The Biological Efficacy of Natural Products against Acute and Chronic Inflammatory Diseases in the Oral Region

The oral inflammatory diseases are divided into two types: acute and chronic inflammatory diseases. In this review, we summarize the biological efficacy of herbal medicine, natural products, and their active ingredients against acute and chronic inflammatory diseases in the oral region, especially stomatitis and periodontitis. We review the effects of herbal medicines and a biscoclaurin alkaloid preparation, cepharamthin, as a therapy against stomatitis, an acute inflammatory disease. We also summarize the effects of herbal medicines and natural products against periodontitis, a chronic inflammatory disease, and one of its clinical conditions, alveolar bone resorption. Recent studies show that several herbal medicines such as kakkonto and ninjinto reduce LPS-induced PGE2 production by human gingival fibroblasts. Among herbs constituting these herbal medicines, shokyo (Zingiberis Rhizoma) and kankyo (Zingiberis Processum Rhizoma) strongly reduce PGE2 production. Moreover, anti-osteoclast activity has been observed in some natural products with anti-inflammatory effects used against rheumatoid arthritis such as carotenoids, flavonoids, limonoids, and polyphenols. These herbal medicines and natural products could be useful for treating oral inflammatory diseases.


Introduction
Oral inflammatory disease is a general term for the inflammatory lesions developed in oral mucosa. The pathogenesis of oral inflammatory diseases is non-uniform due to the involvement of various factors-such as external and mechanical stimuli, the presence of microorganisms, and the overall physical conditions-that play a role in the onset of inflammation. There is a wide range of variations in the aspect of oral inflammatory diseases, and the aspect is unequal. Therefore, we construed the oral inflammatory diseases as a symptom of inflammation, and categorized them into acute and chronic inflammatory diseases. In the oral region, the representative example of acute inflammatory diseases is stomatitis (also named as oral mucositis), and that of chronic inflammatory diseases is periodontitis. Several Japanese herbal medicines (also known as kampo medicines) are clinically used for the treatment of inflammatory diseases. Recent reviews have summarized the therapeutic application of herbal medicines for oral diseases such as stomatitis and periodontitis [1]. For example, hangeshashinto (TJ-14) is used for inflammatory diseases such as acute or chronic gastrointestinal catarrh, nervous gastritis and stomatitis.
In this review, we aim to summarize the biological efficacy of herbal medicine, natural products, and their active ingredients against acute and chronic inflammatory diseases in the oral region, especially stomatitis and periodontitis.

Stomatitis (Oral Mucositis)
Stomatitis is an inflammatory condition of the oral and oropharyngeal mucosa with both pain and ulcers in severe cases. The causes of stomatitis is classified into (1) bacterial or viral infection, (2) chemotherapy and/or radiation for the treatment of cancers, (3) autoimmune disease (such as lichen planus and pemphigus vulgaris), and (4) unknown (such as recurrent aphthous stomatitis). Recurrent aphthous stomatitis is a common condition characterized by the repeated formation of benign and non-contagious mouth ulcers (aphthae). However, the cause of aphthous stomatitis is still unknown.

