Clinical Applications of Catechin in Dentistry : A Review

Studies on plant and food phytochemistry and its potential benefits to human health are becoming the focus of the research community. Researchers are turning to alternatives drugs in treating human diseases using natural products from plants and foods. Polyphenols are one of the largest groups in the plant family and consist of many subgroups. One of them is catechin, which is generally acknowledged to be part of a compound in tea. Over the years, investigations have shown that catechin has anti-oxidant, anti-inflammatory and antibacterial properties. In dentistry, documented evidence have shown the use of catechin in treatment of dental caries, periodontal disease, pulp pathology, and oral cancer. Other crucial areas of research include advancements in dental material incorporated with catechin. This review article explores the current studies on the potential use of catechin in dentistry.


Introduction
The role of natural products in the pharmaceutical industry is undeniable. Prior to commercialization of drugs, people from ancient civilizations have used plant-based extracts to cure certain illnesses 1 . The Egyptians have documented the use of more than 1000 extracts derived from plants such as the oils from Cedrus species (cedar) 2 while Hippocrates has described the development of an anesthetic using the extract from Atropa belladonna 3 . The modernization and evolution in the fields of medicine and chemistry provide better insights to the mechanism of action of the natural products, thus providing a better platform for the researcher to find and develop products based on the extracts from plants.

Abstract
Studies on plant and food phytochemistry and its potential benefits to human health are becoming the focus of the research community. Researchers are turning to alternatives drugs in treating human diseases using natural products from plants and foods. Polyphenols are one of the largest groups in the plant family and consist of many subgroups. One of them is catechin, which is generally acknowledged to be part of a compound in tea. Over the years, investigations have shown that catechin has anti-oxidant, anti-inflammatory and antibacterial properties. In dentistry, documented evidence have shown the use of catechin in treatment of dental caries, periodontal disease, pulp pathology, and oral cancer. Other crucial areas of research include advancements in dental material incorporated with catechin. This review article explores the current studies on the potential use of catechin in dentistry.
The extracts of Gingko biloba, for example, have been used in various herbal medicinal products since it has been proven to have anti-oxidant and memoryenhancing effects 4,5 . In dentistry, the extract of propolis and miswak are added to toothpaste as studies have proven that these compounds have antibacterial effects 6,7 and promote gingival tissue health.
Another potential natural compound to be explored in dentistry is catechin. Catechin is a secondary plant metabolites, which is a flavonoid. It can be found in abundance in the human diet and plants, such as green tea, cocoa and beans. Chemically, catechin is featured as a compound with two benzene rings ( Ring A and B) and a heterocyclic Ring C in between them. A hydroxyl group at position 3 of Ring C which leads to catechin also to be known as flavan-3-ol 8 (Figure 1). Catechin and epicatechin (EC), another derivative of flavan-3-ol with different chemical configurations, as a monomer can form an oligomer called proanthocyanidins (PAC) 9 . Esterification of this monomer with gallic acids produce epigallocatechin (EGC), epicatechin gallate (ECG) and epigallocatechin gallate (EGCG) 10 .
Over the years, reports from the in vitro and in vivo studies in the literature have proven the health benefits of catechin, mainly of its antioxidant and antiinflammatory effects 11 . Both of these effects contribute to the potential usage of catechin in the treatment of certain illnesses. In cardiovascular disease, for example, catechin from red wine have platelet-inhibitory effects 12 while EGCG in green tea increases the blood level of nitric oxide (NO) and subsequently reduces vascular inflammation 13 . This, in turn, minimizes the individual risk of developing a cardiovascular-related disease such as hypertension and myocardial infarct. Owing to these facts, catechin has been increasingly used in foods and supplements for health purposes.
As a result of earlier studies on its biological properties, studies in the dental area have explored the potential application of catechin compounds. This review article aims to explore the current research on the usage of catechin and its derivatives in the field of dentistry.

