Introduction

An increasing number of cell culture, animal, and human studies indicate that boron in nutritional amounts has beneficial health effects. We have found that boron positively regulates genes in MC3T3-E1 cells that are important for bone formation and remodeling [1]. In rabbits fed with a high-energy diet, boron had beneficial effects on bone strength and mineral composition [2]. Rat and mouse studies have shown that boron deficiency impaired injured bone healing by apparently reducing osteogenesis [3, 4]. Boron deficiency decreased alveolar bone remodeling and inhibited bone formation.

The supplements used the studies above showing boron in nutritional or supra nutritional amounts having beneficial effects on bone strength and mineral composition were far from toxic. Intakes of over 100 times nutritional or 10 times supra nutritional are intakes that result in toxicity signs. For example, rats fed with 500 mg boron/kg diet exhibited depressed weight and femur magnesium and zinc, but no significant change in femur calcium and phosphorus concentrations and tibia bone density [5].

Undecalcified histomorphometric evaluation of maturing dental enamel from continuously erupting rat incisor found a reduction in enamel thickness (hypoplasia) [6]. Microchemical characterization by energy-dispersive X-ray spectrometry did not find any alteration in enamel mineralization. In a study histologically examining teeth of rabbits, tooth structure (enamel shape and thickness, dentin, cementum, and pulp) was not significantly affected by boron supplementation [7]. However, micro-CT evaluation found increased alveolar bone mineral density when the rabbits were given boron supplements of 10 and 30 mg/kg diet. The difference in the findings may have been the result of the different experimental model used and/or the boron status of the control animals.

An acidic protein that has been found to play a critical role in the initial stages of ectodermal enamel mineralization is tuftelin [8]. During embryonic development, tuftelin was found to become histologically visible in the mouse molar at the E13 stage [9]. Subsequent to 1991, tuftelin was found in other tissues including craniofacial structures [10, 11]. At the E15 embryonic stage, when alveolar bone cells and vascularization appears [12], osteoblasts were found to be positive for tuftelin. Interestingly, unlike teeth, tuftelin in alveolar bone is intracellular, apparently not a matrix component, and changes location during development. Initially, tuftelin is abundant and occurs in the cytoplasm of differentiating/differentiated osteoblasts. At later pre- and post-natal stages, tuftelin-positive osteoblasts decrease with the localization expanding into the perinuclear/nuclear regions.

Bobek et al. (2019) further characterized the spatial-temporal expression of tuftelin in the mineralizing and non-mineralizing tissues of the craniofacial complex in the developing mouse embryo [13]. Two tuftelin mRNA transcripts and a single 64 Kda protein were detected throughout embryonic development. Tuftelin mRNA and protein already were expressed at stage 10.5 before the initiation of tooth formation. Tuftelin was detected in tissues that develop from embryonic ectoderm, ectomesenchyme, and mesoderm origins. This finding in addition to a shift between cytoplasmic and perinuclear/nuclear expression suggests that tuftelin has a role in the regulation of transcription.

The objective of the following study was to determine whether physiological boron influences the mRNA expression related to osteogenic differentiation of MC3T3-E1 cells. Particular attention was given to tuftelin, which has been found to be involved in alveolar bone formation and development.

Material and Methods

Cell Culture

The MC3T3-E1 (subclone 4) cells were kindly provided by Dr. Martha J. Somerman, NIDCR, NIH, USA, and Renny T. Franceschi, University of Michigan, USA. The cells were derived from newborn mouse calvaria [14]. This subclone was chosen because it was suitable for differentiation study [1]. Cells were maintained in an α-MEM medium with 10% fetal bovine serum and 1% penicillin, streptomycin, and L-glutamine (Biological Industries, Israel) in a humidified incubator at 37 °C with 5% CO2 in the air.

Preparation of Boric Acid

Boric acid (Sigma-Aldrich, Saint Louis, MO, USA) was dissolved in double-distilled water and shaken for 30 min. The solution was adjusted to a pH of 7.0 with NaOH, filtered through 0.22 μm cell culture filter, and sterilized. This stock solution containing 10 μg/mL boric acid was used to make final concentrations of 0.1, 1, 10, 100, and 1000 ng/mL boric acid in the mRNA expression assay medium. These boric acid concentrations provided 0.0175, 0.175, 1.75, 17.5, and 175 ng/mL of boron. Safe/optimum boric acid concentrations were selected according to the studies reported in the literature [1, 15]. Cells were treated with the different concentrations of boric acid every other day.

Gene Analysis

RNA Isolation

Cells at passages 4–7 were used for the gene analysis. Cells were cultured in 60 mm plates at a density of 5 × 104/cm2. Total RNAs of cells were isolated on day 3 by using a monophasic solution of phenol and guanidine isothiocyanate (Invitrogen, Carlsbad, CA, USA) [7, 16] and quantified at 260 nm by NanoDrop.

