Introduction

Dental pulp mesenchymal stem cells (DPSCs/DP-MSCs) are a heterogeneous and non-hematopoietic population of mesenchymal stem cells (MSCs) that were first isolated in 2000 from dental pulp tissue by Gronthos and coworkers [1]. According to the ISCT (International Society for Cell & Gene Therapy) criteria, DPSCs are characterized by high proliferation capacity, multilineage differentiation ability, expression of specific mesenchymal markers (CD90, CD73 and CD105) and non-expression of haematopoietic markers (CD19, CD34 and CD45) [2, 3].

It had been reported that these cells compared to other sources of MSCs possess higher capabilities in terms of proliferation, immunomodulation, tissue repair and regeneration [4,5,6,7,8]. The immunomodulation potential is due to direct interaction with immune cells by PD-L1 and -L2 (programmed cell death ligand -1 and -2), HLA-G1 (human leukocyte antigen-G1) and also secretion of cytokines and growth factors including IDO (indolamine 2,3-dioxygenase), HLA-G5 (human leukocyte antigen-G5), IL-10 (interleukin 10), TGF-β (transforming growth factor beta) and PGE2 (prostaglandin E2) [9,10,11].

Dental pulp is a crucial tissue for longevity and homeostasis of teeth. Tooth decay as one of the most common health problems can lead to pulp damage and necrosis and eventually tooth loss. For this reason, one of the important goals in regenerative dentistry is maintenance and repair of inflamed and damaged pulp and also avoid invasive treatments such as pulpectomy [12]. Increasing the expression of DSPP (dentin sialophosphoprotein), ALP (alkaline phosphate), DMP-1(dentin matrix protein 1) and VEGF (Vascular endothelial growth factor) genes which involve in regeneration or repair dental pulp-dentin complex, is a new opportunity to improve the stemness status of these cells for treatment of the damaged pulp in patients with pulpitis [1, 12,13,14,15]. Furthermore, researches have proposed that the high expression of CD106/VCAM1 (vascular cell adhesion molecule-1) and STRO-1 (stromal precursor antigen-1) biomarkers as well as STAT (signal transducer and activator of transcription) family signaling molecules and also low/non-expression of the RelA (nuclear factor-kB p65) transcription factor enhance migration, differentiation and immune modulation properties of MSC cells [16,17,18,19,20,21,22,23,24,25,26].

In this regard, the usage of herbal extracts or compounds have been introduced as ideal gene stimulators [27, 28]. Curcumin is a natural compound obtained from Curcuma longa plant with numerous therapeutic and biological activities, including anti-inflammatory and tissue repair and regenerative activities. However, poor solubility and instability of curcumin, plus its rapid metabolism profile, limit its therapeutic properties [29,30,31,32].

Nano-material technology is a growing field with very promising potentials and applications in modern medicine. Recently, phytosomal curcumin (PC), a nanoparticle type of curcumin into phytosomal formulation has been considered as novel candidate for overcoming some limitations of curcumin, improving the drug delivery into the cell and crossing unavailable different barriers. It was observed that phytosomes are highly stable compared to liposomes and other nanoparticles due to their strong covalent bonds between phosphatidylcholine and curcumin [33,34,35,36].

Because of the precise molecular mechanisms of immunomodulation and regeneration in DP-MSCs are still uncertain, more investigation is needed to support clinical applications of DP-MSCs and optimize transplantation conditions. Based on the above, it was the aim of this study to evaluate the role of PC on DSPP, VEGF-A, HLA-G5, VCAM1, RelA and STAT3 gene expression levels of DP-MSCs, targeting their application as inducers/modifiers of their immunomodulatory and regenerative potential in dental pulp tissue regeneration.

Materials and methods

Isolation and culture of DP-MSCs

Healthy and non-carious third molars (n = 3) were taken with informed consent from patients (20–25 years old) at Dental center of Imam Reza Hospital in Birjand (Iran) according to the guidelines of ethics committee of the Birjand University of medical sciences (ethical number: IR.BUMS.REC.1399.090). DP-MSCs from pulp organ tissues were isolated as described previously [37]. Briefly, after the separation of pulp tissue from dental root and crown parts, the pulp digestion process was performed with collagenase enzyme type I (Gibco, USA). Then, digested tissue suspension was centrifuged and resulted pellet was cultured in complete medium including the Dulbecco’s modified Eagle’s medium mixture F12 (DMEM/F12) with 50 mg/ml streptomycin, 50 unit/ml penicillin and 15% fetal bovine serum (all from Gibco, USA). Cells were incubated at 37 °C under water-saturated atmosphere plus 5% CO2. The medium was refreshed two times in week until cells confluence was obtained. All subsequent analyses were done with cells in passages of 3–5.

Multilineage differentiation potential of DP-MSCs

To induce DP-MSCs osteogenic differentiation, 105 cells were implanted in each well of 6-well plates. When the cells achieved 100% confluency, they were induced with an osteogenic medium in DMEM containing 15% FBS, 10 mM β-glycerol phosphate, 0.1 µM dexamethasone, and 50 µM ascorbic acid (all from Sigma-Aldrich, USA) for 21 days. The differentiated cells were then fixed in 4% paraformaldehyde and stained for Alizarin Red [37]. In addition, to induce adipogenesis, the cells were treated with an inductive medium contain 0.5 mM isobutyl-methyl-xanthine, 1 µM dexamethasone, 200 µM indomethacin (all from Sigma-Aldrich, USA), for 14 days. Then, the cultures were stained using Oil Red solution after cell fixation with paraformaldehyde [37]. Cell staining results were investigated under a phase-contrast microscope (Olympus Microsystems).

Immunophenotyping of DP-MSCs by flow cytometric analysis

Immunophenotyping analysis was performed according to the optimized protocol by Al-Habib et al. with a slight change [38]. Briefly, 106 cells were harvested and washed with PBS solution and subsequently stained with human conjugated antibodies at 4 °C in the dark. Finally, after washing the cells with PBS, they were used for flow cytometric analysis by CyFlow Cube 6 cytometer (Sysmex Partec, Germany). The following antibodies were used to identify cell-specific markers: CD105-PE, CD90-APC, CD73-PE CY7 and CD45-FITC (all from eBioscience, USA).

Cell treatments

Stock solutions in dimethyl sulfoxide (DMSO, Sigma) were prepared, according to the molecular mass and solubility of the PC. The stock solutions were then diluted with complete medium to obtain the different concentrations PC (i.e., 10–100 μM). In addition, as a control group, the cells were incubated in a complete medium containing 1% DMSO. Therefore, there were two groups of control samples: DP-MSCs + DMSO and un-treated DP-MSCs. PC was received from Sami Labs Ltd (Bangalore, India).

Evaluation of cell viability

The optimum and highest non-toxic concentration of PC on the DP-MSCs was determined by MTT procedure [39]. Briefly, the cells were seeded in 96-well microplates (5 × 103 cells in each well) and allowed to adhere for 24 h. First, the cells were treated to PC media as described earlier at a broad spectrum of concentrations for 24 h to obtain approximately the appropriate non-toxic dose. Then, 30 μL of MTT (Sigma-Aldrich, USA) solution with 5 mg/mL concentration was subjected to every well 24, 48, and 72 h after the PC treatment in various concentrations (30, 35, and 40 μM) and the plates were maintained in the dark for 4 h at 37 °C. The medium was slowly discarded and DMSO was subjected to the wells. Absorbance was assessed at 570 nm by the Epoch Microplate spectrophotometer (BioTek Instrument, USA).

Quantitative real-time PCR analysis

Total RNA extraction and cDNA synthesis were done according to the manufacturer’s guidelines (both from Pars Tous, Iran), at 24 and 48 h after DP-MSCs treatment with PC. SYBR Green-based quantitative real-time PCR by the 2−ΔΔCt method was utilized for investigation of the relative expression of DSPP, VEGF-A, HLA-G5, VCAM1, RelA and STAT3 genes. In the analysis, target genes were normalized to the internal control gene GAPDH and each assay was done in triplicate (n = 3). The sequence of primers used were as follows: DSPP, forward: 5′-TTCCGATGGGAGTCCTAGTG-3′ and reverse: 5′-TCTTCTTTCCCATGGTCCTG-3′; VEGF-A, forward: 5′-AGGGCAGAATCATCACGAAGT-3′ and reverse: 5′-AGGGTCTCGATTGGATGGCA-3′; HLA-G5, forward: 5′ CTGAGATGGAAGCAGTCTT-3′ and reverse: 5′-GCTCCCTCCTTTTCAATCT-3′; STAT3, forward: 5′-GAAGAATCCAACAACGGCAG-3′ and reverse: 5′-TCACAATCAGGGAAGCATCAC-3′; RelA, forward: 5′-AGGCTTCTGGGCCTTATGTG-3′ and reverse: 5′-TGCTTCTCTCGCCAGGAATAC-3′; VCAM1, forward: 5′-ATGTCAATGTTGCCCCCAGA-3′ and reverse: 5′-ACAGGATTTTCGGAGCAGGA-3′; GAPDH, forward: 5′-CGAACCTCTCTGCTCCTCCTGTTCG-3′ and reverse: 5′-CATGGTGTCTGAGCGATGTGG-3′.

Statistical analysis

Statistical significance of the cell viability test data was checked using SPSS Ver. 26 software and one-way ANOVA variance (Tukey’s test). The real-time PCR data were analyzed with the Rest software (T-test). In this study, all data were indicated as the mean ± standard deviation at statistical power of P < 0.05.

Results

Culture and characterization of DP-MSCs

Following 5–6 days of digested pulp incubation, DP-MSCs around pulp tissue along with non-adherent small cells were observed. Almost after three cell passages, a pure population of fibroblast-like, spindle-shaped MSCs was obtained (Fig. 1a, b). The multi-differentiation capacity of extracted cells was ascertained by adipogenesis and osteogenesis induction. Following 14 days adipogenic induction, fat droplets were observable which were acknowledged using Oil Red staining (Fig. 1c). Moreover, DP-MSCs in the osteogenic condition established cellular aggregates that were distinguished by the existence of calcium material deposits. The calcium deposits were stained by Alizarin Red on 21st day of osteo-differentiation (Fig. 1d). The expression of DP-MSCs markers was evaluated by flow cytometry which demonstrated that these cells were positive for CD105, CD90, and CD73 and negative for CD45 (Fig. 2).

Fig. 1
figure 1

Isolation, culture and multipotential differentiation of DP-MSCs in vitro. a, b Migration of MSCs from pulp tissue and change morphologically into pure fibroblast-like, spindle-shaped cells after 3 passages; c adipocyte differentiation and lipid vacuole formation as manifested using Oil Red stain solution after stimulation for 14 days; d osteocyte differentiation and calcium deposit formation as manifested using Alizarin Red stain solution after stimulation for 21 days. DP-MSC, dental pulp mesenchymal stem cell; MSC, mesenchymal stem cell

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Fig. 2
figure 2

Immunophenotyping of DP-MSCs isolated from freshly extracted, non-carious human third molar tooth. Flow cytometry histograms display expression of specific mesenchymal markers and no-expression of hematopoietic markers by the DP-MSC population. The isolated and cultured cell phenotype was CD90 + /CD73 + /CD105 + /CD45−. DP-MSC dental pulp mesenchymal stem cell

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The lethal effects of PC at high doses and times on DP-MSCs

According to MTT results, 45–60 μM concentrations of PC had significant cell toxicity effects on the DP-MSCs after 24 h (P < 0.05), while no statistically considerable effects were detected in this regard for the concentrations less than or equal to 40 μM (P > 0.05). In addition, the results revealed that there were remarkable differences in cell viability between PC treatment and control groups at concentrations above 30 μM after 72 h (P < 0.05). Therefore, for next experiments, it was selected the concentration of 30 μM in 24 and 48 h based on the results where PC had no effect on hDPSC viability (P > 0.05). Moreover, cell treatment with 1% DMSO did not represent any significant effects (Fig. 3).

Fig. 3
figure 3

Effects of PC on the DP-MSCs viability and proliferation were determined by MTT assay. The column charts show the viable cell percentage with respect to control group (Ctrl, 100%) after the treatment with various concentrations of PC. Data are mean ± SD at least three independent triplicated experiments. *P < 0.05; NS no-significant, PC phytosomal curcumin, DP-MSC dental pulp mesenchymal stem cell, SD standard deviation

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The effects of PC on the gene expression of regenerative and immunomodulatory markers in DP-MSCs

Relative expression of DSPP, VEGF-A, HLA-G5, VCAM1, RelA and STAT3 genes were evaluated in the cells cultured in complete medium with or without PC at 24 and 48 h. According to the real-time PCR analysis (Fig. 4), cell treatment with PC resulted in the great up-regulation of VEGF-A gene up to 3.01-fold and 1.61-fold, respectively, in 24 and 48 h (P < 0.001). In addition, significant down-regulation of HLA-G5 (24 h = 0.438-fold, 48 h = 0.674-fold), VCAM1 (24 h = 0.543-fold, 48 h = 0.258-fold), RelA (48 h = 0.376-fold) and STAT3 (24 h = 0.23-fold, 48 h = 0.57-fold) genes was observed after treatment with the nanoparticle (all P < 0.001). In this study, the expression of DSPP mRNA increased in the treatment groups, but this increase was not statistically significant compared to the un-treated groups (24 h = 1.12-fold, 48 h = 1.3-fold and P > 0.05).

Fig. 4
figure 4

Quantitative real-time PCR analysis for DSPP, VEGF-A, HLA-G5, VCAM1, RelA and STAT3 genes in DP-MSCs after the treatment with 30 μM PC. Data are mean ± SD of triplicate experiments. *P < 0.05; NS no-significant, PC phytosomal curcumin, DP-MSC dental pulp mesenchymal stem cell, SD standard deviation, DSPP dentin sialophosphoprotein, VEGF-A vascular endothelial growth factor-A, HLA-G5 human leukocyte antigen-G5, VCAM1 vascular cell adhesion molecule-1, RelA nuclear factor-kB p65, STAT3 signal transducer and activator of transcription-3

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Discussion

Pulp regeneration is a new and promising biological strategy in regenerative dentistry for treating the damaged dental pulp with irreversible inflammation. DP-MSCs have been introduced as a prime candidate for the treatment of inflammatory and destructive diseases, including pulpitis [15, 40, 41]. The role of DP-MSCs in pulp regeneration is due to their ability to modulation of immune responses and also multi-differentiation especially into osteo/odontogenic lineages [40, 42]. Change in the gene expression profile of progenitor cells is a widely researched strategy in regenerative medicine. This strategy aims to facilitate cell growth, improve cell survival as well as promote therapeutic effectiveness and stemness status of the cells [27, 40, 43].

The current study was conducted to assess the immunoregulation and repair/regeneration capabilities of DPSCs after treatment with PC on the premise that these cells can potentially be optimized by the nanoparticle. Therefore, DP-MSCs were isolated from pulp tissue using an enzymatic method. The isolated cell biological characteristics including cell surface markers expression and differentiation into various tissues demonstrated that these cells were of mesenchymal stem cell lineage. According to the results of the cell viability test, PC had the time/dose-dependent lethal effects on DP-MSCs. In this study, we performed the real-time PCR analysis on the cells under the condition of treatment with 30 μM of PC.

Our results showed that PC can enhance the expression of DSPP and VEGF-A genes up to many times over, although this enhance was statistically significant only for VEGF-A. In confirmation of the importance of improving these genes by PC, it has been reported that DSPP protein expression and then proteolytic processing of it to little fragments are key steps in dentin formation. So that inhibition of DSPP processing caused hypomineralization defects of dentin in mice and its expression perfectly relieved the dentin weakness in DSPP-deficient mouse model [13].

On the other hand, recent studies have demonstrated that angiogenesis is necessary for maintain pulp homeostasis and survival of transplanted DPSCs to pulp regeneration in vivo and modulation of the immune system in vitro [15, 24, 41, 44]. In this regard, Nakashima and his colleagues [12] introduced DPSC cells as a potential candidate for pulp regeneration and cerebral and limb ischemia treatment due to the high properties of angiogenic and neurogenic.

In accordance with our goals, cell treatment with PC decreased significantly the expression of the RelA molecule which its inhibition may help to improve bone regeneration [45, 46]. More recently, at the ex vivo level, Shuxiang et al. clearly demonstrated that RelA deletion can promote osteogenesis and chondrogenesis in bone marrow-MSCs [23] and it was agreed with the results published by Wang and coworkers [47].

Results of our study also indicated that PC significantly reduces the expression of STAT3, VCAM1 and HLA-G5 genes in DPSCs. In a study conducted by Demircan, it was reported that DP-MSCs exert potent immunoregulatory functions through the increased expression of HLA-G, VCAM1 and VEGF markers in co-culturing with T lymphocyte cells [24]. Although previous studies have shown the role of STAT3 in improving the differentiation and immune modulation potentials of MSCs [25, 26], but similar to our results, it was reported that curcumin exhibits significant effects in promoting the proliferation of neural stem cell (NSC) by down-regulation of STAT3 and glucocorticoid receptor (GR) [48]. In addition, in this regard, Cao et al. suggested that elimination of STAT3 stimulates neural-differentiation in NSCs isolated from mouse embryos [49]. On the other hand, researchers in various research fields have attributed the protective effects of curcumin to the inhibition of signaling pathways associated with STAT3 molecule [50, 51].

A recent article revealed that treatment with liposomal curcumin (CurLIP) causes the improvement in proliferation and immunomodulatory capacities of DPSCs through the negative regulation of NF-kB factor-associated signaling pathways [52]. Another study supported that curcumin therapy can enhance osteo-differentiation in MSCs and optimize them for bone tissue regeneration [53].

Therefore, from these results, it may be inferred that although, PC reduces expression of the HLA-G5 and VCAM1 genes which are important in modulating immunity by MSCs but probably is able to improve stemness capacities of DPSC via up-regulation of DSPP and VEGF-A as well as down-regulation of RelA and STAT3.

Conclusion

These findings reveal that PC can affect the stemness capability of DPSCs in the in vitro level and it may facilitate the development of MSCs-based therapeutics for the degenerative and inflammatory diseases especially maintenance and treatment of inflamed and damaged pulp. However, more studies are required to exactly verify the efficiency PC as a novel strategy to promote tissue regeneration and immunomodulatory properties of MSCs.