Bone Homeostasis Research Articles

Exercise and osteoimmunology in bone remodeling.

Bones can form the scaffolding of the body, support the organism, coordinate somatic movements, and control mineral homeostasis and hematopoiesis. The immune system plays immune supervisory, defensive, and regulatory roles in the organism, which mainly consists of immune organs (spleen, bone marrow, tonsils, lymph nodes, etc.), immune cells (granulocytes, platelets, lymphocytes, etc.), and immune molecules (immune factors, interferons, interleukins, tumor necrosis factors, etc.). Bone and the immune system have long been considered two distinct fields of study, and the bone marrow, as a shared microenvironment between the bone and the immune system, closely links the two. Osteoimmunology organically combines bone and the immune system, elucidates the role of the immune system in bone, and creatively emphasizes its interdisciplinary characteristics and the function of immune cells and factors in maintaining bone homeostasis, providing new perspectives for skeletal-related field research. In recent years, bone immunology has gradually become a hot spot in the study of bone-related diseases. As a new branch of immunology, bone immunology emphasizes that the immune system can directly or indirectly affect bones through the RANKL/RANK/OPG signaling pathway, IL family, TNF-α, TGF-β, and IFN-γ. These effects are of great significance for understanding inflammatory bone loss caused by various autoimmune or infectious diseases. In addition, as an external environment that plays an important role in immunity and bone, this study pays attention to the role of exercise-mediated bone immunity in bone reconstruction.

Skeletal stem cells in bone development, homeostasis and disease.

Tissue-resident stem cells are essential for development and repair, and in the skeleton this function is fulfilled by recently identified skeletal stem cells (SSCs). However, recent work has identified that SSCs are not monolithic, with long bones, craniofacial sites, and the spine being formed by distinct stem cells. Recent studies have utilized techniques such as fluorescence-activated cell sorting (FACS), lineage tracing and single-cell sequencing to investigate the involvement of SSCs in bone development, homeostasis and disease. These investigations have allowed researchers to map the lineage commitment trajectory of SSCs in different parts of the body and at different time points. Furthermore, recent studies have shed light on the characteristics of SSCs in both physiological and pathological conditions. This review focuses on discussing the spatiotemporal distribution of SSCs and enhancing our understanding of the diversity and plasticity of SSCs by summarizing recent discoveries.

Smart osteoclasts targeted nanomedicine based on amorphous CaCO3 for effective osteoporosis reversal

BackgroundOsteoporosis is characterized by an imbalance in bone homeostasis, resulting in the excessive dissolution of bone minerals due to the acidified microenvironment mediated by overactive osteoclasts. Oroxylin A (ORO), a natural flavonoid, has shown potential in reversing osteoporosis by inhibiting osteoclast-mediated bone resorption. The limited water solubility and lack of targeting specificity hinder the effective accumulation of Oroxylin A within the pathological environment of osteoporosis.ResultsOsteoclasts’ microenvironment-responsive nanoparticles are prepared by incorporating Oroxylin A with amorphous calcium carbonate (ACC) and coated with glutamic acid hexapeptide-modified phospholipids, aiming at reinforcing the drug delivery efficiency as well as therapeutic effect. The obtained smart nanoparticles, coined as OAPLG, could instantly neutralize acid and release Oroxylin A in the extracellular microenvironment of osteoclasts. The combination of Oroxylin A and ACC synergistically inhibits osteoclast formation and activity, leading to a significant reversal of systemic bone loss in the ovariectomized mice model.ConclusionThe work highlights an intelligent nanoplatform based on ACC for spatiotemporally controlled release of lipophilic drugs, and illustrates prominent therapeutic promise against osteoporosis.

Targeting bone homeostasis regulation: potential of traditional Chinese medicine flavonoids in the treatment of osteoporosis.

Osteoporosis is a systemic metabolic disease characterized by disrupted bone formation/resorption and homeostasis. Flavonoids extracted from traditional Chinese medicinal plants regulate bone homeostasis by intervening in differentiating bone marrow mesenchymal stem cells, balancing the bone immune system, inhibiting oxidative stress response, and reversing iron overload. The target molecules and signaling pathways, such as Wnt/β-catenin and OPG/RANKL/RANK, directly affect osteoblast/osteoclast activity, exhibiting significant potential in the treatment of OP. Therefore, this study presents a systematic review of the recent literature to provide comprehensive information on the traditional Chinese medicine flavonoids involved in the regulation of bone homeostasis. Also, the molecular mechanisms and pharmacological uses of these metabolites are summarized, and their clinical translation and development potential are discussed.

