Introduction

Osteoblasts derived from mesenchymal stem cells are differentiated by various hormones, growth factors, cytokines and cell signaling mediators [1, 2]. The bone morphogenetic protein (BMP) pathway, a major signaling cascades in bone formation, is involved in stimulating the expression of osteogenic transcription factors such as Runt-related transcription factor 2 (Runx2) through the activation of Smad1/5/8 and Mitogen-activated protein kinases (MAPKs) pathway [3]. MAPKs are important signaling mediators that regulate cellular responses to various biological stimuli [4, 5]. Extracellular signal-regulated kinase (ERK) is part of MAPK family, and participates in cellular functions such as cell proliferation, differentiation, survival, and senescence [6, 7]. In addition, ERK plays an essential role in bone development by regulating growth factor activation and osteoblast differentiation [8, 9]. When ERK is inactivated, osteoblast proliferation, differentiation and mineralization is inhibited [10]. ERK phosphorylates Runx2, which is highly expressed in osteoblasts of all bones during the early stages of bone formation and stimulates transcriptional activity to enhance the expression of genes specific for osteoblast differentiation [11, 12].

Bone is a dynamic tissue; the process of bone remodeling is controlled by the balance of activity between osteoblasts and osteoclasts [13]. The interaction between osteoblast-mediated osteogenesis and osteoclast-mediated bone resorption becomes imbalanced due to various stresses including aging, and hormones [14, 15]. If the imbalance persists, bone remodeling fails and bone density decreases, which can lead to bone diseases such as osteoporosis [16]. The number of patients suffering from bone diseases such as osteoporosis and the duration of treatment is expected to increase as the human lifespan increases in an aging society [17]. In particular, postmenopausal women may be at an increased risk of bone disease due to hormone declines [18]. Currently, antiresorptive therapies such as estrogen and bisphosphonates are widely used for treatment of osteoporosis [19]. However, these therapies have side effects during long-term treatment [20,21,22]. Therefore, it is important to develop new therapeutic agents and functional health foods to better treat bone diseases [23].

Some natural products have few side effects and have beneficial effects on bone remodeling [24,25,26]. Essential oils extracted from plants contain highly fragrant monoterpene compounds as their main component. Monoterpene compounds are widely distributed in the plant kingdom, and have antibacterial, anti-inflammatory, and immunomodulatory properties [27]. They are also reported to have a positive effect on bone metabolism, but the exact mechanism is not well understood [28, 29]. Some monoterpene compounds prevent bone resorption and potentially modulate bone metabolism in mice by inhibiting of osteoclast activity [30, 31]. Eucalyptol (EU), the major organic compound in the essential oil extracted from Eucalyptus globulus, is a monoterpene oxide [32]. EU is found in various plant species such as Salvia, Psidium, and Croton, and has a fresh mint flavor and a cooling taste. Furthermore, EU exhibits anti-inflammatory and antioxidant effects in respiratory diseases such as bronchitis, sinusitis, chronic rhinitis, and asthma [33, 34]. However, the effects of EU on osteoblast differentiation and bone formation have not yet been studied. Therefore, the present study investigated the effect of EU on osteoblast differentiation and bone formation.

Materials and methods

Reagents and chemicals

EU and dimethyl sulfoxide (DMSO) were purchased from Sigma-Aldrich (St. Louis, MO, USA). Phosphate buffered saline (PBS), α-minimum essential medium (α-MEM), penicillin–streptomycin, and 0.25% trypsin-ethylenediaminetetraacetic acid (EDTA) were purchased from Gibco (Grand Island, NY, USA). Fetal bovine serum (FBS) was purchased from Atlas Biologicals (Fort Collins, CO, USA). PD98059 was purchased from Calbiochem (San Diego, CA, USA). Emerald Amp GR PCR master mix was purchased from TaKaRa (Shiga, Japan), and Ampigene qPCR green mix Hi-Rox was purchased from Enzo (Farmingdale, NY, USA).

Cell culture

MC3T3-E1 cells were obtained from ATCC (Manassas, VA, USA). All cells were maintained in α-MEM supplemented with 10% FBS and 1% antibiotics (100 U/mL penicillin, 100 g/mL streptomycin) at 37 °C with 5% CO2. The culture medium was changed every 2 days.

