**5. Implications of periodontitis in oral carcinogenesis**

#### **5.1 Epidemiological associations between periodontitis and head and neck squamos cell carcinoma**

Cancer is one of the leading causes of worldwide mortality. In 2016, 1.1 million new cases were reported, and the prevalence was of 4.1 million cases. Head and neck squamous cell carcinomas (HNSCCs) constituted 5.7% of global cancerrelated mortality, the equivalent of 512.770 deaths. Also the global mortality burden is expected to increase; by 2030 an estimated 705.902 people worldwide will be expected to die due to HNSCC [43]. This great burden of cancer worldwide strongly indicates the need for implementing rigorous screening and control programs in order to facilitate the identification of all potential HNSCC related risk factors, including periodontitis, and the early detection of HNSCC.

Squamous cell carcinoma is the most common histological subtype of oral and oropharyngeal malignant tumors, accounting for about 90% of the cases. The incidence of HNSCCs are reported to be on the rise, with a global estimated incidence of over 300.000 new cases registered each year [44, 45].

Emerging evidence indicates periodontitis as a potential independent risk factor for the development of premalignant lesions and HNSCC [46, 47]. Current evidence regarding a positive association between periodontitis and oral and oropharyngeal cancers are currently controversial [48]. Some studies suggest an increased susceptibility to HNSCC in periodontitis patients [49]. Extended and severe forms of periodontitis have been frequently identified among patients with HNSCC [48].

A meta-analysis reported a 2.66-fold higher risk for oral cancer in patients with periodontal disease as compared to periodontal healthy controls [50]. A hospitalbased case-cohort study, after controlling for important confounders, found that individuals with periodontitis were 3.7 times more likely to have oral squamous cell carcinoma as compared to individuals without periodontitis [51].

Positive associations between periodontitis and other cancers such as digestive tract cancer, pancreatic cancer, prostate cancer, breast cancer, corpus uteri cancer, lung cancer, hematological cancer, and Non-Hodgkin lymphoma have been reported [46].

The relationship between periodontitis and HNSCC has been reported based on the specific periodontal parameters or surrogate markers of periodontal disease. An increased risk of HNSCC development, with respect to the number of missing teeth has been observed. A 2.7-fold increased risk of oral cancer in patients having 11 to 16 missing teeth as compared to dentate subjects has been reported [52]. A 5.23-fold increase in the risk of tongue cancer and a 4-fold increased risk of head and neck squamous cell carcinoma for each millimeter of alveolar bone loss, after the adjustment for important confounders has been reported [53, 54]. This data suggest that the severity and extension of periodontitis might be a risk indicator for HNSCC [55].

#### **5.2 The influence of chronic periodontal inflammation in carcinogenesis**

The investigation of the causal role of periodontitis in the cancer development is challenging due to the major independent and shared risk factors of these conditions. Periodontitis may have a direct effect on the rise of cancer risk or may impact through shared genetic and environmental factors [56]. Smoking is one of the main independent risk factors for periodontitis and HNSCC, while smoking and alcohol consumptions account for up to 85% of all oral cancers. The synergic effect of alcohol and tobacco consumption increases the risk for of oral cancer by 15 times [57, 58] However, about 15% to 20% of the HNSCCs occur independently to any of these risk factors, and the clinical features and evolution of the cancer in this group is often particularly aggressive [57–60].

Some molecular pathogenic mechanism could associate periodontitis to oral cancers and are briefly detailed below.

#### *5.2.1 Destruction of epithelial barrier promotes the passage of toxic compounds*

Chronic periodontal inflammation might be an extrinsic pathway in cancer development [46, 47, 61, 62]. The damage of the junctional epithelium mediated by periodontal pathogens alters its protective function promoting the absorption of toxic compounds from alcohol and tobacco, which in turn create a chronic inflammatory environment [61, 63]. The inflammatory process and the presence of cell-stimulating signals may create an optimal environment for the atypical cell proliferation and differentiation, which may eventually lead to cancer development [46, 61, 63].

#### *5.2.2 Increased pro-inflammatory periodontal mediators damage DNA*

In periodontitis, the complex inflammatory response developed to eliminate periodontal pathogens leads to the accumulation of excessive levels of endogenous compounds such as cytokines, chemokines, prostaglandins, reactive oxygen and nitrogen species, matrix metalloproteinases, and endothelial and epidermal growth factors. The anarchic and excessive release of these molecules is responsible for the indirect destruction of periodontal tissues [64]. In the meantime, these proinflammatory molecules may irreversibly damage the cellular DNA, deregulate the mechanisms of DNA repair and subvert the cell cycle regulatory mechanisms [65, 66]. Thus, cumulative, permanent genetic alterations lead to oncogenes activation or tumor suppressor genes inactivation [67].