Functionalized microporous sulfonated polyetheretherketone: Osteogenesis and anti-osteoclastogenesis effects via ROS and NLRP3-mediated pyroptosis in macrophages

The treatment of osteoporotic implant failures tends to be challenging due to an imbalance in bone homeostasis. Recently, the accumulation of intracellular reactive oxygen species (ROS) and activation of the inflammasome containing nucleotide-binding oligomerization domain-like-receptor family pyrin domain-containing 3 (NLRP3) protein have been shown to indirectly affect osteogenesis and osteoclastogenesis. Owing to the release of multi-metal ions from laponite (LAP), we fabricated a multifunctional LAP coating on a three-dimensional microporous sulfonated polyetheretherketone (SPEEK) surface via polydopamine (PDA) modification (SPEEK@PDA-LAP). The balance between osteogenesis and osteoclastogenesis was examined using macrophages in an osteo-immune microenvironment. Employing in vitro assays, we showed that the SPEEK@PDA-LAP alleviated intracellular ROS accumulation and inhibited the activation of NLRP3 inflammasome-mediated pyroptosis in macrophages, creating an osteoimmunomodulatory microenvironment to stimulate osteogenesis and inhibit osteoclastogenesis. The micro-computed tomography and histological results of in vivo experiments indicated that SPEEK@PDA-LAP implants reversed the balance between osteogenesis and osteoclastogenesis to promote the formation of new bone around the implant even under osteoporotic conditions. The results indicate that inhibiting the NLRP3 inflammasome and regulating intracellular ROS in macrophages can be an effective therapy for osteoporosis. These results highlight the potential of functional PEEK materials for bone regeneration in patients with osteoporosis.

Improvement of Bone Homeostasis Imbalance in Osteoporotic Fractures by Mesoporous Silica Carrying miR-302b

miR-302b and DKK1 are two molecules related to the regulation of bone metabolism. Mesoporous silica is a potential drug carrier. This article aims to study the mechanism of mesoporous silica carrying miR-302b targeting DKK1 regulation to improve bone homeostasis imbalance in osteoporotic fractures. Mesoporous silica nanoparticles were synthesized and characterized. miR-302b was loaded into mesoporous silica to form composite nanoparticles. In vivo rat model experiments were performed to evaluate bone metabolism. X-ray examination and μCT scan were used to detect the bone content and trabecular bone status of rats. Alcian blue/hematoxylin/Orange G staining was used to observe changes in trabecular bone in the tibial metaphysis. Immunohistochemical staining showed the formation of trabecular bone in rats in each group and changes in the number of bone cells. Calcein double labeling experiment showed the bone mineralization speed of mice in each group. Pure and stable mesoporous silica nanoparticles were successfully synthesized and miR-302b was successfully loaded into the nanoparticles. The osteoporotic fracture rat model was successfully created. In vivo experimental results showed that after injecting composite nanoparticles into mice, bone density and bone strength were significantly increased and osteoporotic fractures were improved. Mesoporous silica carries miR-302b to target DKK1 regulation, which can improve bone homeostasis imbalance in osteoporotic fractures. Composite nanoparticles can inhibit the expression of DKK1, promote bone formation, and inhibit bone resorption, thereby improving bone density and bone strength.

Modulation of the microRNA-6089/E2F transcription factor2 axis by querceting: implications for osteoblast viability, proliferation, migration, and osteogenic differentiation in fracture healing.

Quercetin is widely distributed in plants as a flavonol compound with multiple biological activities. It has been found that quercetin can regulate bone homeostasis through multiple pathways and targets. This study investigated the role and specific molecular mechanisms of quercetin in regulating osteoblast viability, proliferation, migration and osteogenic differentiation. A mouse model of traumatic fracture was established and then 100 mg/kg quercetin corn oil suspension was gavaged at the same time every day for 28 days. miR-6089 and E2F transcription factor 2 (E2F2) expression levels in mice were measured. Fracture healing in mice was observed. MC3T3-E1 cells were transfected with plasmids targeting miR-6089 and E2F2, and cell viability, proliferation, migration, apoptosis, and osteogenic differentiation were determined. The targeting relationship between miR-6089 and E2F2 was verified. In vivo experiments showed that quercetin significantly increased osteocalcin (OCN) expression (P<0.05) and promoted fracture healing in traumatic fracture (TF) mice. miR-6089 expression was down-regulated (P<0.05) and E2F2 expression was up-regulated (P<0.05) in TF mice. Quercetin promoted miR-6089 expression and inhibited E2F2 expression (both P<0.05). In vitro results showed that quercetin promoted miR-6089 expression and inhibited E2F2 expression in a dose-dependent manner (both P<0.05). Quercetin dose-dependently promoted MC3T3-E1 cell viability, proliferation, migration, and osteogenic differentiation, and inhibited MC3T3-E1 cell apoptosis (all P<0.05). Up-regulating miR-6089 further promoted MC3T3-E1 cell viability, proliferation, migration and osteogenic differentiation, and inhibited MC3T3-E1 cell apoptosis (all P<0.05). miR-6089 targeted and regulated E2F2 expression. Up-regulating E2F2 attenuated the promoting effect of up-regulated miR-6089 on MC3T3-E1 cell viability, proliferation, migration, osteogenic differentiation, and inhibition of apoptosis (all P<0.05). We conclude that quercetin enhances osteoblast viability, proliferation, migration, and osteogenic differentiation by modulating the miR-6089/E2F2 axis, thereby promoting fracture healing.

Gut microbially produced tryptophan metabolite melatonin ameliorates osteoporosis via modulating SCFA and TMAO metabolism.

Osteoporosis (OP)is a severe global health issue that has significant implications for productivity and human lifespan. Gut microbiota dysbiosis has been demonstrated to be closely associated with OP progression. Melatonin (MLT)is an important endogenous hormone that modulates bone metabolism, maintains bone homeostasis, and improves OP progression. Multiple studies indicated that MLT participates in the regulation of intestinal microbiota and gut barrier function. However, the promising effects of gut microbiota-derived MLT in OP remain unclear. Here, we found that OP resulted in intestinal tryptophan disorder and decreased the production of gut microbiota-derived MLT, while administration with MLT could mitigate OP-related clinical symptoms and reverse gut microbiota dysbiosis, including the diversity of intestinal microbiota, the relative abundance of many probiotics such as Allobaculum and Parasutterella, and metabolic function of intestinal flora such as amino acid metabolism, nucleotide metabolism, and energy metabolism. Notably, MLT significantly increased the production of short-chain fatty acids and decreased trimethylamine N-oxide-related metabolites. Importantly, MLT could modulate the dynamic balance of M1/M2 macrophages, reduce the serum levels of pro-inflammatory cytokines, and restore gut-barrier function. Taken together, our results highlighted the important roles of gut microbially derived MLT in OP progression via the "gut-bone" axis associated with SCFA metabolism, which may provide novel insight into the development of MLT as a promising drug for treating OP.

The Effect of Clopidogrel Treatment on Osseointegration of Titanium Implants: A Histomorphometric Study in Rabbits.

To evaluate the effect of continuous perioperative clopidogrel treatment on the osseointegration of titanium implants. A total of 32 New Zealand rabbits were randomly divided between two groups: a clopidogrel group (n = 16) and a control group (n = 16). For 1 week prior to the surgical placement of a titanium implant in their medial femoral condyle, rabbits in the clopidogrel group received 3 mg/kg of clopidogrel daily, and the control group received only the vehicle. This treatment was continued for another 6 weeks postoperatively. At 6 weeks, the rabbits were euthanized and postmortem histologic and histomorphometric evaluation of the implants was performed. The surgical procedures and postoperative period were uneventful and well tolerated by all animals without any surgical wound dehiscence, signs of infection, or other complication. No implant failure was observed in any of the groups. Histomorphometric analysis showed that bone-to-implant contact (BIC) was 48.77% for the clopidogrel group and 34.65% for the control group, with statistically significant difference between them (P < .001). Moreover, clopidogrel group had significantly greater bone tissue density (40.52% vs 28.74%, respectively; P <.001) and mean trabecular thickness (284.7 μm vs 180.7 μm, respectively; P < .001) in proximity to the implant surface than the control group, while the mean trabecular number had no difference between groups (1.56 vs 1.60, respectively; P = .961). The present study showed that continuous clopidogrel treatment does not negatively affect osseointegration, but rather promotes it in terms of BIC and bone density around the titanium implants. Further studies on the effect of the P2Y12 receptor and its antagonists on peri-implant bone homeostasis may provide useful information or applications for long-term success of dental implant therapy.

Peripheral nerve-derived Sema3A promotes osteogenic differentiation of mesenchymal stem cells through the Wnt/β-catenin/Nrp1 positive feedback loop.

Sensory nerves play a crucial role in maintaining bone homeostasis by releasing Semaphorin 3A (Sema3A). However, the specific mechanism of Sema3A in regulation of bone marrow mesenchymal stem cells (BMMSCs) during bone remodelling remains unclear. The tibial denervation model was used and the denervated tibia exhibited significantly lower mass as compared to sham operated bones. Invitro, BMMSCs cocultured with dorsal root ganglion cells (DRGs) or stimulated by Sema3A could promote osteogenic differentiation through the Wnt/β-catenin/Nrp1 positive feedback loop, and the enhancement of osteogenic activity could be inhibited by SM345431 (Sema3A-specific inhibitor). In addition, Sema3A-stimulated BMMSCs or intravenous injection of Sema3A could promote new bone formation invivo. To sum up, the coregulation of bone remodelling is due to the ageing of BMMSCs and increased osteoclast activity. Furthermore, the sensory neurotransmitter Sema3A promotes osteogenic differentiation of BMMSCs via Wnt/β-catenin/Nrp1 positive feedback loop, thus promoting osteogenesis invivo and invitro.

Mechanical protein polycystin-1 directly regulates osteoclastogenesis and bone resorption

Mechanical loading is required for bone homeostasis, but the underlying mechanism is still unclear. Our previous studies revealed that the mechanical protein polycystin-1 (PC1, encoded by Pkd1) is critical for bone formation. However, the role of PC1 in bone resorption is unknown. Here, we found that PC1 directly regulates osteoclastogenesis and bone resorption. The conditional deletion of Pkd1 in the osteoclast lineage resulted in a reduced number of osteoclasts, decreased bone resorption, and increased bone mass. A cohort study of 32,500 patients further revealed that autosomal dominant polycystic kidney disease, which is mainly caused by loss-of- function mutation of the PKD1 gene, is associated with a lower risk of hip fracture than those with other chronic kidney diseases. Moreover, mice with osteoclast-specific knockout of Pkd1 showed complete resistance to unloading-induced bone loss. A mechanistic study revealed that PC1 facilitated TAZ nuclear translocation via the C-terminal tail-TAZ complex and that conditional deletion of Taz in the osteoclast lineage resulted in reduced osteoclastogenesis and increased bone mass. Pharmacological regulation of the PC1-TAZ axis alleviated unloading- and estrogen deficiency- induced bone loss. Thus, the PC1-TAZ axis may be a potential therapeutic target for osteoclast-related osteoporosis.

Biological Activities of Deer Antler-Derived Peptides on Human Chondrocyte and Bone Metabolism.

Orally administered "tortoiseshell and deer antler gelatin" is a common traditional medicine for patients with osteoporosis or osteoarthritis. From the pepsin-digested gelatin, we previously isolated and identified the osteoblast-stimulating pentapeptide, TSKYR. Its trypsin digestion products include the dipeptide YR, enhancing calcium ion uptake, and tripeptide TSK, resulting in remarkable 30- and 50-fold increases in mineralized nodule area and density in human osteoblast cells. These peptides were chemically synthesized in this study. The composition of deer antler preparations comprises not only proteins and peptides but also a significant quantity of metal ion salts. By analyzing osteoblast growth in the presence of peptide YR and various metal ions, we observed a synergistic effect of calcium and strontium on the effects of YR. Those peptides could also stimulate the growth of C2C12 skeletal muscle cells and human chondrocytes, increasing collagen and glycosaminoglycan content in a three-dimensional environment. The maintenance of bone homeostasis relies on a balance between osteoclasts and osteoblasts. Deer antler peptides were observed to inhibit osteoclast differentiation, as evidenced by ROS generation, tartrate-resistant acid phosphatase (TRACP) activity assays, and gene expression in RAW264.7 cells. In summary, our findings provide a deep understanding of the efficacy of this folk medicine.

Vitamin D3 deficiency and osteopenia in spastic paraplegia type 5 indicate impaired bone homeostasis

Hereditary spastic paraplegia type 5 (SPG5) is an autosomal recessively inherited movement disorder characterized by progressive spastic gait disturbance and afferent ataxia. SPG5 is caused by bi-allelic loss of function mutations in CYP7B1 resulting in accumulation of the oxysterols 25-hydroxycholesterol and 27-hydroxycholesterol in serum and cerebrospinal fluid of SPG5 patients. An effect of 27- hydroxycholesterol via the estrogen and liver X receptors was previously shown on bone homeostasis. This study analyzed bone homeostasis and osteopenia in 14 SPG5 patients as a non-motor feature leading to a potential increased risk for bone fractures. T-Scores in CT bone density measurements were reduced, indicating osteopenia in SPG5 patients. Further, we analyzed various metabolites of bone homeostasis by ELISA in serum samples of these patients. We identified a lack of vitamin D3 metabolites (Calcidiol and Calcitriol), an increase in Sclerostin as a bone formation/mineralization inhibiting factor, and a decrease in cross-linked N-telopeptide of type I collagen (NTX), a marker indicating reduced bone resorption. As statin treatment has been found to lower oxysterol levels, we evaluated its effect in samples of the STOP-SPG5 trial and found atorvastatin to normalize the increased sclerostin levels. In summary, our study identified osteopenia as a non-motor feature in SPG5 and suggests the need for vitamin D3 substitution in SPG5 patients. Sclerostin may be considered a therapeutic target and biomarker in upcoming therapeutical trials in SPG5.

ECM-based Ca2+/l-arginine/NO periosteum nourishes bone defect microenvironment, directs macrophage polarity, and accelerates osteogenesis and angiogenesis

The natural periosteum is responsible for supporting bone homeostasis and initiating repair, but is often damaged during bone fracture, causing acute inflammation and loss of a functioning substratum for induction of new bone formation. Here, we propose an ECM-based Ca2+/l-arginine/nitric oxide (NO) electrospun periosteum to reinitiate bone regeneration. The multifaceted action of NO could promote osteoblastic differentiation, angiogenesis and polarization of macrophages, while accelerating new bone formation, enhancing bone quality and reducing inflammation in a rat critical size calvarial defect model, acting as a mainspring that drives the rest of the PI3K-AKT pathway including the downstream action of VEGF, MAPK and mTOR signalling. The modulation of NO cycle by periosteum scaffold is envisioned to be highly efficient at supporting bone grafting surgeries, with simple and clean clinical translation and implications for anti-inflammatory healing.

Stress reduction through cortical bone thickening improves bone mechanical behavior in adult female Beclin-1+/- mice.

Fragility fractures, which are more prevalent in women, may be significantly influenced by autophagy due to altered bone turnover. As an essential mediator of autophagy, Beclin-1 modulates bone homeostasis by regulating osteoclast and chondrocyte differentiation, however, the alteration in the local bone mechanical environment in female Beclin-1+/- mice remains unclear. In this study, our aim is to investigate the biomechanical behavior of femurs from seven-month-old female wild-type (WT) and Beclin-1+/- mice under peak physiological load, using finite element analysis on micro-CT images. Micro-CT imaging analyses revealed femoral cortical thickening in Beclin-1+/- female mice compared to WT. Three-point bending test demonstrated a 63.94% increase in whole-bone strength and a 61.18% increase in stiffness for female Beclin-1+/- murine femurs, indicating improved biomechanical integrity. After conducting finite element analysis, Beclin-1+/- mice exhibited a 26.99% reduction in von Mises stress and a 31.62% reduction in maximum principal strain in the femoral midshaft, as well as a 36.64% decrease of von Mises stress in the distal femurs, compared to WT mice. Subsequently, the strength-safety factor was determined using an empirical formula, revealing that Beclin-1+/- mice exhibited significantly higher minimum safety factors in both the midshaft and distal regions compared to WT mice. In summary, considering the increased response of bone adaptation to mechanical loading in female Beclin-1+/- mice, our findings indicate that increasing cortical bone thickness significantly improves bone biomechanical behavior by effectively reducing stress and strain within the femoral shaft.

Exploring the Implications of Golgi Apparatus Dysfunction in Bone Diseases.

The Golgi apparatus is an organelle responsible for protein processing, sorting, and transport in cells. Recent research has shed light on its possible role in the pathogenesis of various bone diseases. This review seeks to explore its significance in osteoporosis, osteogenesis imperfecta, and other bone conditions such as dysplasias. Numerous lines of evidence demonstrate that perturbations to Golgi apparatus function can disrupt post-translational protein modification, folding and trafficking functions crucial for bone formation, mineralization, and remodeling. Abnormalities related to glycosylation, protein sorting, or vesicular transport in Golgi have been associated with altered osteoblast and osteoclast function, compromised extracellular matrix composition, as well as disrupted signaling pathways involved with homeostasis of bones. Mutations or dysregulation of Golgi-associated proteins, including golgins and coat protein complex I and coat protein complex II coat components, have also been implicated in bone diseases. Such genetic alterations may disrupt Golgi structure, membrane dynamics, and protein transport, leading to bone phenotype abnormalities. Understanding the links between Golgi apparatus dysfunction and bone diseases could provide novel insights into disease pathogenesis and potential therapeutic targets. Future research should focus on unraveling specific molecular mechanisms underlying Golgi dysfunction associated with bone diseases to develop targeted interventions for restoring normal bone homeostasis while decreasing clinical manifestations associated with these issues.

Fluorescent Probe for Imaging Hydrogen Polysulfide Dynamics of Macrophage Inflammatory Regulation in Bone Homeostasis

Herein, a fluorescent probe BFO-HPS was developed for imaging the H2Sn dynamics of the macrophage inflammatory regulation in bone homeostasis. The probe indicated advantages including large response scale, rapid response, steady optical performance in various pH conditions, high selectivity, and good selectivity. BFO-HPS showed a dose-dependent fluorescence response at 570 nm for the H2Sn concentration with the linear range from 0.1 to 100 µM, limit of detection of 0.11 µM, and limit of quantitation of 0.10 µM. Based on the low cyto-toxicity, BFO-HPS provided the intracellular monitoring of the exogenous H2Sn level in uninduced RAW264.7 cells. In the receptor activator of nuclear factor-κB ligand (RANKL)-induced and RANKL-induced inhibited groups, BFO-HPS achieved the monitoring of the H2Sn dynamics to indicate the macrophage-involved bone homeostasis. This work was informative for further investigations on macrophage inflammatory regulation in bone homeostasis.

Osteoprotective Effect of the Phytonutraceutical Ormona® on Ovariectomy-Induced Osteoporosis in Wistar Rats

The phytonutraceutical Ormona® is a product composed of purified oil of Bixa orellana Linné, dry extract of Myrciaria dubia McVaugh, dry extract of Trifolium pratense L., and dry extract of Euterpe oleracea Mart. obtained using Evolve® technology. This study evaluated the effects of Ormona® on Wistar rats affected by ovariectomy-induced osteoporosis. Pre-treatment was conducted for 15 days before surgery and continued for a further 45 days after the surgical procedure. The experimental design consisted of five groups (n = 5): OVW: treated with distilled water (1 mL/kg, p.o); ADS: alendronate sodium (4 mg/kg p.o); EST: conjugated estrogen (2 µg/kg, p.o); ORM: Ormona® (20 mg/kg, p.o); ORM + EST: Ormona® (20 mg/kg, p.o) + conjugated estrogen (2 µg/kg, p.o). Biochemical and hormonal parameters of bone histopathology and trabecular and femoral diaphysis size were evaluated through scanning electron microscopy (SEM) and bone calcium quantification by atomic absorption spectrophotometry. The results show that ovariectomy caused bone alterations such as loss of femoral microarchitecture, decreased bone homeostasis parameters, and changes in the lipid profile. Estrogen supplementation reduced parameters such as cholesterol, LDL, and Ca2+ concentration. However, Ormona® showed higher serum estradiol levels (p &lt; 0.01), effects on the lipid profile, including parameters that estrogen replacement and alendronate sodium did not affect, with an increase in HDL, and positive modulation of bone metabolism, increasing osteocytes and the presence of osteoblasts. Ormona®, therefore, produced better results than the groups treated with estrogen and alendronate sodium.

Abstract 6866: Investigating the role of Epha2 in breast cancer-mediated myeloid cell expansion and function

Abstract Bone metastases are present in over 70% of metastatic breast cancer patients, and current therapy reduces osteolysis in tumor-induced bone disease (TIBD) but does not improve overall survival. Frequently, TIBD is characterized by an increase in osteoclast proliferation and bone-resorbing activity, and patients often experience pain and pathologic fractures. Increased bone resorption releases growth factors that aid in the progression of tumor growth causing the ‘vicious cycle’. Additionally, bone metastatic patients are often resistant to first- and second-line therapies highlighting an importance for research in this area. The receptor tyrosine kinase, Ephrin-type-A 2 receptor (EphA2), participates in both forward and reverse signaling during receptor/ligand engagement and participates in many physiological and pathological processes, including bone homeostasis, immunity, and cancer. Interestingly, EphA2 is highly expressed in bone metastases, and previous evidence suggests that EphA2 can promote TIBD and reduce anti-tumor immunity. We therefore hypothesize that increased expression of EphA2 in breast cancer cells at the site of bone metastasis promotes expansion of myeloid cells, leading to TIBD. Previous data shows a reduction in osteoclast number and bone lesions following EphA2 knockout. However, the mechanism by which EphA2 mediates the expansion of osteoclasts is not well understood. In agreement with previous work, our data shows that EphA2 deletion in murine breast cancer (BC) cell line, 4T1, reduces bone lesion area and number and the presence of bone metastasis-associated macrophages. Additionally, we observed a significant reduction in Ly6Chi cells. To assess the effects of EphA2 deletion on the behavior of 4T1 cells, we conducted a wound healing assay and observed a decrease in wound healing capacity in 4T1 cells following EphA2 deletion. This suggests that EphA2 expression in 4T1 cells may increase their metastatic potential. We will repeat these studies and incorporate factors from the bone marrow. Our future studies will investigate the role of EphA2 signaling and reverse signaling in BC cell lines, myeloid progenitor cells, osteoclasts, and bone metastasis-associated macrophages. We will utilize murine and human BC cell lines to elucidate the effects of EphA2 deletion in BC on immune cell populations, bone metastasis, and tumor progression. Additionally, we will investigate how EphA2 reverse signaling impacts osteoclasts’ ability to modulate immune cells. We aim to reduce TIBD and tumor burden by reducing the expansion of osteoclasts and shifting the immune cells to an anti-tumorigenic phenotype. Our studies will provide additional insight into the underlying mechanisms of the vicious cycle which will offer new treatment strategies for patients with osteolytic breast cancer. Citation Format: Dominique Parker, Verra Ngwa, Jin Chen, Julie Rhoades. Investigating the role of Epha2 in breast cancer-mediated myeloid cell expansion and function [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts); 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84(6_Suppl):Abstract nr 6866.

The osteoblast in regulation of tumor cell dormancy and bone metastasis

Breast and prostate cancer are among the most common malignancies worldwide. After treatment of the primary tumor, distant metastases often occur after a long disease-free interval. Bone is a major site for breast and prostate cancer metastasis and approximately 70% of patients with advanced disese suffer from osteolytic or osteoblastic bone metastases, a stage at which the disease is incurable. In bone, the disseminated tumor cells (DTCs) can become quiescent or “dormant”, a state where they are alive but not actively dividing. Alternatively, the cancer cells can proliferate, disturb the bone homeostasis, and form metastatic lesions. The fate of cancer cells is largely dependent on the bone microenvironment, particularly the bone forming osteoblasts and bone resorbing osteoclasts. Osteoblasts originate from mesenchymal precursors through a tightly regulated cascade. The main function of osteoblasts is to synthesize bone matrix, coordinate mineralization and maintain bone remodeling by regulating osteoclast activity and bone resorption. In metastatic bone environment, osteoblasts can create a niche within the bone where DTCs cells become dormant and induce quiescence in cancer cells keeping them in a non-proliferative state. Osteoblasts also contribute to metastatic outgrowth and actively promote tumor growth in bone. In this article, we review the recent literature on the role of osteoblasts in cancer cell dormancy and bone metastasis and describe the underlying mechanisms by which osteoblasts regulate cancer cell fate in bone. In addition, we discuss the possibility of targeting osteoblasts to treat osteolytic bone metastases.