PCR Array Analysis

The gene expression profiles of the MC3T3-E1 cells were evaluated and were determined by using the RT2 Profiler PCR array system for mouse osteogenesis (PAMM-026 f-12, Qiagen, Germantown, USA) that includes 84 genes related to osteogenic differentiation. The array contains the gene groupings for skeletal development, bone mineral metabolism, cell growth and differentiation, extracellular matrix proteins, and cell adhesion molecules that are shown in Table 1.

Table 1 Functional gene grouping, symbols and the description of the genes in mouse osteogenesis PCR array
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PCR Array

Diluted first strand cDNA synthesis reaction (102 μL) was combined with 2448 μL of RT2 qPCR Master Mix (Qiagen, Germantown, USA) to make a final volume of 2550 μL (1275 μL superarray mix + 1173 μL ddH20 + 102 μL cDNA). From this mixture, 25 μL were added to each well of a 96-well plate of the PCR array [16].

cDNA Synthesis and Quantitative RT-PCR Analysis

Using a cDNA synthesis kit (Applied Biosystems, Foster City, California, USA), cDNA was synthesized from 1.0 μg of total RNA. For the RT-PCR analysis (Stratagene, California, TX, USA), 1.0 μg of cDNA per 25 μL final reaction volume was used. PCR reactions were performed using the Tuftelin1 universal library probe (04688112001-Roche). Glyceraldehyde-3-phosphate dehydrogenase (GAPDH) served as a housekeeping/reference gene for normalization. The amplification profile for the tuftelin and GAPDH was 95 °C for 10 min followed by 40 cycles of 95 °C/15 s and 60 °C/1 min using Stratagene MX3000P.

Statistical Analysis

For PCR array data analysis; web portal was used for calculations (tabular format, a scatter plot, a three-dimensional profile, and a volcano plot) and interpretation of the control wells upon including threshold cycle data from a real-time instrument.

For mRNA expression analysis, one-way analysis of variance and Tukey HSD (honestly significant difference) multiple comparison tests were used to compare groups (GraphPad Software, San Diego, CA). The data are presented as mean ± standard deviation. A value of p < 0.05 was considered statistically significant, and a value of p < 0.01 was considered statistically highly significant.

Results

As indicated by Figs. 1 and 2, all boron as boric acid treatments compared with controls upregulated tuftelin mRNA. In addition to tuftelin, the boric acid treatments modulated integrin α2 (Itga2), and intercellular adhesion molecules (ICAM) mRNA expressions. The 0.1, 1, 10, and 1000 ng/mL treatments upregulated Itga2 mRNA expression which is involved in cell adhesion and cell-surface-mediated signaling. While 100 ng/ml has no effect on the Itga2 transcript and 1 ng/ml boric acid upregulated Itga2 mRNA expression (2.1-fold), 0.1, 10, and 1000 ng/ml boric acid downregulated the Itga2 gene transcript 2.2-, 1.5-, and 2.1-fold, respectively. The 0.1, 1, and 10 ng/ml boric acid treatments decreased ICAM mRNA expression, which is involved inflammation, immune response, and intracellular signaling. In addition to tuftelin, Itga2, and ICAM mRNA expression, other changed expressions were inconsistently found. While 0.1 ng/ml boric acid induced BMP6 (2.1-fold), 1000 ng/ml boric acid treatment increased BMP1r mRNA expression (1.5-fold). Decreased Epidermal Growth Factor mRNA expression (1.7-fold) was observed in 0.1 ng/ml boric acid group. While 1.0 ng/ml boric acid treatment increased Col61a, 10 ng mL boric acid treatment decreased Col7a1 mRNA expressions (4.4-fold) (Figs. 1 and 2).

Fig. 1
figure 1

Comparison and fold changes of the genes diagram of genes associated with osteogenesis of MC3T3-E1 cells with different boric acid concentrations (0.1, 1, 10, 100, and 1000 ng/ml). Please note the layout of the PCR array (bottom right)

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Fig. 2
figure 2

Significant up and downregulated genes of MC3T3-E1 cells exposed to different concentrations of boric acid

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The quantitative RT-PCR results shown in Fig. 3 confirmed the PCR-array tuftelin findings. All boric acid treatments except the 1000 ng/ml treatment statistically significantly increased tuftelin mRNA expression.

Fig. 3
figure 3

Tuftelin mRNA expression of MC3T3-E1 cells treated with 0.1, 1, 10, 100 and 1000 ng/ml boric acid. Significant increase was observed in 0.1, 1, 10, and 100 ng/ml boric acid treated groups when compared with untreated control group (p < 0.05)

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Discussion

The present findings are consistent with previous studies showing that boron promotes bone formation and remodeling through modulating genes involved in osteogenesis. The upregulation of tuftelin by boron indicates that boron has its effect in the initial stages of bone and tooth formation and mineralization. In embryonic development, tuftelin mRNA was already expressed in the E10.5 stage before the initiation of tooth development [13]. At the E15 stage when alveolar bone and vasculation appears, tuftelin was histologically seen in osteoblasts [12]. Tuftelin is abundant in the cytoplasm of differentiating osteoblasts, which suggests that tuftelin has a critical role in the differentiation of mesenchymal cells into osteoblasts. The increase in tuftelin mRNA in pre-osteoblastic MC3T3-E1 cells by boron suggests that a major influence of boron on bone and tooth formation and mineralization is through genetically enhancing the differentiation or formation of osteoblasts. This suggestion is supported by the findings that boron deprivation depresses the number of osteoblasts in alveolar bone repair [3, 4]. Enhancing the development of pre-osteoblasts into osteoblasts might be a basis for boron increasing mRNA expression and amounts of osteocalcin, bone sialoprotein, type 1 collagen, alkaline phosphatase, and bone morphogenetic proteins in MC3T3-E1 cells [1]. Osteoblast differentiation also might be the basis for some of the significant changes found for other osteogenetic expressions found in the present study.

An effect on tuftelin mRNA expression also might be a basis for apparently essential actions of boron. Tuftelin is present in morula and embryonic stem cells [17, 18] and found in other soft organs including lung, liver, kidney and testis [19, 20]. These findings suggest that tuftelin is critical for embryo development. Boron has been described as essential for embryo development of zebrafish [21] and the African clawed frog [22].

Similar to MC3T3-E1 cells, physiological amounts of boron has been found to modulate gene expression in DU-145 prostrate cells [15]. In these cells, boron activated transcription factors involved in the induction anti-inflammatory genes, the differentiation of osteoblasts, the mineralization of teeth, tumor suppression, and retinal degeneration prevention. All these are beneficial actions for which boron has been reported [23]. Boric acid activation of genes may be the basis for wide range of diverse phenotypic responses associated with boron deprivation or supplementation.

The mechanism through which boric acid induces gene activation has not definitively established. However, the findings with DU-145 prostate cells indicate that boron can modulate intracellular Ca2+ concentration involved in the signaling for gene activation [15]. The findings of the present study suggest that boron affects the regulation of the tuftelin1 gene in osteoblastic cells. This effect might be the result of boron influencing signaling pathways, perhaps those mediated by Ca2+. This speculation may explain increased Cd11 as well since it modulates Ca2+-dependent cell-cell adhesion.

Tuftelin is thought to be involved with adaptation to hypoxia, mesenchymal stem cell function, and neurotrophin nerve growth factor-mediated neuronal differentiation. According to our knowledge, this is the first study detecting the importance of boron regulation on the tuft1gene in the osteoblasts. This might have secondary effects on the vicinity of the osteoblasts. Further studies are required to understand the pathway and signaling mechanisms of tuft1 gene. Since this study only evaluated mRNA level, protein levels of Tuft should be also checked. Other cells, i.e., mesenchymal stem cells, should also be evaluated regarding tuft gene to clarify the effects on the osteogenic/neurogenic/adipogenic differentiation.

In a previous study (Hakki et al. 2010), we observed that boron as boric acid increased bone morphogenetic protein (BMP), BMP-4, -6 and -7 protein levels at 0.1, 1, 10, and 100 ng/ml boric acid concentrations [1]. In this experiment, Tuft1 regulation seems definitely more apparent when compared with the other genes related to osteogenesis. Inductive effects of boric acid concentrations below 1000 ng/ml were also the findings of this study. In summary, the upregulation of tuftelin mRNA in pre-osteoblastic MC3T3-E1 cells provides further evidence that nutritional or physiological amounts of boron has beneficial, perhaps critical or essential, effects in animals and humans. This effect is at the molecular level and thus can have numerous beneficial phenotypic effects including being beneficial to alveolar bone and tooth development and mineralization. Since only MC3T3-E1 cell line was used in this study, these finding should be confirmed in other cells like human osteoblasts, primary osteoblastic cells, and mesenchymal stem cells isolated from bone marrow and dental tissues to ensure tuftelin regulation by boric acid and its importance on the mineralized tissues. Further in vivo studies, tuftelin knockout (tuftelin-null mice) animals also could provide more evidence for tuftelin expression in alveolar/other bones and dental tissues during development and postnatal term of the organisms and possible boron interaction.