Effect of Hangeshashinto on Stomatitis
Recently, clinical administration of herbal medicine, such as the treatment of recurrent aphthous stomatitis, has been increasing in Japan. Herbal medicines are chosen according to the patient's condition, called "sho" (pattern), for example "excess pattern" or "deficiency pattern." Among the herbal medicines, some products such as hangeshashinto , orengedokuto (TJ-15), orento (TJ-120), inchinkoto (TJ-135), byakkokaninjinto (TJ-34), juzentaihoto (TJ- 48), and shosaikoto (TJ-9) are selected in the treatment against oral inflammatory diseases, including recurrent aphthous stomatitis, according to the patient's pattern [2]. In addition, it seems that hangeshashinto is considered effective in the treatment of stomatitis caused by anti-tumor agents and radiation therapy [2].
In a preliminary study, rinsing with hangeshashinto reduced the grade of stomatitis [by Common Terminology Criteria for Adverse Events (CTCAE) version 4.0, National Cancer Institute, Bethesda, MD] [3]. Moreover, in a double-blind, placebo-controlled, random, phase II study, the rinsing of the oral cavity with hangeshashinto showed a trend to reduce the risk of chemotherapy-induced stomatitis in patients with gastric cancer [4]. In this study, hangeshashinto reduced the risk of grade 1 stomatitis but did not reduce those of more than grade 2 [4]. In a retrospective study, rinsing and gargling with hangeshashinto prevented grade 3/4 stomatitis induced by (chemo)radiation in patients with head and neck cancers (odds ratio = 0.21, 95% CI: 0.045-0.780, hangeshashinto: n = 27, placebo: n = 32) [5]. In addition, hangeshashinto also improved the rates of the treatment of stomatitis [5].
In an animal model, free intake of diet mixed with 2% hangeshashinto prevented radiation-induced mucositis within the buccal mucosa in hamsters [6]. In addition, hangeshashinto inhibited the infiltration of neutrophils and COX-2 expression in irradiated buccal mucosa [6]. Moreover, in an in vitro study using oral keratinocytes, hangeshashinto was suggested to be effective in the treatment of chemotherapy-induced stomatitis [7]. As just described, hangeshashinto is effective for the improvement of stomatitis although there is little evidence in in vivo and in vitro studies.

Effect of Cepharanthin R on Stomatitis
A biscoclaurin alkaloid preparation, Cepharanthin R (CE), has also been used for the cure of oral mucosal disease, such as recurrent aphthous stomatitis, leukoplakia, and oral lichen planus. CE is a drug product, prepared from extracts of Stephania cephalantha Hayata, and has been widely used for several decades to treat a range of acute and chronic diseases in Japan [8,9]. As CE is reported to elicit an anti-inflammatory effect and increase blood stem cell count, immuno-enhancing effects, and anti-allergic properties, it has seen clinical application against inflammatory diseases as well as post-radiation-therapy leukocytopenia, pit viper bite, alopecia areata, and bronchial asthma. Nakase et al. reported that the rate of excellent or moderate efficacy was 100% for aphthous stomatitis and 25.0% for reducing the size of oral lichen planus, and its efficacy for glossodynia was 83.4% by CE treatment with gargle-internal use (15 mg/day) for two weeks [10]. Moreover, Saki et al. also reported-regarding the efficacy of CE against these oral mucosal diseases-that the improvement rate by oral administration of CE (20 mg/day) for 4 weeks or more was 83.3% for aphthous stomatitis, 87.0% for oral lichen planus, 77.8% for glossodynia, and 80.0% for leukoplakia. In this case, they evaluated the clinical response and rated according to the assessment points such as the degrees of pain, ulcer, erosion, and erythema [11]. Taken together, it is considered that CE is beneficial in the cure of aphthous stomatitis, according to previous clinical reports [12].
CE is a biscoclaurin alkaloid preparation, and the main active ingredients are four alkaloids: cepharanthine (26%), isotetrandrine (32%), berbamine (13%), and cycleanine (10%) (Figure 1). Using a mixture of these four active ingredients in CE exhibits almost an equal effect as that of CE [13,14]. Functional mechanisms of CE and its main active ingredients for inflammatory diseases have been reported in previous studies. For example, CE reduced the production of superoxide anion (O − 2 ) by neutrophils [15] and by macrophages [16], and decreased the levels of several types of reactive oxygen species (O − 2 , H 2 O 2 , OH · ) by behaving as a reactive oxygen species (ROS) scavenger [17]. According to previous literatures corresponding to the application of the four main active ingredients, cepharanthine was reported to inhibit the synthesis of leukotriene B4 through the reduction of arachidonic acid release [18]. Moreover, each of the four main active ingredients reduced NO production by activated macrophages [19]. However, there was a difference in the efficacy against the O − 2 and TNF-α production among main active ingredients; the efficacy of cepharanthine and isotetrandrine seemed to be more than that of berbamine and cycleanine in the reduction of O − 2 production by neutrophils [20]. It was also reported that cepharanthine, isotetrandrine, and cycleanine, but not berbamine, significantly reduced the level of TNF-α or acute lethal toxicity induced by lipopolysaccaride (LPS) in mice [13,21]. Additionally, Matsuno et al. reported that the decreasing effect of O − 2 production through neutrophil stimulation by arachidonic acid and N-formylmethionine-leucyl-phenylalanine (FMLP) was more evident in cepharanthine than in opsonized zymogen [20]. This finding indicates the cell membrane to be a possible operating point of CE, and this hypothesis is supported in the following study by Sugiyama et al., who reported that cepharanthine could inhibit histamine release from mast cells through the stabilization of the membrane by decreasing membrane fluidity via interaction with the lipid bilayer of the cell membrane [22].
Interestingly, the pharmacological actions of CE on living bodies vary depending on the method of administration. We reported that the single injection of CE reduced the LPS-induced histidine decarboxylase (HDC) activity, although contrastingly, LPS-induced HDC activity in mice spleens increased after consecutive administration of CE [23]. Moreover, it was considered that mast cell was closely associated with this reduction of HDC activity, because LPS-induced HDC activity in mast-cell-deficient mice increased, but decreased in normal mice following a single administration of CE [23]. CE has immuno-enhancing effects as well as anti-inflammatory effects. The inhibition of mast cells may be closely related to the difference of CE action.
Conclusively, CE is considered to be an effective treatment of oral inflammatory diseases, such as recurrent aphthous stomatitis, through the reduction of various function in immunocytes closely related to inflammation.

