Background
Currently, impaired alveolar bone healing in diabetic patients still remains a great problem in dentists’ clinical work, and it can impede the implementation of different oral disease treatments, such as tooth extraction and dental implant surgery. These patients often exhibit high blood glucose, which has detrimental effects on alveolar bone repair [
1,
2]. Although the exact cellular mechanisms of this impaired healing are unclear, high levels of proinflammatory cytokines, like interleukin-1β (IL-1β), in periodontal tissues under the condition of hyperglycemia are considered to be potentially important contributors [
1]. These cytokines can exacerbate inflammation and subsequently inhibit tissue formation.
NLRP3 (NACHT [nucleotide-binding oligomerization], LRR [leucine-rich repeat], and PYD [pyrin domain] domains-containing protein 3) plays a crucial role in inflammatory cytokine production and inflammation response [
3]. It assembles a multi-protein complex called inflammasome, which is composed of NLRP3, apoptosis-associated speck-like protein containing a CARD (ASC), and caspase-1 [
4]. Upon activation, NLRP3 recruits ASC by pyrin domain interactions and subsequently recruits procaspase-1 through CARD-CARD interactions [
5,
6]. Then, procaspase-1 autocatalyzes its cleavage and results in the maturation of the precursor forms of IL-1β, leading to the secretion of other different proinflammatory cytokines [
7]. Overexpression of NLRP3 inflammasome has been found in various inflammatory diseases, including delayed soft tissue wound healing and periodontitis under diabetic conditions [
7,
8]. These findings suggest the inhibition of NLRP3 inflammasome may promote alveolar bone repair in diabetes.
In this work, to detect the effect of NLRP3 inflammasome suppression on the alveolar bone healing under the diabetic condition, we established a diabetic rat model of alveolar bone defect, applied a lentiviral short hairpin RNA (shRNA) targeting NLRP3 in the defect, and then examined the mRNA and protein expression of NLRP3 inflammasome, proinflammatory cytokine IL-1β, and bone formation-related factors in the bone healing site.
Discussion
The mechanisms of diabetes-related impaired wound healing in soft tissues have been extensively explored [
7,
17,
18], but much remains unknown on delayed alveolar bone healing under diabetic conditions. In this work, we observed that diabetic rats showed a significantly impaired alveolar bone defect healing, compared with their normal controls. Additionally, the expression of NLRP3 inflammasome was enhanced in the defect sites of diabetic rats. After the treatment of lentiviral shRNA targeting NLRP3, the activation of NLRP3 inflammasome was reduced, and the bone repair was improved. These results suggest that the delayed healing of alveolar bone in diabetes was closely correlated with elevated activation of NLRP3 inflammasome in the defect sites. An experiment on diabetic mice has demonstrated upregulation of proteins in the NLRP3 inflammasome in skin wound tissues and accelerated healing after decreasing the activity of the inflammasome [
7]. Another study on diabetic animals has also shown increased NLRP3 inflammasome activity in tooth extraction sockets, which is associated with delayed wound healing around sockets [
19]. Upon blocking the NLRP3 inflammasome by Ac-YVAD-cmk, a specific caspase-1 inhibitor, impeded mucosal healing with bone necrosis around the extraction sockets can be markedly improved [
19].
Increasing evidence has shown that the activation of NLRP3 inflammasome cascade plays a crucial role in sustained inflammation in different inflammatory diseases, including atherosclerosis, chronic obstructive pulmonary disease, and diabetes [
20‐
22]. NLRP3 activation can cause pro-caspase-1 cleavage and mature IL-1β release [
6,
7]. Subsequently, NLRP3-dependent IL-1β secretion induces immune cells, such as macrophages, to become more proinflammatory and to release a greater amount of proinflammatory cytokines like tumor necrosis factor-α and interleukin-6 [
23,
24]. Then, these proinflammatory cytokines amplify inflammatory responses in the diseased site, resulting in exacerbated tissue damage or impaired tissue regeneration [
25]. In the current study, we also demonstrated the high expression level of IL-1β in the alveolar bone defect in diabetic mice, in comparison with normal control mice. Upon NLRP3 shRNA treatment, the reduced expression of IL-1β was observed accompanied with improved healing. These findings implicate a key role of NLRP3-induced IL-1β production in diabetes-associated impairment of alveolar bone defect healing. Several studies on bone diseases have shown the adverse effects of IL-1β production induced by NLRP3 inflammasome activation on the bone. NLRP3 inflammasome-caused IL-1β expression has been found positively associated with the development of bisphosphonate-related osteonecrosis in diabetes [
19]. High expression levels of NLRP3 inflammasome and IL-1β deteriorate osteoporosis under estrogen deficiency by inhibiting osteogenic differentiation [
26]. Moreover, the suppression of IL-1β expression by blockade of NLRP3 inflammasome activity can repress the progression of these two bone diseases [
19,
26]. In periodontal diseases, upregulation of IL-1β protein and NLRP3 inflammasome has also been demonstrated in periodontal tissues of uncontrolled type 2 diabetic patients with periodontitis, who exhibit more alveolar bone damage, compared with nondiabetic periodontitis patients [
8].
Bone repair is a process involving proliferation and differentiation of different osteogenic cells [
27]. Several biological factors, such as RUNX2 and OCN, are expressed during the process of bone healing, and the alteration of their expression levels can affect the progression of bone restoration [
28]. In this work, we observed that the expression levels of RUNX2 and OCN were reduced in the alveolar bone defect in diabetic mice, compared with the normal controls. However, the expression levels of these factors in diabetic mice were elevated after the treatment of shRNA targeting NLRP3. These findings further demonstrate the improvement of bone healing by the inhibition of NLRP3 inflammasome and IL-1β (Additional file
1: Figure S1). RUNX2 has been known as a critical transcription factor in osteoblast differentiation [
29], and it is located upstream to many other osteogenic factors including OCN [
30]. OCN is a bone-specific protein synthesized by osteoblasts and is vital in the formation of hydroxyapatite crystals and the progression of mineralization [
31]. Previous studies have shown that the repression of RUNX2 and OCN expression is associated with the reduction of bone formation [
32,
33]. IL-1β has been reported to stimulate proliferation of osteoblasts and production of mineralized bone matrix [
34], but high concentrations of this cytokine suppress osteogenesis [
35]. Additionally, IL-1β can stimulate the secretion of tumor necrosis factor-α and interleukin-6 by immunoregulatory cells and osteoblasts, subsequently inhibiting bone formation [
25]. Reduced Runx2 expression and impaired osteogenic differentiation are observed in preosteoblastic cells after excessive IL-1β treatment [
36]. Sustained IL-1β expression accompanied with delayed OCN production and osteoblast differentiation are also found in periodontal bone tissues around dental implants under diabetic conditions [
1]. These published reports also implicate that the impaired bone formation may be correlated with the impeded osteogenic gene expression caused by IL-1β overexpression.
Acknowledgements
The authors would like to acknowledge all the personnel from Experimental Center for Medical Sciences, Guangxi Medical University, and State Key Laboratory of Oral Diseases, West China Hospital of Stomatology, Sichuan University, for their technical support.