Dental pulp is the soft tissue inside tooth, which plays an indispensable role in the homeostasis of vital teeth [1]. The function of dental pulp is to support dentin formation and regeneration [2]. A range of injuries or diseases including caries, pulpitis, periapical periodontitis, tooth trauma, etc. can result in pulp necrosis, further teeth losing [3]. For therapeutic strategies for the kind of teeth above, vital pulp therapy (VPT) and regenerative endodontic treatment (RET) attracted recent attention to remain functional of teeth. The aims of VPT are to preserve and save dental pulp vitality, to induce dental pulp stem cells (DPSCs) to differentiate into osteoblasts and odontoblasts and ultimately form the hard tissue such as the tertiary dentin. RET known as revascularization aims to promote normal physiological development in teeth with necrotic pulp and act as a substitute for injured dental structures. DPSCs have been regarded as an important candidate for such treatment. Their capability of differentiation into odontogenic and osteogenic stem cells associated with biomaterials and growth factors is critical for dental pulp regeneration [4, 5]. Mesenchymal stem cells (MSCs) have a significant role in pulp regeneration therapy for the reconstruction of tissues. Due to the easily available source of dental tissues, dental stem cells are considered good candidates for tissue engineering applications.
Human MSCs are a serviceable therapeutic tool for tissue engineering. DPSCs are identified as a type of MSCs that were originally isolated from human dental pulp tissue [6]. DPSCs have a lot of merits of easy access with the least invasive procedures and without any ethical controversy, and retain capacities of clonogenic formation, high proliferation, excellent regeneration, multilineage differentiation potential, and little inherent immunogenicity. As has been widely acknowledged, DPSCs can be induced to differentiate into a good many of directions, including osteogenic, dentinogenic, adipogenic, chondrogenic, myogenic, and neurogenic differentiation [7]. Compared with other MSCs from tissues as follows: bone marrow [8], peripheral blood [9], adipose tissue [10], and umbilical cord blood [11, 12], DPSCs demonstrate higher clonogenic and proliferative potential. Therefore, DPSCs become an engaging tool cell source for tissue engineering and regenerative medicine.
During the developmental process of MSCs, extracellular cues tend to exert its function in determining the fate of MSCs. Previous studies have revealed that in chronic inflammatory bone diseases, bone regeneration can be inhibited, and the osteogenic differentiation of DPSCs can be influenced by inflammatory microenvironments [13]. Interleukin 1 family member 7 (IL-1F7), a novel anti-inflammatory cytokine was recently proposed to be renamed Interleukin-37 (IL-37) [14]. IL-37 functions as a natural inhibitor of inflammatory and immune responses [15]. IL-37 is an anti-inflammatory factor that affects other pro-inflammatory signals, such as those mediated by tumor necrosis factor α (TNF-α), IL-1β, and IL-18 [16]. It was reported that IL-37 can promoted BMMSCs to differentiate into osteogenic lineage cells in vitro [17]. Recent studies have shown that abnormal expression of IL-37 in several autoimmune orthopedic diseases, such as ankylosing spondylitis and rheumatoid arthritis [18]. Most recently, studies demonstrated a connection between IL-37 and several bone metabolism-related inflammatory cytokines and reported that recombinant IL-37 (rhIL-37) inhibited the expression of pro-inflammatory cytokines, such as IL-6, TNF-α, IL-17, and IL-23 in patients with ankylosing spondylitis [19]. The new study has indicated that IL-37 suppresses osteoclast formation and bone resorption in vivo [20].
A better understanding of the molecular mechanisms that govern odontogenesis and osteogenesis might provide us with new perspectives on treatment of many oral diseases. Recent reports have provided evidence that macromolecular degradation in stem cells in a process of differentiation occurs through autophagy [21]. Autophagy or “self-eating”, is a conservative cellular degradation pathway that recycles cellular content [22]. Upon activation of autophagy, degradation of intracellular protein and organelles via a process that involves the delivery of cytoplasmic cargo to lysosomes and release of metabolites required for anabolic processes, such as cell growth, proliferation and differentiation [23]. When cells are subjected to external stress, such as nutrition deficiency, oxidative stress, hypoxia, tumor formation, aging or infection, autophagy plays an important role as a cell survival mechanism [24, 25]. Recent studies have indicated that autophagy is an essential part in the functioning and maintenance of stem cells, acting to maintain their stemness, regulate their self-renewal, and mediate their differentiation capacity [26, 27] [28]. In addition, accumulating evidence has demonstrated that autophagy is also involved in osteogenesis and bone development [29, 30]. Autophagy is also an essential process that maintains mineralizing capacity, and balance the population of osteoblasts and osteoclasts [31]. The members in our team have demonstrated that autophagy is involved in the odontogenic and osteogenic differentiation in stem cells of apical papilla and rBMMSCs [32, 33].
We hypothesis that autophagy might be involved in IL-37-mediated DPSCs osteogenic and odontogenic differentiation. Therefore, the aim of the present study was to verify whether IL-37 is able to promote the osteogenic and odontogenic differentiation of DPSCs and whether autophagy was involved in the regulation of osteo/odontogenic differentiation of DPSCs in vitro.