Periodontitis
Periodontal disease (periodontitis) comprises a group of infections that leads to inflammation of the gingiva and destruction of periodontal tissues, and is accompanied by alveolar bone loss in severe clinical cases. The tissue destruction is the result of activation of the host's immuno-inflammatory response to virulent factors such as LPS and peptidoglycan. In inflammatory responses and tissue degradation, prostaglandin E 2 (PGE 2 ), interleukin (IL)-6, and IL-8 play important roles. As PGE 2 has several functions in vasodilation, the enhancement of vascular permeability and pain, and osteoclastogenesis induction, PGE 2 participates in inflammatory responses and alveolar bone resorption in periodontitis [24].
Generally, periodontitis is a chronic inflammation, and the elimination of these virulent factors by initial preparation is very important for the treatment of periodontitis. However, during the acute advanced stage, non-steroidal anti-inflammatory drugs (NSAIDs) are administrated to improve gingival inflammation. In fact, many studies have demonstrated that systemic administration of acid NSAIDs prevented gingival inflammation and alveolar bone resorption in animals and humans [25]. However, acid NSAIDs are well known to have side effects such as gastrointestinal dysfunction and bronchoconstriction. Therefore, the usage of alternative agents is necessary for patients with gastrointestinal ulcer or bronchial asthma. Previously, we suggested that several herbal medicines are effective for the improvement of periodontitis. In this review, we focused on the anti-inflammatory effects of herbal medicines on mainly periodontitis -in particular, about the effects on human gingival fibroblasts (HGFs). In addition, we summarized the effects of ingredients in herbs and their mechanism against arachidonic acid cascade.
Here, we will explain the importance of HGFs in the study of periodontitis. (1) HGFs are the most prominent cells in periodontal tissue. LPS-treated HGFs produce inflammatory chemical mediators, such as PGE 2 , and inflammatory cytokines such as IL-6 and IL-8. (2) More importantly, unlike macrophages, HGFs continue to produce PGE 2 [26], IL-6, and IL-8 [27] in the presence of LPS. From these findings, the large amount of chemical mediators and cytokines derived from HGFs may be contained in periodontal tissues. Therefore, we believe that examining the effects of pharmaceuticals on HGFs is needed in the study of periodontitis.

Brief Summary of Arachidonic Acid Cascade
At first, we explain arachidonic acid cascade briefly, focusing on sites of action for herbs and ingredients. PGE 2 is produced by arachidonic acid cascade ( Figure 2). Phospholipids in plasma membrane are digested by phospholipase A 2 (PLA 2 ), producing arachidonic acid. Cyclooxygenases (COXs) convert arachidonic acid into PGH 2 , and thereafter PGE synthase converts into PGE 2 . PLA 2 is the most upstream enzyme in the arachidonic acid cascade and releases arachidonic acid from the plasma membrane. PLA 2 forms a superfamily and is classified into cytosolic PLA 2 (cPLA 2 ), calcium-independent PLA 2 (iPLA 2 ), secretory PLA 2 (sPLA 2 ), and others [28]. Among these isoforms, cPLA 2 is the primary isoform in HGFs from the results using PLA 2 inhibitors [29]. cPLA 2 activity is directly regulated by extracellular signal-regulated kinase (ERK). The active form of ERK (phosphorylated ERK) phosphorylates Ser505 of cPLA 2 and activates cPLA 2 [30][31][32]. Therefore, the suppression of ERK phosphorylation leads to the suppression of cPLA 2 activation and the reduction of PGE 2 production [30][31][32]. In contrast, annexin1, also named as lipocortin, is an anti-inflammatory mediator produced by steroidal anti-inflammatory drugs (SAIDs) that inhibits PLA 2 activity [33,34].
COX is classified into COX-1 and COX-2. COX-1 is constitutive expressed at low level, and is involved in normal functions such as protection of gastric mucosa. In contrast, COX-2 is induced by the various stimuli such as LPS and peptidoglycan, and involved in inflammatory response. The expression of COX-2 is upregulated by NF-κB. The reduction of PGE 2 by anti-inflammatory drugs is one of the important mechanisms. Acid NSAIDs inhibit both COX-1 and COX-2 activities. The inhibition of COX-2 improve inflammatory response, while the inhibition of COX-1 causes gastric irritation. SAIDs also have powerful anti-inflammatory effects, and inhibit NF-κB activity and suppress COX-2 expression.
Recently, protein kinase A (PKA) pathway is reported to regulate LPS-induce PGE 2 production in HGFs [35]. PKA inhibitor (H-89) reduced LPS-induced PGE 2 production in a concentration-dependent manner. In contrast, PKA activator (dibutyryl cAMP; dbcAMP) and drugs which increased intracellular cAMP (adrenaline and aminophylline) increased LPS-induced PGE 2 production in a concentration-dependent manner. However, the effects of PKA pathway on arachidonic acid cascade have not been examined in this report [35].
• Shosaikoto (TJ-9) inhibited COX-2 activity and suppressed COX-2 expression, but did not alter cPLA 2 expression (the effects on annexin1 expression and ERK phosphorylation were not examined) [37]. Hangeshashinto (TJ-14) inhibited both COX-1 and COX-2 activities, and suppressed cPLA 2 and COX-2 expressions and ERK phosphorylation [38]. Therefore, these herbal medicines are suggested to inhibit arachidonic acid cascade at multiple points. • Shinbuto (TJ-30) and ninjinto (TJ-32) enhanced annexin1 expression, but did not alter ERK phosphorylation and COX activity [39]. However, the contribution of enhancement of annexin1 expression is considered to be small because shokyo, which is the main herb in shinbuto to reduce PGE 2 production, did not affect annexin1 expression. • Kakkonto (TJ-1) suppressed ERK phosphorylation, but neither inhibited COXs activities nor suppressed the expression of molecules in arachidonic acid cascade [36]. In addition, orento (TJ-120) suppressed ERK phosphorylation, but neither inhibited COXs activities nor suppressed the expression of molecules in arachidonic acid cascade, but rather increased COX-2 expression [40]. However, its contribution in the suppression of ERK phosphorylation is considered to be small as described at keihi (Cinnamomi Cortex). Indeed, we did not examine the direct effect of herbal medicines on cPLA 2 activity. Nevertheless, we consider that these herbal medicines inhibit cPLA 2 activity and that this effect is due to shokyo (Zingiberis Rhizoma) and kankyo (Zingiberis Processum Rhizoma) as described below.

Effect of Herbs on Arachidonic Acid Cascade
Next, we will demonstrate the experimental results at the herb level. The ingredients in the formula of herbal medicines that were used are shown in Tables 2-7. In our experiments at the herb level, shokyo (Zingiberis Rhizoma), kankyo (Zingiberis Processum Rhizoma), kanzo (Glycyrrhizae Radix), and keihi (Cinnamomi Cortex) reduced PGE 2 production (Figures 3 and 4) [29,39]. We summarized major ingredients in herbs and their mechanism against arachidonic acid cascade in Table 8. In addition to these four herbs, ogon (Scutellariae Radix), and oren (Coptidis Rhizoma) are shown in Table 8 because ogon (included in shosaikoto and hangeshashinto) and oren (included in hangeshashinto and orento) also have several bioactive ingredients such as flavonoids, saponin, and chalcones. We will describe the effects and mechanisms of these herbs, particularly shokyo and kankyo, on arachidonic cascade.           [76][77][78] (a) MAP kinases; (b) AMP-activated protein kinase.

Shokyo (Zingiberis Rhizoma)/Kankyo (Zingiberis Processum Rhizoma)
Shokyo is the powdered rhizome of Zingiber offinale Roscoe (ginger), and kankyo is the steamed and powdered rhizome of ginger. Both shokyo and kankyo are the aqueous extracts of ginger. Among the herbal medicines shown in Table 1, shokyo is included in kakkonto (TJ-1), shosaikoto (TJ-9), shinbuto (TJ-30), saireito (TJ-114), and orento (TJ-120), and kankyo is included in hangeshashinto (TJ-14) and ninjinto (TJ-32). Many reports have demonstrated that ginger possesses anti-inflammatory effects in human [80,81] and animal models [82][83][84], and in vitro [85]. Ginger has been widely used in diet and as a treatment for rheumatoid arthritis, fever, emesis, nausea, and migraine headache [80]. A recent systematic review shows that the extracts of ginger including turmeric, ginger, Javanese ginger, and galangal are clinically effective as hypoanalgesic agents [81]. In an animal model, the aqueous extract of ginger significantly reduced serum PGE 2 level by oral or intraperitoneal administration in rats [82]. Moreover, crude hydroalcoholic extract of ginger reduced the serum level of PGE 2 , and improved tracheal hyperreactivity and lung inflammation induced by LPS in rats [83]. Ethanol extract of ginger reduced the tissue level of PGE 2 , and improved acetic acid-induced ulcerative colitis in rats [84].
Gingerols and shogaols inhibit PLA 2 activities [57]. In more detail, iPLA 2 activity was inhibited by 6-, 8-, and 10-gingerol and 6-, 8-, and 10-shogaol, whereas cPLA 2 activity was inhibited by 6-gingerol and 6-, 8-, and 10-shogaol. In particular, IC 50 values of 10-shogaol against iPLA 2 and cPLA 2 were 0.7 µM and 3 µM, respectively, in U937 cells. As aforementioned, many reports have examined the effects of ginger. However, there is little report using ginger as "shokyo" and "kankyo." For this reason, we examined the mechanism of the actions of shokyo and kankyo on the reduction of PGE 2 production in HGFs. Shokyo and kankyo concentration-dependently reduced LPS-induced PGE 2 production by HGFs, and the effects of kankyo were slightly stronger than those of shokyo ( Figure 4) [39]. The effects of shokyo and kankyo on arachidonic cascade in HGFs are described as follows.

•
Both shokyo and kankyo only slightly increased cPLA 2 expression, and did not alter annexin1 expression [39]. • Shokyo did not alter LPS-induced ERK phosphorylation in HGFs [29] (but we have not examined the effect of kankyo). Therefore, shokyo (and perhaps kankyo) may have little to no effect on cPLA 2 activation, and the subsequent arachidonic acid production.

•
Both shokyo and kankyo did not inhibit COX-2 and PGE synthase activities, and did not alter LPS-induced COX-2 expression in HGFs [29,39]. These findings suggest shokyo and kankyo primarily inhibit cPLA 2 activity in HGFs. Although we have no direct data to show that shokyo and kankyo inhibit cPLA 2 activity, this assumption is consistent with the results that ginger (and gingerols/shogaols) inhibits both iPLA 2 and cPLA 2 activities [57].
As described above, our data that shokyo did not alter COX-2 activity and COX-2 expression are different from those of gingerols and shogaols in Table 8. Although there is no obvious evidence, the reason may be the preparation method of shokyo and kankyo. Gingerols and shogaols are extremely hydrophobic by their structures. These ingredients were extracted from hydrophobic phase, whereas shokyo and kankyo were prepared by decoction. Therefore, hydrophobic ingredients such as gingerol and shogaol are unlikely to be extracted, and their concentration might be lower than those in previous reports. Quantification of these ingredients is needed to explain these discrepancies.
Licorice contains triperpene saponin such as glycyrrhizin (glycyrrhizin acid), and chalcones such as liquiritin and isoliquiritigenin. Their structures are indicated in Figure 6. Glycyrrhizin, liquiritin, and isoliquiritigenin are reported to reduce PGE 2 production. The effects of these ingredients on arachidonic acid cascade are briefly summarized in Table 8.
We examined the mechanism of the action of kanzo on the reduction of PGE 2 production in HGFs. However, the effects of kanzo on arachidonic acid cascade in HGFs cannot be explained by those of glycyrrhizin, liquiritin, and isoliquiritigenin.
Therefore, these effects of kanzo were different from those of glycyrrhizin, liquiritin, and isoliquiritigenin, suggesting that other ingredients may contribute to our findings. In addition, not all herbal medicines which contain kanzo increased annexin1 as kakkonto, hangeshashinto, and orento did not alter annexin1 expression.

Keihi (Cinnamomi Cortex)
Keihi is the powdered bark of Cinnamomum cassia (cinnamon). Among the herbal medicines shown in Table 1, keihi is included in kakkonto (TJ-1), saireito (TJ-114), and orento (TJ-120). Cinnamon has been widely used for the treatment of fever and inflammation [28]. Cinnamon improves nephritis, purulent dermatitis, and hypertension, and it also enhances wound healing. Cinnamon extracts have been used for the improvement or prevention of common cold, diarrhea, and pain [28]. Ethanol-extract of C. cassia reduced LPS-induced PGE 2 production by RAW264.7 cells, and it suppressed NF-κB activity and the following COX-2 expression [66].
Keihi contains the ingredients such as cinnamic aldehyde, cinnamic alcohol, cinnamic acid, and coumarin. The structure of cinnamic aldehyde is indicated in Figure 6. Cinnamic aldehyde is reported to reduce PGE 2 production. The effects of cinnamic aldehyde on arachidonic acid cascade are briefly summarized in Table 8.

4.
Cinnamic aldehyde inhibited TLR4 oligomerization and downstream signal pathway, which include NF-κB. Sulfhydryl modification is suggested to be an important contributing factor for the regulation of TLR4 activation [69].
We examined the mechanism of action of keihi on the reduction of PGE 2 production in HGFs. However, the effects of keihi on arachidonic acid cascade in HGFs cannot be explained by that of cinnamic aldehyde.

•
Keihi inhibited COX-2 activity because keihi decreased LPS-induced PGE 2 production when arachidonic acid is added [29]. This mechanism is accounted for by that of cinnamic aldehyde. In contrast, keihi did not inhibit PGE synthase activity as well as kanzo.

•
As well as kakkonto [81] and orento [40], keihi suppressed ERK phosphorylation in LPS-treated HGFs [29], leading to inhibit cPLA 2 activation. However, the contribution of suppression of ERK phosphorylation is considered to be small because the ability of keihi to decrease LPS-induced PGE 2 production was weak ( Figure 3).
Therefore, these effects of keihi are different from that of cinnamic aldehyde, suggesting that other ingredients may contribute to our findings.
The major ingredients of ogon are flavonoids such as baicalin, baicalein, and wogonin. Their structures are indicated in Figure 7. Baicalin is the glucuronide of baicalein and is an inactive form. Administered baicalein is metabolized to baicalin, which is an active form. Baicalin, baicalein, and wogonin reduce PGE 2 production in human oral keratinocytes [7] and RAW264.7 cells [94].
Berberine, one of benzylisoquinoline alkaloid, is the major ingredient of oren. The structure of berberine is indicated in Figure 7. Berberine is reported to reduce PGE 2 production. The effects of berberine on arachidonic acid cascade are briefly summarized in Table 8.

Conclusion about Herbal Medicines and Herbs
We have described the effects of herbal medicines, herbs, and their ingredients on arachidonic acid cascade in this review. Several herbal medicines show reduced LPS-induced PGE 2 production by HGFs. These results suggest that these herbal medicines may be effective in the improvement of the inflammatory symptoms in periodontitis. Herbal medicines must be properly selected by the patterns of each patient -excess patterns, medium patterns, or deficiency patterns. Among the herbal medicines in our studies, kakkonto (TJ-1) and orento (TJ-120) are used for the patients with excess patterns. Shosaikoto (TJ-9), hangeshashinto (TJ- 14), and orento are used for the patients with medium patterns. Shinbuto (TJ-30) and ninjinto (TJ-32) are used for the patients with deficiency patterns. Therefore, it may be possible to use appropriate herbal medicines to patients with any pattern.
As shown in the above-mentioned descriptions, not all effects of herbal medicines are explainable by the effects of herbs constituting herbal medicines. Similarly, not all effects of herbs are explainable by the effects of ingredients contained in herbs. Experiments using "herbal medicines" or "herbs" themselves may be important rather than those using ingredients. The concentrations of these hydrophobic ingredients may also be low because the herbs that we used are water-soluble fractions. Therefore, it is considered that the concentrations of their ingredients need to be measured. Moreover, the unanalyzed ingredients other than those explained in this review are likely to be present. It is to be desired that further analyses reveal the novel ingredients and their action of mechanisms.

Anti-Osteoclastogenic Effects of Natural Products
Like periodontitis (PD), rheumatoid arthritis (RA) is a disease associated with inflammation and bone destruction. Although therapeutics of RA have recently advanced with the development of antibody drugs, natural substances displaying anti-inflammatory and anti-osteoclast characteristics against RA are still being used as widely as they have been in the past.
Some studies have revealed a relationship between PD and RA. RA prevalence is increased in patients with PD [98,99]. The presence of PD may contribute to the progression of RA; that is, RA patients with PD receiving non-surgical periodontal treatment resulted in a noteworthy improvement in the clinical outcome for RA [100]. From the aspect of the clinical marker, RA and PD are similar in cytokines and mediators involved in inflammation and bone destruction [101]. For example, TNF-α, receptor activator of nuclear factor-κB ligand (RANKL), and matrix metalloproteinase (MMP) family increase in production in RA and PD [102][103][104][105][106]. Due to these similarities, natural products used for RA are probably effective for PD.
The structures of natural products described in this review are indicated in Figure 8. Epidemiological studies have revealed a positive correlation between bone health and increased consumption of fruits and vegetables [107,108]. Some fruits and vegetables contain components that inhibit both inflammation and osteoclast activity.
β-Cryptoxanthin is a carotenoid present in a wide range of citrus fruits and in Diospyros kaki Thunb., Physalis alkekengi L., etc. β-Cryptoxanthin has a potent inhibitory effect on osteoclast-like cell formation in mouse marrow culture [109]. Moreover, in a mouse model of PD, β-cryptoxanthin suppressed bone resorption in the mandibular alveolar bone in vitro and restored alveolar bone loss induced by LPS in vivo [110].
Naringenin is a flavonoid contained in citrus fruits such as oranges and grapefruits. Accumulating evidence has suggested that naringenin modulates chronic inflammation [111]. In a murine model of collagen-induced arthritis, naringenin inhibited pro-inflammatory cytokine production by decreasing MAPK and NF-κB signaling activation [112]. La et al. showed naringenin thus holds promise as a therapeutic and preventive agent for bone-related diseases such as PD [113]. Thus, there are cases in which components demonstrating anti-osteoclast behavior are demonstrated to be effective against PD. In addition to naringenin, citrus fruits contain components that suppress osteoclast activity via MAPK. Nomilin, a limonoid present in citrus fruits, displays inhibitory effects on osteoclastic differentiation through the suppression of MAPK signaling pathways [114].
Ellagic acid is a polyphenol contained in berries, pomegranates, nuts, etc. Ellagic acid has an anti-inflammatory effect in various organs such as the liver, stomach, small intestine, and skin [115][116][117][118]. Moreover, ellagic acid has anti-osteoclast activity and significantly reduced serum levels of pro-inflammatory cytokines, TNF-α, IL-1β, and IL-17 in RA model mice [119]. A recent study supported the traditional use of Geum urbanum L. root contained ellagic acid derivatives in cavity inflammation including mucositis, gingivitis, and PD [120].
Additional useful components against both RA and PD have been found in tea. (-)-Epigallocatechin-3-gallate (EGCG) is a major catechin derivative present in green tea. Previous studies have also suggested that EGCG decreases MMP-1, MMP-2, and MMP-3 production by RA synovial fibroblasts, thereby preventing further cartilage and bone destruction [121,122]. Moreover, it has been reported that EGCG selectively inhibited IL-1β-induced IL-6 synthesis in RA synovial fibroblasts and suppressed IL-6 trans-signaling via upregulation of an endogenous inhibitor, a soluble gp130 [123]. Clinical study of EGCG suggested that local drug delivery utilizing green tea extract could be used as an adjunct in the treatment of chronic PD [124].
Traditional medicine in Ayurveda also presents useful teas against RA and PD. Salacia reticulata Wight is a plant native to Sri Lanka that has been used for the prevention of RA, gonorrhea, and skin disease. We previously reported that leaf of S. reticulata alleviates collagen antibody-induced arthritis in RA model mice [125]. S. reticulata contains a polyphenol known as mangiferin that inhibits osteoclastic bone resorption by promoting ERβ mRNA expression in mouse bone marrow macrophage cells [126].
In conclusion, natural products displaying both anti-inflammation and anti-osteoclast characteristics are suggested to be useful for the prevention and treatment of PD.
Author Contributions: C.S. conceived, designed, proved, and edited the whole review manuscript. N.S. and C.S. wrote "Introduction" and the topic of "Biological Efficacy of Natural Products against Acute Inflammatory Disease: Stomatitis", T.A. wrote the topic of "Biological Efficacy of Natural Products against Chronic Inflammatory Disease: Periodontitis", S.N. and K.K. wrote a second half part of the topic "Anti-Osteoclastogenic Effects of Natural Products".

Funding:
The work was supported in part by funding from JSPS KAKENHI Grant Number JP16H05144, the Nagano Society for the Promotion of Science, and a Scientific Research Special Grant from Matsumoto Dental University.