Isolation of primary mouse calvarial cells

Primary cells were isolated from the calvaria of 2-day-old ICR mice. Isolated calvaria were incubated in PBS supplemented with 0.1% collagenase (Gibco) and 0.2% dispase (Gibco). After 20 min incubation, the primary cells were isolated by centrifugation for 8 min. The incubation and centrifugation processes were repeated five times. Cells were maintained in α-MEM supplemented with 10% FBS and 1% antibiotics (100 U/mL penicillin, 100 g/mL streptomycin) at 37 °C with 5% CO2. All animal procedures used to isolate the primary cells were carried out in accordance with the guidelines and regulations approved by the Committee for Laboratory Animal Care and Use of Daegu University (approval number: DUIACC-12020/4–0313-006).

Cell viability assay

MC3T3-E1 cells were seeded in 48-well plates at a density of 2 × 104 cells/well and cultured in medium containing EU of 1–50 M concentration. Cells were incubated with 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) solution (Sigma-Aldrich) for 1 h. Formed formazan was dissolved by adding DMSO to each well. The absorbance was measured at 570 nm using an Infinite M200 Pro multiplate reader (Tecan, Mannedorf, Switzerland).

Reverse-transcriptase PCR (RT-PCR) and quantitative PCR (qPCR)

RT-PCR and qPCR were performed to confirm the mRNA expression levels of osteogenic genes for EU. Total RNA was extracted from cultured cells using TRI-solution (Bio Science Technology, Daegu, Korea) according to the manufacturer’s protocols. The cDNA was synthesized by reverse transcription from equal amounts of total RNA (3 g) with TOPscript RT DryMIX (Enzynomics, Daejeon, Korea). Each reaction consisted of initial denaturation at 95 °C for 5 min followed by a three-step cycle: denaturation at 95 °C for 30 s, annealing at the optimal temperature for each primer pair for 30 s, and extension at 72 °C for 30 s. After 30–35 cycles, a final extension step was carried out at 72 °C for 5 min. The relative expression level of each gene was normalized relative to the expression of β-actin. Primer sequences used for PCR are listed in Table S1 and Table S2.

Western blot analysis

Cultured cells were lysed in RIPA buffer (ATTO Technology, Tokyo, Japan) supplemented with protease and phosphatase inhibitors (Roche, Mannheim, Germany). Total protein was quantified using Bradford protein assay reagent (Sigma-Aldrich). Quantified proteins were boiled in loading buffer and separated by 10% sodium dodecyl sulfate–polyacrylamide gel electrophoresis (SDS-PAGE). Proteins were transferred to polyvinylidene difluoride (PVDF) membranes, which were then blocked with 5% skim milk prepared in Tris-buffered saline (TBS) containing Tween-20. Membranes were then incubated with a specific primary antibody for overnight at 4℃. After washing, membranes were incubated with a secondary antibody at room temperature for 2 h. Signals were detected using ECL reagent (Advansta, Menlo Park, CA, USA) according to the manufacturer’s protocol. The amount of detected signal was visualized as a band using a Fusion Solo analyzer system (Vilber Lourmat, Eberhardzell, Germany) and densitometry was quantified through ImageJ. Antibodies used for western blot are listed Table S3.

Alkaline phosphatase (ALP) activity

Measurement of ALP activity was performed using ALP staining. Cells were seeded in 24-well plates at a density of 5 × 104 cells/well and cultured for 10 days. Cultured cells were fixed with 4% formaldehyde (Duksan Pure Chemicals, Gyunggi-do, Korea). The fixed cells were washed with distilled water and stained with 5-bromo-4-chloro-3-indolyl phosphate/nitro blue tetrazolium (BCIP/NBT) solution (Sigma-Aldrich) at room temperature. ALP activity was evaluated by imaging the stained area with an Epson Perfection V37 scanner (Seiko Epson, Suwa, Japan) and quantifying with the ImageJ program.

Mineralization

Measurement of extracellular matrix mineralization was performed using alizarin red S (ARS) staining. Cells were seeded in 24-well plates at a density of 5 × 104 cells/well and cultured for 3 weeks. Cultured cells were fixed with 4% formaldehyde, washed with distilled water, and stained with 2% ARS solution (Sigma-Aldrich) at room temperature. The levels of mineralization were evaluated by imaging the stained area with an Epson Perfection V37 scanner (Seiko Epson, Suwa, Japan) and quantifying with the ImageJ program.

Quantitative analysis of zebrafish caudal fin ray regeneration

Wild-type zebrafish (Danio rerio) (length = 3.4–4.4 cm) were maintained under standard conditions in tanks (temperature 30 ± 1℃, 300 mL: 3 fish per tank) with 14 h light/10 h dark cycle. Zebrafish were fed daily with commercial flakes (Tetra Bits Complete, Tetra, Melle, Germany) and Artemia nauplii (Artemia salina). Zebrafish were stunned in cold water, and approximately half of the caudal fin ray (lepidotrichia) was amputated using a scalpel. Zebrafish were divided into three groups (control, 5 M EU, 50 M EU; n = 3 per group). EU was added to the tank water according to the relevant concentration for each group, and zebrafish were incubated for 10 days. The tank water was changed every 2 days. All zebrafish were sacrificed and fixed in 10% neutral-buffered formalin for 24 h. Fixed specimens were treated with 25% saturated sodium tetraborate for 2 h. After washing, specimens were stained with 1% KOH and 1 mg/mL ARS. To visualize the mineralized structures, specimen tissues were cleared with 1% KOH and 3% H2O2 for 12 h. The caudal fins bone rays of the zebrafish were imaged through a scanner and microscope. The level of bone regeneration was measured by quantifying the images through the ImageJ program. Zebrafish studies were carried out in accordance with the guidelines and regulations approved by the Committee for Laboratory Animal Care and Use of Daegu University (approval number: DUIACC-12020/4- 0313–006).

Ovariectomy (OVX) mouse model

Female 7-week-old C57BL/6 mice (average weight = 18 g) were purchased from Koatech (Pyeongtaek, Korea). Mice were maintained under specific pathogen-free conditions (12 h light/dark cycle, temperature 22 ± 3℃, humidity 50 ± 20%), and had free access to water and food. After acclimatization for 1 week, mice (n = 16) either underwent bilateral ovariectomy (n = 12, OVX group) or sham surgery (n = 4, sham group) in which the ovaries were not removed. The ovariectomized mice were divided into three groups (OVX, OVX + EU 10 mg/kg, OVX + EU 50 mg/kg; n = 4 per group). After inducing osteoporosis for 4 weeks, the test substances were orally administered to mice every day for 8 weeks. Sham group and OVX control group were given the same amount of vehicle. All procedures were carried out in accordance with the guidelines and regulations approved by the Institutional Animal Care and Use Committee of Laboratory Animal Center of Daegu-Gyeongbuk Medical Innovation Foundation (approval number: DGMIF-20060304–02).

Micro-computed tomography (micro-CT) image analysis

The mouse femurs were examined using a microcomputed tomography system. Scanned images were obtained at a tube voltage of 90 kVp and tube current of 180 μA using a Quantum FX micro-CT scanner (Perkin Elmer, Waltham, MA, USA), set at an image voxel size of 19 m and field of view of 10 mm. After scanning, the regions of interest (ROI) were selected at a 0.3 mm interval from the growth plate reference. Trabecular bone 3D images were visualized based on the ROI. Morphometric parameters including bone mineral density (BMD), bone volume/tissue volume (BV/TV), trabecular thickness (Tb. Th), and trabecular number (Tb. N) were measured using Analyze 12.0 software (AnalyzeDirect, Stilwell, KS, USA).

Histological and immunohistochemical analyses

Mice were sacrificed, and femurs were isolated. The isolated femurs were fixed with 10% neutral-buffered formalin and decalcified in 10% EDTA. The femur tissue was embedded in a paraffin block and sectioned to a thickness of 5 µm. Paraffin sections were then deparaffinized, and hematoxylin and eosin (H&E) and immunohistochemistry (IHC) staining was performed. H&E staining was conducted according to standard procedures. For IHC staining, sections were incubated with an antibody to P-ERK (#4370, Cell Signaling Technology). The 3,3-diaminobenzidine (DAB) reaction was used to detect the antibody, and hematoxylin was used as a counterstain. Stained sections of tissue were scanned using an AxioScan.Z1 slide scanner (ZEISS, Thuringia, Germany).

