Introduction
Periodontal disease, the most common chronic disease in humans, is characterized by inflammation of the supporting tissues around the teeth [
1]. Alveolar bone loss is one of the major hallmarks for disease progression, and combatting bone loss is therefore key to treating periodontal disease [
2]. Normal bone remodeling is a dynamic process maintained by a balance between bone formation and resorption [
3,
4]. In periodontal disease, chronic inflammation disrupts the homeostatic balance between bone formation and bone resorption, in favor of bone loss [
5]. There is evidence that the periodontal ligament (PDL) plays an important role in the remodeling of alveolar bone [
5].
Periodontal ligament cells (PDLCs) are specialized spindle-shaped cells responsible for maintaining the integrity of the ligament that is connecting the tooth root cementum to the alveolar jaw bone [
6]. However, in chronic inflammation (i.e., periodontitis), the phenotype of PDLCs changes, leading to a tissue-destructive response [
7]. Microorganisms (e.g.,
Porphyromonas gingivalis,
Fusobacterium nucleatum, and
Escherichia coli) promote the PDLCs to interact with osteoclast progenitor cells (OPCs), which leads to the differentiation of these OPCs into mature osteoclasts [
8,
9]. During this interaction, PDLCs attract and stimulate the differentiation of OPCs towards mature osteoclasts via expression of adhesion molecules, such as adhesion molecule-1 (ICAM-1), and expression of osteoclastogenesis stimulatory factors, such as RANKL, M-CSF, and tumor necrosis factor-alpha (TNFα) [
10]. In vivo, this leads to retraction of PDLCs and migration of OPCs to the bone surface; where the OPCs mature into bone-resorbing osteoclasts [
11‐
14]. Consequently, the cascade of infection, inflammation mediated by the PDLCs, and osteoclast differentiation presents an important drug target for therapeutic agents aimed at suppressing bone loss.
Over the last decade, there has been an increasing number of studies on the anti-inflammatory effect of specialized pro-resolving mediators (SPMs), and especially of lipoxins (LXs) [
15,
16]. More recently, other effects of LXs have been observed, including the role of LXs in preventing bone resorption [
17]. LXs are endogenous metabolic products derived from arachidonic acid metabolism, which have important pro-resolving and anti-inflammatory properties [
18‐
20]. Lipoxin-type A4 (LXA4) binds specifically to a G protein-coupled N-formyl peptide receptor 2 (FPR2/ALXR) expressed by a variety of inflammatory cells, and which is also expressed in PDLCs [
21]. The binding of LXA4 to its receptor induces a pro-resolving effect mainly by suppressing the expression of inflammatory mediators (e.g., IL-1β and IL-6) via inhibition of multiple signaling pathways, including receptor activator of nuclear factor-κB (NF-κB), which opposes inflammation [
17,
22,
23]. The anti-inflammatory effect of LXA4 is well documented in the literature [
22,
24]. However, the effect of LXA4 on bacterial-induced osteoclastic differentiation during cell-cell contact between PDLCs and OPCs remains elusive.
Given the relationship between inflammation and osteoclast formation and function, we hypothesized that (1) LXA4 would have an inhibitory effect on bacterial-induced osteoclastogenesis, and (2) this effect would be reversed by FPR2/ALXR antagonist, Boc-2. To test this hypothesis, we developed a coculture model derived from a murine RAW264.7 osteoclast cell line, in direct contact with primary human PDLCs in vitro. After induction of an inflammatory phenotype of the PDLC (by the addition of E. coli lipopolysaccharide; LPS), the osteoclastic differentiation was evaluated by means of TRAP staining, TRAP enzymatic activity assay, resorption of a calcium phosphate substrate, and expression levels of osteoclast-specific genes.
Discussion
Previous investigations into the pathogenesis of periodontal disease have typically centered on the role of bacterial infection. However, over the past decade, there has been an increasing interest in the inflammatory response that occurs after infection and which gives rise to osteoclastic bone resorption [
29]. In view of this, it has been demonstrated that PDLCs, although not commonly regarded as inflammatory cells, might also play a role and are contributing to the differentiation of osteoclasts by their interaction with osteoclast progenitor cells [
30‐
32]. In periodontal disease, bacterial colonization alters the phenotype of PDLCs by activating inflammatory signaling pathways, which promote tissue-destructive responses [
7,
33]. Subsequently, osteoclastic differentiation is promoted by cell-cell-mediated interactions between PDLCs and OPCs—a phenomenon facilitated by adhesion molecules (e.g., ICAM) and release of inflammatory factors (i.e., cytokines, chemokines) [
29,
34,
35]. Cell-cell crosstalk between PDLCs and OPCs leads to the migration of TRAP-positive OPCs to the bone surface where osteoclastogenesis occurs, resulting in bone resorption [
29,
34,
35]. In the current study, we aimed to set up a similar model, yet in an in vitro environment, to test novel types of medication, which can aid in the treatment of periodontitis. Therefore, a coculture system was set up, closely mimicking the in vivo situation described above.