#### *5.2.3 Increased proinflammatory periodontal mediators promote epigenetic modifications*

Epigenetic changes occur more frequently than gene mutations and may persist for the entire cell life and even for multiple generations. Extensive exposure of oral mucosa to bacteria and chemokines may contribute to carcinogenesis by causing epigenetic alterations. The epigenetic changes, which refer to any heritable modifications in gene expression without alterations of the DNA sequence, also promote genomic instability. Epigenetic mechanisms include DNA methylation, posttranslational modifications of histone proteins and post-transcriptional gene downregulation by microRNAs. Any of these three distinct epigenetic mechanisms leads to inappropriate gene expression and cancer development [61, 67, 68]. A relatively stable epigenetic change may induce the increase of carcinogenesis mechanisms such as faster cellular proliferation, stimulation of an increased angiogenesis, and inhibition of apoptosis [61].

#### *Periodontal Medicine: Impact of Periodontal Status on Pregnancy Outcomes and Carcinogenesis DOI: http://dx.doi.org/10.5772/intechopen.96147*

The methylation of DNA refers to the covalent addition of a methyl group to the 5-carbon position of cytosine base from a CpG dinucleotide. Hypermetilation of CpG islands of growth-regulatory genes promoter regions causes the transcriptional "silencing" of tumor suppressor genes and promotes tumor progression [68]. Although some aberrant methylation patterns have been already identified, the complex underlying molecular mechanisms that address the association of chronic periodontal inflammation and oral cancers are still not fully understood [61].

Histones, proteins binding to the DNA in the nucleus and condensing it into chromatin, can undergo multiple aberrant post-translational modifications, which induce structural and functional modification in the chromatin and thus alterations of the pattern of gene expression directly contributing to the initiation of neoplasia and its subsequent course [68].

#### *5.2.4 The increased release of pro-inflammatory mediators*

The increased release of inflammatory mediators in periodontitis, such as interleukin-6 (IL-6), interleukin-8 (IL-8), and tumor necrosis factor–α (TNF–α) [61], may trigger epithelial-mesenchymal transition and activation of inflammatory cells which facilitate cancer invasion [61, 69].

#### *5.2.5 Increased levels of IL-8 regulate carcinoma growth*

IL-8 is primarily produced by periodontal cells in response to periodontal bacteria, like *Porphyromonas gingivalis*, and bacterial toxins [13, 70]. One of the possible links between *Porphyromonas gingivalis* and oral squamous cell carcinoma may be the increased IL-8 levels in the periodontal microenvironment and the subsequent overexpression of MMPs. IL-8 has long been recognized as an autocrine regulator of oral squamos cell carcinoma growth, and a contributor of increased cell motility. Thus, salivary IL-8 has been proposed to be a discriminative diagnostic biomarker for oral cancer detection [69, 70].

#### *5.2.6 The influence of inflammasomes-mediated inflammation in cancer*

More recent studies investigated the topic of inflammasome-mediated inflammation in cancer. The inflammasome is a part of the innate immune system and it responds to microbial challenge through regulation of caspase-1 activation and induction of inflammation. The most studied and best characterized inflammasome, Nucleotide-Binding Domain, Leucine-Rich–Containing Family, Pyrin Domain–Containing-3 (NLRP3), is an emerging, key player in the development and progression of cancer. Activation of NLRP3 may promote inflammation induced tumor growth and metastasis in HNSCC [71]. Certain periodontal pathogens, such as *Porphyromonas gingivalis* and *Aggregatibacter actinomycetemcomitans* have the ability to modulate inflammation and potentially induce carcinogenesis by controlling interleukin-1β (IL-1β) secretion through NRLP3 inflammasome complex activated by adenosine 5 '- triphosphate (ATP). IL-1β is directly involved in several chronic pathologies and various types of cancers, including oral cancer [72, 73].

#### *5.2.7 The role of transcription factors*

Inflammatory mediators, microbes, as well as environmental factors (tobacco and alcohol consumption) could activate some transcription factors, such as the nuclear signal transducers and activators of transcription-3 (STAT-3), the activator protein-1 (AP-1) and the nuclear factor-kB (NF-kB). These transcription factors

activate oncogenes that regulate apoptosis, cell proliferation and angiogenesis as well as genes regulating the production of pro-inflammatory molecules. These oncogenic changes drive a tumor-promoting inflammatory milieu through the intrinsic pathway that favors the development of already established tumors. Moreover, the inflammatory microenvironment favors the tumor to escape from immune surveillance and alters the response to chemotherapy [65, 66, 74].

#### *5.2.8 Periodontitis associated-oxidative stress promoting carcinogenesis*

Oxidative stress occurs as a state of disturbance between free radical production and the capability of antioxidant system to counteract the free radicals. The activity of periodontal bacteria induces oxidative stress through free radical release, and decreased plasma antioxidant capacity. On the other hand, oxidative stress causes inflammation, which can increase the production of free radicals. Patients with chronic periodontitis showed low serum and salivary antioxidants levels and elevated oxidative stress biomarkers such as 8-isoprostane and malondialdehyde. Moreover, assessment of blood and gingival tissues of chronic periodontitis patients also revealed mitochondrial DNA deletion mediated by lipid peroxidation [73].