Source of Catechin
Catechin can be found in our normal diet especially in fruits, vegetables, tea, and wine. Fruits such as plum, apple, peach, strawberry, and cherry are reported to have an abundance of catechin. The oligomeric form of catechin like proanthocyanidins is present in grape and berries mainly in their skins and seeds, while epicatechin is high in apple, cherry, and black grape.
In vegetables, catechin and its derivatives are highly found in grains and legumes compared to leafy vegetables. Catechin and procyanidins make up almost 70% of the total phenolic compound found in cranberry beans and lentils 14 . Pinta bean was reported to have high concentrations of catechin 15 while raw cranberry beans are enriched with both catechin and proanthocyanidins 16 .
Also, several studies have shown that tea contains catechin. Green, black and oolong teas are reported to have a high level of catechin. Green tea comprises of 60 to 90% of total flavonoids while a lower percentage between 6 to 24% was found in black tea 17 . The bioactive components of tea include catechin, epicatechin, epigallocatechin (EGC), epigallocatechin gallate (EGCG) and epicatechin gallate (ECG) 18 .
Catechin can also be found in wines, particularly the red ones. This is owing to the use of grapes in the winemaking industry. The quality of red wine, determined by its astringency and bitterness are attributed to the presence of proanthocyanidins, a polymeric form of catechin and epicatechin 19 . However, the phenolic structure of wines differs from each other, depending on the type of grapes, how they are grown, and also the techniques used during the winemaking process 20 .
Besides tea and wines, cocoa also contains catechin and epicatechin. Both these compounds were isolated from both unroasted and roasted cocoa beans. Factors like high temperatures and alkalization of cocoa powder during the manufacturing process play a crucial role in this conversion 21 . Interestingly, dark chocolate, a product made from cocoa powder also has been reported to be rich in catechin and epicatechin 21,22 .

Clinical Application of Catechin in Dentistry
Catechin and its derivatives possess antiinflammatory, antioxidant and antimicrobial effects which are beneficial for human health. These properties trigger the researchers to further investigate the potential application of catechin in dentistry. Numerous studies in the field of dentistry have indicated the beneficial effects of catechin, mainly EGCG in the treatment of oral diseases. Tables 1 and 2 summarise the studies related to the use of catechin in dentistry.

Dental Caries
Tea contains an abundance of catechin. One of them is EGCG, which possesses strong bactericidal activity 23 .   24,25 . It is found that catechin from the tea extract can damage the gram-positive bacterial cell membrane by binding directly to its lipid bilayer. However, it is less efficacious against gram-negative bacteria due to the presence of the negatively charged lipopolysaccharides (LPS) on its outer membrane 26 .
Virulence factor consists of protein metabolites produced by the bacteria which enables it to invade and cause damage to the host 27  mutans, for example produces glucosyltransferase (GTF), one of the virulence factors that leads to the production of intracellular polysaccharides (IPS) and extracellular polysaccharides (EPS). EPS helps in initial adherence of Streptococcus mutans and other oral bacteria on the tooth surface and forms mature dental plaque biofilm 28,29 . Apart from GTF, other virulence factors produced by Streptococcus mutans include the membrane-bound F1-F0 ATPase system and the enzyme enolase and lactate dehydrogenase 30 . EGCG hampers the effects of the virulence factors produced by Streptococcus mutans at both transcriptional and enzymatic levels, leading to reduced acidogenicity and stress tolerance of the bacteria 31 . EGCG also inhibits the action of the membrane-bound ATPase and lactate dehydrogenase (LDH) enzymes, affecting the sugar transport and acid secretion of the bacteria 32 . In addition, EGCG can suppress the GTF expression in Streptococcus mutans, therefore, inhibiting the cell adherence ability of Streptococcus mutans and reduce its biofilm production 33 . Other forms of catechin like proanthocyanidins (PAC) in cranberry extract was also found to cause a reduction in the biofilms formation and subsequently minimizes the risk of caries development both in vitro and in vivo studies 34 .
Clinical studies also proved the efficacy of catechin-rich products in the prevention of dental caries. The use of a tea-based 35 and catechin 36 mouth rinse showed a remarkable reduction in the number of Streptococcus mutans and Lactobacilli ssp. colony respectively. While the incorporation of green tea extract in chewing gum reduced the DMFS (decayed, missing, and filled surfaces) score 37 .

Periodontal Disease
Periodontitis is the disease of the tooth-supporting tissue, which involves inflammation and sometimes infection of the gingiva, periodontal ligament, and alveolar bone 38 . The ability of catechin to exert an effect on the periodontal pathogen will be beneficial in preventing the occurence and treating periodontal disease. The majority of the studies available have investigated the effects of catechin, mainly in the form of EGCG on Porphyromonas gingivalis-the principle reason in the development of chronic and aggressive periodontitis.
Porphyromonas gingivalis produces a variety of virulence factors that can penetrate the gingiva and cause tissue destruction, which includes gingipains, FimA fimbriae, HtrA protease, and lipid A phosphatase 39 . The most potent amongst these are the gingipains, which produce fibrillin that helps the bacteria to directly bind and adhere to the extracellular matrix proteins of the host 40 .
Studies on the effects of catechins towards Porphyromonas gingivalis showed that EGCG extracted from green tea suppress the growth and prevent the adherence of the bacteria to the epithelial cells 41, 42 , mainly due to the action of the gallic acids within the phenolic compound of the EGCG 41 . Catechin 43 and proanthocyanidins 44 on the other hand, inhibit the production of gingipains which leads to the inability of Porphyromonas gingivalis to attach and invade the host. EGCG can also destroy the biofilms established by the bacteria 42 and inhibit the gene expression of the virulence factor produced by Porphyromonas gingivalis, mainly on the genes that involve in host colonization, tissue destruction, and heme acquisition 43 .
Besides these, catechins have been reported to exhibit potential synergistic effects with conventional antibiotics directed against Porphyromonas gingivalis, specifically metronidazole 43 .
Apart from studies related to Porphyromonas gingivalis , researchers also have investigated the effects of catechins on matrix metalloproteinases (MMPs). MMPs, released by the inflamed connective tissue in the periodontium cause destruction of the gingival collagen and periodontal ligament, and alveolar bone resorption 45 . EGCG was found to be able to reduce the secretion of MMPs released by the inflamed periodontal tissue 46 , which in turn limits the progression of the disease itself.
Clinically, the researcher has incorporated catechin in topical agents used in the management of periodontal disease. Incorporation of catechin in dentifrice, from the green tea extract has reduced the periodontal inflammation 47 while catechin from the grape seed extract, applied in a gel form into the periodontal pocket resulted in improvement of the

Pulp Pathology
When caries invades deeper into dentine and closer to the pulp, dental pulp cells start to produce proinflammatory cytokines and inflammatory cells. This, in turn, will cause pain as the pulp tissue undergoes some inflammatory changes. Catechin is known to posses anti-inflammatory properties which would be beneficial in reducing and treating symptoms that arise from pulpal inflammation. EGCG and ECG from the tea extracts also have the ability in reducing the expression of pro-inflammatory mediators such as interleukin 1(IL-1) and interleukin 8 (IL-8) found in inflamed pulp 49 -51 . In vivo study on animal model also showed the inhibitory effects of EGCG towards pain conduction that occurs during pulpal inflammation by inhibiting the production of prostaglandin E2 (PG-E2) and the subsequent release of transient receptor potential vanilloid (TRPV1) and substance P 52 .
Moreover, EGCG also suppresses the growth of Enterococcus faecalis, a potent bacteria that commonly results in root canal infections 53 . In vitro studies using extracts from Uncaria tomentosa 54 and Uncaria gambir 55 showed that the extracts of both plants managed to suppress the growth of Enterococcus faecalis. Extracts of Uncaria tomentosa, used in a gel form produced similar antibacterial activity as compared to chlorhexidine when tested against Enterococcus faecalis in the infected root dentine. Interestingly, the substantivity of the antibacterial effects were longer compared to the sodium hypochlorite (NaOCl) 54 .
Regenerative endodontic has emerged as one of the methods that has been reviewed and suggested to replace the conventional apexification procedure. Advancement in tissue engineering technology opens more opportunities and options in the regeneration of the pulp-dentine complexes. The use of stem cells, scaffolds, and suitable growth factors have been reported in the literature as part of the mechanism to induce regeneration of the dentine and pulp tissue.
The study on epicatechin and EGCG 56 found that it can establish a cross-linkage with the collagen from the collagen scaffolds. This mechanism, which is regulated by the extracellular signal-regulated kinase (ERK) signaling pathway, provides a strong platform for the differentiation and proliferation of the pulp cells 57 . EGCG enhanced the strength and surface roughness of the collagen scaffolds besides showing the antibacterial activity against Streptococcus mutans, Fusobacterium nucleatum, and Enterococcus faecalis 56 . All of these desirable effects will improve the environment of the collagen scaffolds that is suitable for pulp cell's attachment, differentiation, and growth.
Proanthocyanidins, on the other hand, enhanced the differentiation of the pulp cells and promoted dentinogenesis and biomineralisation 58 which in turn stimulated the growth of the dentine-pulp complex as part of regenerative endodontic therapy.

Dental Restorative Material
The adhesive system in a dental restorative material creates a solid and durable bond between resin and tooth structure. The bond, known as a hybrid layer, consists of embedded collagen fibrils of the dentine matrix in the adhesive resin. Despite the strong bond of the resin onto the dentine, it tends to deplete over time because of the degradation of collagen fibrils by the oral environment both chemically and mechanically 59 , which consequently leads to the failure of the restoration.
Exogenous cross-linking agents such as formaldehyde and glutaraldehyde can contribute in modifying the structure of collagen fibrils and improve degradation resistance. However, these agents have shortcomings in terms of cytotoxicity, illmatched mechanical properties, and poor long-term stability 60 . Hence, a natural cross-linking agent such as proanthocyanidins from the catechin subunit is a useful alternative due to its biocompatibility to biological tissue and its health-promoting effect.
Proanthocyanidins were reported to have the ability to form a cross-linkage with the collagen fibrils of the dentine and therefore improved the tensile strength and stability of dentine matrix mainly at the hybrid layer interface 61 -63 . In addition, the application of proanthocyanidins-rich agents improved the quality of the hybrid layer in terms of susceptibility towards enzymatic degradation and water absorption, which Journal of Natural Remedies | ISSN: 2320-3358 http://www.informaticsjournals.com/index.php/jnr | Vol 20 (1) | January 2020 in turn reduced the material creep rupture and fatigue over time 64 . Besides that, the proanthocyanidinsrich agent also exhibited an inhibitory effect against proteases that is responsible for the degradation of the material 65 .
The potential usage of EGCG has also been vastly investigated in the dental restorative field. An in vitro study was conducted looking into the effects of EGCG that has been incorporated as part of the glass ionomer cement (GIC) particle. Although the antibacterial effects were minimal compared to the control, EGCG improved the mechanical property of GIC and did not interfere with the fluoride released by the material 66 .
The incorporation of EGCG extracts as a copolymer in composite resin showed that it did not affect the polymerization of the resin monomer 67 and has the ability to inhibit the enzymatic degradation of the material 68, 69 and growth of Streptococcus mutans 70 . This, in turn, improves the long-term durability of the material. It is believed that the inhibitory effects against enzymatic degradation are due to the fact that ECGC carries a strong inhibition against MMPs 71 . The action against MMPs have also triggered researchers to investigate the potential development of an anti-erosive gel 72 which will benefit the patient with severe reflux disease.
A similar concept of MMPs inhibition by EGCG also has been investigated in the development of endodontic sealers. Good sealing and bonding abilities are crucial requirements for an endodontic sealer. The resinbased sealer has been used for decades to meet these demanding criteria. However, root canal irrigation with ethylenediaminetetraacetic acid (EDTA) and sodium hypochlorite before the obturation procedure can lead to collagen degradation, which usually stems from host-derived proteases MMPs 73 . The degradation of collagen will affect the covalent bond between the epoxy resin sealer and the root dentine. EGCG has been hypothesized to prevent the action of MMPs in root dentine thus increase the bond of the sealer to the root dentine 74 .

Oral Cancer
The antioxidant and anti-inflammatory properties of catechin suggest that catechin can play an essential role in the prevention and treatment of cancer. Catechin, mainly EGCG in green tea has been extensively studied as a potential chemopreventive and therapeutic agent of oral cancer.
Literature has reported the antitumor effects of EGCG by preventing the early stages of cancer development 75 , hindering tumor cell proliferation 76,77 and preventing the metastasis of cancer 78,79 . These effects are due to the ability of the extract to interfere with the cancer cell activity at the molecular level.
EGCG suppresses the expression of mRNA and transcription of transporter genes that is involved in the self-renewal ability (stemness) of the head and neck cancer stem cells 75,80 . Besides that, EGCG directly blocks the tumor signaling pathways regulated by nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB) 81 . Other pathways that are affected include the epidermal growth factor receptors (EGFR) 82 , activator protein-1 (AP1) and signal transducers and activators of transcription (STATs) 83 . The angiogenesis ability of the cancer cells are also affected by inhibiting the expression of vascular endothelial growth factor (VEGF) and MMP9 genes 83 .
Hsu, et al. explained that catechin could induce cell cycle arrest by up-regulating the cyclin-dependent kinase inhibitor p21 WAF1 76 . Besides that, Lee et al. in their study had concluded that EGCG enhanced the expression of BTG2 gene that arrests cell cycle at the G1 phase and depreciated cell proliferation via p38 mitogen-activated protein kinase (MAPK) signal pathway 77 . They also highlighted the ability of EGCG in inducing apoptotic cell death via inactivation of protein kinase B (AKT) 84 and alteration of the Bcl-2/ bax ratio 85 .
EGCG also has been reported to show selective inhibitory activity against oral cancer cells. The study has shown that it induced the formation of mitochondrial ROS leading to the dysfunction of the organelle and subsequently resulted in early cell apoptosis 86 . At the same time, EGCG also acts as an antioxidant in healthy cells and protects the cell from damage. This selective effect is related to the differential modulation of NAD-dependent deacetylase sirtuin-3 (SIRT3) and its downstream targets 87 .
In addition, Hsu et al. described the selective induction of cancer cell apoptosis by EGCG involving the expression of p57, a cyclin-dependent kinase and Journal of Natural Remedies | ISSN: 2320-3358 http://www.informaticsjournals.com/index.php/jnr | Vol 20 (1) | January 2020 apoptosis inhibitor 76 . Lacking the p57 gene expression will lead to caspase-3 activation and cell apoptosis. They postulated that EGCG has the ability to selectively induce and increase the production of p57 in normal cells, leading to the survival pathway but induced the pro-apoptosis pathway in cancer cells.
Aside from hindering tumor cell proliferation, evidence showed that EGCG has a potential effect on the prevention of metastasis of cancer. Hwang et al. discovered that the administration of EGCG into an in vitro 3-D culture system of oral squamous cell carcinoma (OSCC) cells led to the inhibition of the cells' growth and suppressed the activation of the invadopodia protein that is responsible for the invasion of the cancer cells 78 . Chen and his colleagues in another study also proved the ability of EGCG in causing complete inhibition of the growth and invasion of OSCC cells, this time by reducing the expression of matrix metalloproteinase-2 and urokinase-type plasminogen activator 79 .
Clinically, EGCG has been shown to suppress oral premalignant lesion, blocking the angiogenesis stimulation towards the dysplastic epithelial cells 88 . Moreover, the synergistic application of green tea and anticancer drugs have shown promising results. The application of EGCG enhanced the effects of the anticancer drugs 89,90 , where a study found that it not only reduced the weight of the tumor formed by the human cancer stem cells 80 , but it also inhibits the stemness ability and viability of the cells 75 .
All of these findings and a better understanding of the molecular effects of EGCG towards cancer cells provide a new perspective on the direction of future cancer therapy. The use of catechin-based products can be an alternative option in prolonging the survival rate of cancer patients.

Conclusion
Catechin and its derivatives possess biological properties that have a promising potential in the field of dental caries, periodontal disease, pulpal pathology, dental restorative materials, and oral cancer research. However, most of the studies conducted were laboratory-based. Thus, more future in vivo studies is required to validate its use in the clinical setting.