Statistical analysis

All experiments were performed at least three times and data are expressed as mean ± SEM. Comparisons between two groups were analyzed by unpaired Student’s t test. Comparisons between multiple groups were performed using one-way analysis of variance (ANOVA) followed by Tukey's multiple comparison test. All analyzes were performed using GraphPad Prism 5 (GraphPad Software, La Jolla, CA, USA), and statistical significance was considered at P value < 0.05.

Results

EU induces osteoblast differentiation of MC3T3-E1 cells.

The chemical formula of EU is C10H18O, and the structure is shown in Fig. 1A. MC3T3-E1 cells were exposed to various concentrations (1–50 M) of EU for 1, 2 or 4 days and the potential cytotoxicity of EU was determined by MTT assay. The results showed that EU was not cytotoxic at concentrations below 5 M but was cytotoxic at concentrations above 10 M (Fig. 1B). Therefore, EU was used at a concentration of 5 M in the subsequent experiment. To investigate whether EU can induce osteoblast differentiation, we measured the mRNA and protein expression levels of osteogenic genes such as distal-less homeobox5 (Dlx5) and Runx2 during osteoblast differentiation. As the results, EU significantly increased the mRNA expression of osteogenic genes in MC3T3-E1 cells (Fig. 1C). Western blot analysis showed that EU significantly increased the protein levels of osteogenic genes in a time-dependent manner (Fig. 1D). Also, ALP activity in MC3T3-E1 cells treated with EU for 10 days significantly increased (Fig. 1E). These results indicate that EU induces osteoblast differentiation by increasing the expression of osteogenic genes.

Fig. 1
figure 1

Effect of EU on osteogenic differentiation in MC3T3-E1 cells. A The chemical structure of EU. B The cytotoxicity of EU on cell viability of MC3T3-E1 cells was measured using an MTT assay after incubation with different concentrations of EU (1, 2, 5, 10, 20, and 50 µM) for 1, 2, and 4 days. C Effect of EU (5 µM) on the mRNA expression of osteogenic genes was measured by RT-PCR and qPCR. D The protein levels of osteogenic markers were measured by western blotting. Relative intensity is shown in the lower panel. E ALP staining of cells cultured for 10 days in the absence or presence of EU (5 µM). The upper panel represents the image of staining, and the lower panel indicates the relative intensity. Data are presented as the mean ± SEM of three individual experiments. The statistical significance was determined relative to the control by the Student’s t-test (*P < 0.05, **P < 0.01, ***P < 0.001)

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EU regulates osteogenic differentiation via ERK phosphorylation in MC3T3-E1 cells

The molecular mechanism behind the osteogenic effect of EU in osteoblast was investigated via evaluation of ERK. MC3T3-E1 cells were treated with EU for the specified times. EU significantly increased the protein level of phosphorylated ERK (Fig. 2A). ERK phosphorylation by EU is blocked by an ERK inhibitor (PD98059) (Fig. 2B). We measured mRNA and protein expression of osteogenic genes using PD98059 to determine whether osteoblast differentiation by EU was regulated through ERK phosphorylation or independently. As a result, mRNA expression of EU-induced osteogenic genes was suppressed by the PD98059 (Fig. 2C). The protein expression of EU-induced osteogenic genes was completely reversed by PD98059 (Fig. 2D). Also, EU significantly increased ALP activity and mineralization, which are representative markers of bone formation, but these effects were reversed by PD98059 (Fig. 2E, F). These results demonstrate that EU regulates osteogenic differentiation through ERK phosphorylation.

Fig. 2
figure 2

EU increases osteogenic differentiation via ERK phosphorylation in MC3T3-E1 cells. A MC3T3-E1 cells were treated with EU (5 µM) for the indicated times. B MC3T3-E1 cells were treated with or without EU (5 µM) and PD98059 (10 µM) for 0.5 h. A and B Phosphorylation of ERK were determined by western blotting. Relative intensity is shown in the lower panel. C, D, E and F MC3T3-E1 cells were cultured according to the specified conditions. C mRNA expression of osteogenic genes measured by RT-PCR and qPCR. D Protein expression levels of osteogenic genes measured by western blotting. Relative intensity is shown in the lower panel. E Cells cultured for 10 days were subjected to ALP staining. F Cells cultured for 3 weeks were stained with ARS. The upper panel represents the image of staining, and the lower panel indicates the relative intensity. Data are presented as the mean ± SEM of three individual experiments (*P < 0.05, **P < 0.01, ***P < 0.001 compared with the control group; #P < 0.05, # # P < 0.01, # # # P < 0.001 compared with the EU-treated group)

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EU induces osteogenic differentiation via ERK phosphorylation in primary calvarial osteoblasts

The effect of EU on osteoblast differentiation was confirmed in primary calvarial osteoblasts isolated from neonatal mice. Primary calvarial osteoblasts were treated with 5 µM EU for 1, 2 and 4 days. EU treatment significantly increased the mRNA and protein expression levels of Dlx5 and Runx2 (Fig. 3A, B). In addition, EU treatment for 10 days increased ALP activity in primary calvarial osteoblasts (Fig. 3C). We next determined whether EU regulates the expression of osteogenic genes through ERK phosphorylation. Primary calvarial osteoblasts were treated with EU for the indicated times. Western blot analysis showed that EU treatment for 0.5 h increased ERK phosphorylation (Fig. 3D). In addition, PD98059 suppressed the EU-induced increase in the mRNA and protein expression of osteogenic genes (Fig. 3E, F). These results suggest that EU induces osteogenic differentiation via ERK phosphorylation in primary calvarial osteoblasts.

Fig. 3
figure 3

Effect of EU on osteogenic differentiation in primary mouse calvarial cells. A and B)Primary cells were treated with EU (5 µM) for 1, 2, 4 days. A Effect of EU on the mRNA expression of osteogenic genes was measured by RT-PCR and qPCR. B The protein levels of osteogenic markers were measured by western blotting. C ALP staining of cells cultured for 10 days in the absence or presence of EU (5 µM). D Primary cells were treated with EU (5 µM) for the indicated times. Phosphorylation of ERK were determined by western blotting. Relative intensity is shown in the lower panel. E, F Primary cells were treated with or without EU (5 µM) and PD98059 (10 µM). E The mRNA expression of osteogenic genes measured by RT-PCR and qPCR. F The protein expression levels of osteogenic genes measured by western blotting. Relative intensity is shown in the lower panel. Data are presented as the mean ± SEM of three individual experiments (*P < 0.05, **P < 0.01, ***P < 0.001 compared with the control group; #P < 0.05, # # P < 0.01, # # # P < 0.001 compared with the EU-treated group)

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EU promotes bone regeneration of zebrafish fin rays

The zebrafish injury model is mainly used for regeneration research because it can regenerate various tissues including bone tissue [35,36,37]. In the current study, the injury model indicates the partial amputation of zebrafish fin rays for bone regeneration studies. Amputations of zebrafish fin rays and EU treatment were performed as described in the materials and methods section and are depicted in Fig. 4A, B. To investigate the effect of EU on bone regeneration, amputated zebrafish were treated with EU. The results of our quantitative analysis focusing on ARS staining images, showed that EU dose-dependently increased the level of bone regeneration (Fig. 4D, E). These results show that EU promotes bone regeneration in adult zebrafish.

Fig. 4
figure 4

Effect of EU on the regeneration of zebrafish fin rays. A The schedule and B procedure used for EU treatment in adult zebrafish (n = 3, per group). D The caudal fin rays of zebrafish were amputated C on day 0 and were treated with or without EU (5 or 50 µM) for 10 days. Images of fin ray were scanned after ARS staining. E The level of regeneration of caudal fin rays was quantified using the scanned image. Scale bar, 200 µm (**P < 0.01, ***P < 0.001 compared with the control group)

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EU ameliorates bone loss via ERK phosphorylation in OVX mice

