisms of bone healing, whose most recent findings are reviewed in this article. The pro-osteogenic efficacy of morin (a pale yellow crystalline flavonoid pigment found in old fustic and osage orange trees) has recently been reported, and the combined use of bone morphogenetic protein-9 (BMP9) and leptin might improve fracture healing. Inhibition with methyl-piperidino-pyrazole of estrogen receptor alpha signaling delays bone regeneration. Smoking causes a chondrogenic disorder, aberrant activity of the skeleton’s stem and progenitor cells, and an intense initial inflammatory response. Smoking cessation 4 weeks before surgery is therefore highly recommended. The delay in fracture consolidation in diabetic animals is related to BMP6 deficiency (35 kDa). The combination of bioceramics and expanded autologous human mesenchymal stem cells from bone marrow is a new and encouraging alternative for treating recalcitrant nonunions.