The periodontal microenvironment contains a wide diversity of oral pathogens which play an important role in periodontal health or disease. The periodontal biofilm is heterogenous with approximately 7000 organisms [
36,
37]. Although oral pathogenic bacteria, such as
P. gingivalis, are commonly associated with PD, “non-oral” pathogens (e.g.,
E. coli) have also recently been implicated in the pathogenesis of this disease [
38]. Several studies reported on the presence of non-oral bacterial species after microbiological analysis of the subgingival biofilm of PD patients [
39,
40]. Analysis of the subgingival biofilm samples from a large number of untreated chronic PD patients revealed elevated levels of
E. coli compared with non-PD subjects [
39]. Souto and co-workers revealed a positive correlation with clinical signs of PD with bacterial pathogens, including
E. coli,
Staphylococcus aureus,
and Pseudomonas aeruginosa [
40]. In primary PDLCs, an inflammatory phenotype was induced by adding bacterial
E. coli LPS, which in term should trigger osteoclast formation by inducing RANKL expression in osteoblasts and other stimulatory factors (IL-1, prostaglandins, TNFα) leading up to bone resorption [
41]. While oral pathogens are more pathologically relevant bacterial source,
E. coli-derived LPS is more effective at inducing bone resorption in vitro because it is a strong inflammatory agonist for toll-like receptors (TLRs) [
42‐
44] [
45]. Conversely,
P. gingivalis LPS induces a significantly lower expression of inflammatory cytokines (e.g., IL-6) compared with
E. coli LPS [
46]. Considering PDLCs produce a little inflammatory profile,
E. coli LPS is commonly used to trigger an inflammatory immune response [
47]. The subgingival biofilm of PD patients is complex with higher proportion of periodontal pathogens, including lesser known ones, and knowing the effect of different microorganisms is fundamental to our understanding of the complex mechanisms involved in this multifactorial disease.
The mechanism of LPS-induced osteoclastogenesis can be explained as follows. Under inflammatory conditions, LPS-stimulated PDLCs select and attract osteoclast precursors to fuse into osteoclasts by upregulating the expression of osteoclastogenesis-stimulating molecules [
5]. During this interaction, LPS binds to TLR4 receptors on PDLCs and stimulates the expression of pro-inflammatory mediators, TNFα, IL-1, and PGE
2, which play a crucial role in maturation of OPCs and bone resorption [
47]. It is important to note that LPS does not promote osteoclast differentiation in the absence of osteoblasts/stromal cells (e.g., PDLCs) [
48‐
50]. Thus, LPS-promoted osteoclastogenesis can be attributed to the direct cell-cell interaction between PDLCs and RAW cells and not from direct interaction of LPS [
9,
13]. As a control for our coculture, we also stimulated osteoclast formation artificially, as is commonly done in literature, by the addition of RANKL. The similar results between RANKL stimulation, vs. the effects obtained by LPS/PDLC stimulation, indeed make it plausible that the LPS/PDLC route is a valid representation of the actual situation. Moreover, the subsequent experiments with the CaP coating proved that the RAW cells upon such stimulation became actively matrix-degrading osteoclasts.
After setting up and validating the coculture system with the clinical situation, a suitable drug strategy was selected, for which we focused on the use of SPMs. Among the SPMs released during inflammation, LXs are key to the resolution of inflammation [
15,
51]. LXA4 evokes several important protective responses in vivo, including inhibition of neutrophil recruitment, activation, and chemotaxis, via inhibition of several downstream pathways, such as NF-κB, and activator protein-1 (AP-1) [
17,
52‐
57]. Moreover, LXA4 exerts a potent anti-inflammatory action by modulating leukocyte activity and promoting phagocytosis of apoptotic cells [
58]. The binding of LXA4 to its receptor (FPR2/ALXR) interferes with osteoclastogenesis by suppressing the expression of inflammatory mediators (e.g., IL-1β and IL-6) via inhibition of multiple signaling pathways, including receptor activator of nuclear factor-κB (NF-κB) [
59]. Inhibition of NF-κB suppresses the expression of pro-inflammatory cytokines, which counteracts inflammation-induced bone resorption [
60]. The role of LXA4 in inhibition and function of osteoclasts has been recently described in the literature [
17]. However, the role of LXA4 as a modulator of cell-cell-mediated osteoclastogenesis remains elusive [
61]. Given the lack of studies regarding the role of LXA4 in cell-cell-mediated osteoclast differentiation and function and considering the importance of PDLCs in inflammation-induced osteoclastogenesis, we investigated the effect of LXA4 on osteoclastogenesis promoted by LPS/PDLC challenge, as model system. It was hypothesized that (1) LXA4 would inhibit osteoclast differentiation, and (2) this effect could be reversed by the receptor antagonist, Boc-2. Our data confirmed that both of these hypotheses were tested true.
In the experiments, osteoclastogenesis was analyzed using several complimentary morphological, molecular, and functional assays designed to test all aspects of osteoclast formation, activity, and function. Finally, we performed RT-PCR to fully investigate the mechanism underlying the inhibition of osteoclastogenesis. The results from these studies indicated that RANKL stimulation causes an upregulation in osteoclast differentiation from OPCs. Additionally, osteoclast formation was upregulated during cell-cell contact between PDL and RAW cells when conditioned medium containing bacterial LPS was used. The in vitro experiments demonstrated that PDLCs adapt to bacterial stimuli by upregulating the expression of osteoclastogenesis-stimulating genes, resulting in the release of pro-inflammatory mediators (i.e., cytokines and chemokines) that enhance osteoclast activity and function. Comparison of our results with literature corroborates with most of these effects. For instance, Kanzaki et al. (2001) showed that PDLCs cocultured with peripheral blood monocular cells (PMBCs) exhibited significantly more resorption pits than PMBCs cultured alone [
62]. Furthermore, Bloemen et al. (2010) and Burger & Dayer (2002) showed direct that cell-cell contact increased synergistically the expression of osteoclastogenesis genes in vitro [
9,
63]. Similar effects were further demonstrated in vivo by Kim et al. (2005), who reported about the formation of osteoclasts, independent of RANKL signaling pathway, in response to stimulation with inflammatory mediators (TNFα and IL-1β) [
64]. Considering our results in comparison with the available literature justifies the conclusion that PDLCs contribute to enhanced osteoclast formation in periodontal disease. Evidently, PDLCs can play an important role as a drug target when aiming to maintain hemostasis in the periodontium [
5].
Furthermore, the results of the current study showed inhibition osteoclastogenesis in response to inclusion of LXA4 in the differentiation medium. The reduction of osteoclast formation implied a strong protective role of SPMs against inflammation-induced bone resorption, especially as it could be reversed by the addition of a specific inhibitor. Not only the number but also the function of osteoclasts could effectively be modulated; i.e., the decrease in osteoclastogenesis-associated genes was correlated with absence of resorption pits on the CaP-coated substrates. Apparently, LXA4 carries anti-inflammatory capacity after an in vitro encounter with bacterial LPS. In agreement with our data, Liu et al. (2017) showed that LXA4 treatment reduced osteoclast formation in RANKL-stimulated RAW cells [
17]. Combination of our data and the literature information confirms again that that both our initial hypotheses were true.
The study has potential limitations. TRAP activity, function, and osteoclast-specific gene expression was notably lower for LPS/PDLC compared with RANKL-stimulated RAW cells. There are several explanations for this effect. Firstly, LPS may negatively affect osteoclast formation by promoting the expression of pro-inflammatory mediators (TNFα and nitric oxide), which may negatively affect cell viability (even at low concentrations) [
65‐
67]. Secondly, RANKL is a strong inducer of osteoclast formation [
68,
69], while LPS induces formation of osteoclasts independent of the RANKL pathway by activating pattern recognition receptors, such as TLR4, which can affect activation and survival of osteoclasts [
5,
70]. Still, the RANKL stimulation should be considered as an artificial control, whereas the PDLCs route might be more physiologically relevant. Furthermore, murine RAW264.7 monocytes were integrated into the cell culture system because there is currently a lack of a reliable osteoclast model using a human cell line [
71]. While the human monocytic leukemia cell line (THP-1) would have presented a more clinically relevant model, these cells are unable to consistently develop into multinucleated osteoclasts and are also far less responsive to LPS [
72,
73]. Therefore, it can be challenging to extrapolate the data obtained using this human cell line. On the contrary, murine RAW264.7 cells produce a more robust inflammatory response when challenged with bacterial LPS and have been extensively used to carry out in vitro screens for immunomodulators [
74].
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