We investigated whether EU could improve bone loss in the OVX-induced postmenopausal mouse model. C57BL/6 mice underwent ovarian resection, and then bone loss was allowed to develop for 4 weeks. Figure 5A shows a schematic diagram of EU evaluation in OVX mice. Each group (sham + vehicle, OVX + vehicle, OVX + EU 10 mg/kg, OVX + EU 50 mg/kg) was orally administered EU or vehicle for 8 weeks. OVX mice had a significantly higher body weight than the sham group, and the weight gain in the EU-treated group was lower than that in the OVX group at 12 weeks (Fig. 5B). The effect of EU on bone loss was assessed by micro-CT analysis in the distal femur. OVX surgery significantly decreased trabecular bone mass, but the OVX + EU 50 mg/kg group at 8 weeks had significantly less bone loss (Fig. 6A). As a result of quantitative analysis of trabecular morphological parameters, bone mineral density (BMD) was significantly increased in the OVX + EU 50 mg/kg group compared to the OVX group. BV/TV, Tb.N and Tb.Th showed no statistical significance (Fig. 6B-E). In addition, we confirmed histological morphology of the femurs through H&E staining images. Distal femoral tissues of OVX mice showed lower trabecular bone density and higher number of fat droplets compared to the sham group, but these results were reversed in the OVX + EU 50 mg/kg group (Fig. 7A). H&E staining images showed consistent results with micro-CT scanning. In addition, the expression of phosphorylated ERK was significantly lower in the OVX group than in the sham group but increased in the EU 50 mg/kg treatment group (Fig. 7B). These results suggest that EU ameliorated bone loss by promoting bone formation in OVX mice, and this effect is mediated through ERK phosphorylation.

Fig. 5
figure 5

Effect of EU on the body weight of OVX mice. A Schematic diagram of experimental design used for EU treatment in OVX mice. Bone loss was induced for 4 weeks after surgical removal of the ovaries. OVX mice were orally administered with EU daily for 8 weeks. Mice were sacrificed, and the effect of EU on bone loss was analyzed. B Changes of body weight after ovariectomy and EU treatment in C57BL/6 mice (n = 4, per group) (*P < 0.05, ***P < 0.001 compared with the sham group, #P < 0.05 compared with the OVX + vehicle group)

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Fig. 6
figure 6

Effect of EU on trabecular morphometric parameters in the distal femurs of OVX mice. After 4 and 8 weeks of EU oral administration, micro-CT analysis was performed on the distal femurs. A Representative 3D images of the trabecular bone. Micro-CT images were obtained from sham group, OVX + vehicle group, OVX + EU 10 mg/kg group and OVX + EU 50 mg/kg group (n = 4, per group). Comparative analysis of the bone structural parameters: B bone mineral density (BMD), C bone volume/tissue volume ratio (BV/TV), D trabecular number (Tb.N), and E trabecular thickness (Tb.Th) (***P < 0.001 compared with the sham group; #P < 0.05 compared with the OVX + vehicle group)

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Fig. 7
figure 7

Histological and immunohistochemical evaluation in distal femurs of OVX mice. After all the treatment procedures, mice were sacrificed and femur tissues were isolated (n = 4, per group). A Representative images of H&E staining from femur bone sections of each group. Scale bar, 500 µm. B Representative images of IHC staining with p-ERK antibody from femur bone sections of each group. Scale bar, 100 µm. Abbreviations: BM, bone marrow; TB, trabecular bone

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Discussion

EU is a monoterpenoid compound present in the form of monoterpene oxide. Naturally occurring terpenes, such as EU, are lipophilic molecules that increase the intracellular penetration of drugs, so they may be potentially important drugs in the treatment strategy of chronic diseases [38]. EU has antibacterial, anti-inflammatory and antioxidant effects in chronic diseases such as inflammation, respiratory disease, diabetic disease, and contractile cardiovascular disease [34, 38]. Oxidative stress and inflammation are important processes that act as major etiologies in bone disease [39]. Several plant-derived natural products with anti-inflammatory and antioxidant properties have been reported to be effective in osteoblast differentiation and bone formation [26, 40, 41]. We hypothesized that EU would have a potentially beneficial effects on bone and verified new efficacy on osteoblast differentiation and bone formation. This study is an initial study to identify the new functions of EU. Therefore, it is unclear whether EU mediates the effects of bone through anti-inflammatory and antioxidant effects, and further studies are needed.

In this study, we used pre-osteoblast cell line MC3T3-E1 and primary mouse calvarial cells and showed that EU induces osteoblast differentiation by increasing the mRNA and protein levels of Dlx5 and Runx2. Osteogenic genes such as Dlx5 and Runx2 are essential regulators in osteoblast differentiation. Dlx5 mediates expression of several osteogenic markers to induce osteoblast differentiation [13]. Runx2 produces bone matrix proteins and plays a central role in osteoblast differentiation [42]. Our results indicate that EU regulates osteoblast differentiation by ERK phosphorylation. According to several previous reports, MAPK promotes osteoblast differentiation by inducing phosphorylation of osteogenic proteins such as Dlx5 or Runx2 to increase their stability [43, 44]. EU has been reported to regulate MAPK/ERK signaling in macrophages and podocytes, but the molecular mechanisms for osteoblast differentiation and bone formation have not been studied [38, 45]. MAPK/ERK signaling is an important pathway in promoting osteoblast differentiation and bone formation [9]. As a result of confirming the correlation between osteoblast differentiation and ERK by EU treatment, it was found that PD98059, an ERK inhibitor, suppressed the expression of EU-induced Dlx5 and Runx2. In addition, blockade of ERK phosphorylation by PD98059 reduced ALP activity and extracellular matrix mineralization. This suggests that ERK can not only increase the stability of Dlx5 and Runx2 but also regulate their expression. However, further studies will be needed on how ERK increases the expression of these genes.

Next, we conducted an in vivo study based on the in vitro results. Bony fish such as zebrafish can regenerate various organs and tissues after injury [46]. In particular, the zebrafish caudal fin rays regenerate and maintain the bone rays through a series of dedifferentiation and redifferentiation [35, 36]. Therefore, the regenerative capacity of the caudal fin is a useful system for compounds screening that promote bone formation [47]. In this study, EU promoted the regeneration of the caudal fin rays in a dose-dependent manner. Although zebrafish have high regenerative capacity, complete regeneration is impossible if the injury exceeds a critical size [48]. This suggests that EU promotes osteoblast differentiation in zebrafish caudal fin ray. The OVX mouse model is used mainly to study osteoporosis caused by estrogen deficiency after the menopause [31]. Estrogen promotes the differentiation of osteoblasts from the same embryonic cells and inhibits the differentiation of adipocytes [49]. In other words, ovariectomy activates the differentiation of hematopoietic stem cell-derived osteoclasts and mesenchymal stem cell-derived adipocytes by deficient estrogen secretion in mice. Therefore, weight gain by estrogen deficiency is caused by adipose tissue accumulation, and weight change in OVX mice can be an important predictor of osteoporosis disease because there is a correlation between bone resorption and weight gain [50]. In this study, it was confirmed that the body weight of osteoporosis model mice induced by ovariectomy increased significantly compared to normal mice, but the weight gain of EU-administered mice decreased at 12 weeks. In addition, we found that oral administration of EU ameliorated bone loss in OVX mice. However, while high concentration of EU (50 mg/kg) protected against bone loss, low concentration of EU (10 mg/kg) had little effect. These results suggest that 50 mg/kg is the standard concentration for EU to improve metabolically induced osteoporosis. The MAPK/ERK pathway is essential for skeletal development and homeostasis [9]. Several studies have shown that the ERK pathway promotes osteoblast differentiation and bone formation [7, 8]. Herein, administration of EU (50 mg/kg) prevented the OVX-mediated reduction of ERK phosphorylation in OVX mice femurs. ERK phosphorylation was mainly detected in the trabecular region below the growth plate, which has high bone remodeling activity. We thought that EU prevents bone loss by increasing ERK phosphorylation in femur tissue.

In summary, this study investigated the effect of EU on osteoblast differentiation and an in vivo model for surgically induced bone loss (Fig. 8). EU induced osteoblast differentiation by regulating ERK phosphorylation. In addition, EU promoted regeneration of caudal fin bony rays in amputated zebrafish. In addition, EU increased ERK phosphorylation and ameliorated bone loss in the distal femur of osteoporosis induced OVX mice. We demonstrated that EU treatment induced osteoblast differentiation and osteogenesis via phosphorylation of ERK in vitro and in vivo. Therefore, we suggest that EU is an organic compound that may be useful in the prevention of bone loss.

Fig. 8
figure 8

Schematic diagram showing the effects of EU on osteoblast differentiation and its ameliorative effects on bone loss-induced animal models such as zebrafish and OVX mice. As depicted, EU induces osteoblast differentiation through ERK phosphorylation. EU promotes bone rays’ regeneration in amputated zebrafish caudal fin and ameliorates bone loss in OVX mice through ERK phosphorylation

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