Oxidative stress is also correlated to oral cancer. Increased lipid peroxidation and reduced antioxidants was reported in patients with oral cancer. Lipid peroxidation and irreversible protein modifications are essential molecular mechanisms involved in the oxidative damage of cell structures eventually leading to programmed cell death [73, 75]. Elevated levels of malondialdehyde and low levels of glutathione were observed in the saliva and serum of HNSCC patients [73, 76].

Chronic periodontal inflammation induces a prolonged exposure of oral cells to free radicals that can lead to genomic alterations through DNA damage, lipid and protein peroxidation and activation of signal transduction by post translational modification [66]. Therefore, the accumulation of oxidative stress products in periodontal tissues may significantly contribute to the development of oral cancer.

Oncogene and tumor suppressor pathways are proven intracellular targets for therapies, but recent scientific data are pointing out to new potential, *extracellular* vesicle-based therapeutic targets such as chemokines and chemokines receptors. Anti-inflammatory therapies have been successful in preventing progression and even curing some types of infectious agents associated cancers [66, 74]. Hence, control of chronic periodontal inflammation through specific periodontal therapy could be part of a comprehensive HNSCCs prevention strategy.

#### **5.3 Periodontal bacteria in carciongenesis -** *Porphyromona gingivalis*

*Porphyromonas gingivalis* is a true periodontal pathogen contributing in development of severe chronic inflammations of periodontal tissues. *Porphyromonas gingivalis* has been frequently associate with cancer and the most highly associated organism with oral squamous cell carcinomas [72]. Besides colonizing dental surfaces, the microorganism is also able to colonize various parts of the oral mucosa, which are described to be primary lesion sites during oral squamous cell cancer initiation [72].

*Porphyromonas gingivalis* produces various virulence factors which can modulate oral persistent inflammation and thus the complex physio-pathological network leading to carcinogenesis [72, 77]. *Porphyromonas gingivalis* as well as other periodontal microorganisms can trigger the development of a dysbiotic microbhabitat. *Porphyromonas gingivalis* has the capacity of disrupting the periodontal homeostasis by promoting the transformation of the commensal microbiota into a pathological

#### *Periodontal Medicine: Impact of Periodontal Status on Pregnancy Outcomes and Carcinogenesis DOI: http://dx.doi.org/10.5772/intechopen.96147*

one. Also it can modulate the host's immune system, thus being able to intervene directly in the development of cancers at the oral or distant sites [78].

*Porphyromonas gingivalis* invades oral epithelial cells and could affect cell cycle related molecules at different stages [79]. Epithelial cell responses to *Porphyromonas gingivalis* infection include both changes to apoptosis and cell division [72, 77, 79]; these mechanisms are characteristic to cancer development and progression.

*Porphyromonas gingivalis* lipopolysaccharides could deregulate tumor suppressor gene p53 [72, 77]. Gingipains and cysteine proteinases produced by *Porphyromonas gingivalis*, play a key role in activating MMP-9, which degrades the basement membrane and the extracellular matrix, promoting carcinoma cell migration and invasion, thus allowing dissemination and metastatic growth at remote sites [72, 77, 78, 80]. Also in oral squamous cell carcinoma cells, *Porphyromonas gingivalis* stimulates the release of a variety of cytokines, including IL-8, which can increase MMP-9 production and cell proliferation and invasiveness [80].

*Porphyromonas gingivalis* can also modulate the expression of microRNAs of the epithelial cells, and up-regulation of miR-203 leads to inhibition of the negative regulator suppressor of cytokine signaling 3 and subsequent suppression of apoptosis [77].

*Porphyromonas gingivalis* secretes a nucleoside diphosphate kinase (NDK) having an ATPase function and preventing ATP-dependent apoptosis mediated through the purinergic receptor P2X7. Thus, NDK can suppress the proapoptotic and proinflammatory mechanisms in oral epithelial cells favoring carcinogenesis [72, 77].

The heat shock protein GroEL is another virulence factor of *Porphyromonas gingivalis* that might have a direct carcinogenic effects on certain oral cancer cell lines [72].

*Porphyromonas gingivalis* can induce the expression of the B7-H1 and B7-DC involved in regulating the cell-mediated immune response, but also up-regulated in cells originating from a variety of cancers. B7-H1 expression promotes the event of regulatory T cells that suppress effector T cells. B7-H1 expression might contribute to immune evasion by oral cancers [47, 77, 80].

Currently, among the known virulence molecules of *Porphyromonas gingivalis*, there is not a significantly attributed molecular determinant that could be strongly linked to the illustrated association of *Porphyromonas gingivalis* with orodigestive cancers. It is very likely that the potential synergistic ability of *Porphyromonas gingivalis* with other oral microbial species are contributory to the postulated malignant transformation and progression in the oral cavity and upper digestive tract [72].

### **6. Conclusions**

Understanding the dynamics and common, underlying pathophysiological mechanisms that link periodontitis to systemic diseases is essential to the development of coherent, patient centered diagnostic and therapeutic strategies.

An increased risk for preterm birth and low birth weight of newborns in mothers with periodontitis has been reported. