Pulpitis is an inflammation of the dental pulp tissue primarily caused by dental caries [1, 2]. During the progression of pulpitis, the gram-negative bacteria interacts with the dental pulp through the dentinal tubules, attracting multiple immune cells to the dentin-pulp interface and triggering various inflammatory cytokines and chemokines involved in this process [3–6]. The dental pulp tissue that contains dental pulp stem cells (DPSCs), is frequently invaded by the gram-negative bacteria, which results in pulpitis as a consequence [7]. Lipopolysaccharide (LPS), a major component of the outer membrane of the gram-negative bacteria, has been widely used to investigate the inflammatory response of the cells or tissues and is a potent inducer of pulpitis [8, 9]. Nowadays, the most common treatment method for pulpitis is root canal therapy. The root canal of the affected tooth is cleaned and shaped and finally obturated with the root canal filling materials and sealers [10]. However, the limitation of this method is that it will irreversibly reduce the tooth substance and decrease the fracture resistance. An endodontically treated tooth is devitalized and presents a high risk of biomechanical failure [11, 12].
DPSCs are adult mesenchymal stem cells (MSCs) with high potential of self-renewal and multi-lineage differentiation (including chondrocytes, adipocytes, neural cells, and osteoblasts), which make them an attractive choice for tissue engineering [13, 14]. During the past decades, numerous in-vitro and in-vivo experiments have confirmed that the DPSCs can differentiate into different tissues (including the dentin) under different favorable microenvironmental conditions [15, 16]. Under physiological conditions, the DPSCs can differentiate into odontoblasts and express the dentin-specific proteins DSPP and DMP-1 (Dentin Matrix Acidic Phosphoprotein 1) [17, 18]. Our previous study demonstrated that the TNF-α triggered osteogenic differentiation of the DPSCs via activating the NF-kB pathway [19]. However, many other studies have shown that the odontogenic differentiating ability of the DPSCs decreased in the inflammatory microenvironment, thus affecting the restoration and regeneration of tooth tissue [20]. Therefore, there is an urgent need to explore the underlying mechanism of the impaired odontogenic differentiation capacity of the DPSCs in an inflammatory state, and this knowledge may contribute to the development of more effective clinical treatment strategies.
JunB is a member of the activated protein-1 (AP-1) family, which forms dimeric protein complexes with several protein families, including Jun, Fos, and ATF [21]. It plays a critical role as an activator of transcription. The JunB is involved in the regulation of several basic cellular processes, including cell survival, cell proliferation, senescence, and programmed cell death [22, 23]. Recently, a subset of the MSC transcription factors, including JunB, have been reported to promote osteogenesis and inhibit adipogenesis [24]. Furthermore, the changes in the gene expression profile during the DPSCs differentiation into osteoblastic cells were studied by the microarray technology, and the IGFBP-5, JunB, and NURR1 genes were found to be significantly up-regulated [25]. Another report revealed that the Notch signaling significantly enhanced the cell proliferation but inhibited MSCs osteogenic differentiation induced by the bone morphogenetic protein 9 (BMP 9) via the JunB protein suppression [26]. The JunB severely deficient mice had serious bone problems such as osteopenia and defective endochondral ossification due to the cell-autonomous osteoblast and osteoclast defects [27, 28]. These studies showed that the JunB is widely involved in the osteogenic differentiation of stem cells and the mineralization of bone tissue.
PIN1 is a peptidyl-prolyl cis-trans isomerase that specifically recognizes the phosphorylated Ser/The-Pro motifs and isomerizes only the phosphorylated Ser/Thr-Pro (pS/T-P) peptide bonds [29–32]. The PIN1 regulates diverse cellular processes, including growth-signal responses, cell cycle progression, cellular stress responses, neuronal function and immune responses [33, 34]. The PIN1 has been shown to interact with a number of cancer-related phosphoproteins, which suggested that the PIN1 might link to the signal transduction of the pathogenesis of cancer [31, 35, 36]. Moreover, the PIN1 overexpression is associated with the poor differentiation and survival of oral squamous cell carcinoma [37]. Recently, PIN1 has been reported as a negative regulator of odontogenic differentiation of the DPSCs [38]. However, whether the PIN1 is involved in the regulation of odontogenic differentiation of DPSCs in pulpitis remains elusive.
In this study, we proved that the JunB functioned as a positive regulator of odontogenic differentiation in the DPSCs. The inflammatory state reduced the expression of the JunB, which compromised the odontogenic differentiation ability of the DPSCs. The PIN1 was a negative regulator of odontogenic differentiation of the DPSCs in an inflammatory microenvironment. Our study proved that the JunB could participate in the odontogenic differentiation of the DPSCs through interaction with PIN1 indicating that the JunB and its substrate PIN1 may serve as potential diagnostic markers and/or drug targets for the clinical treatment of pulpitis.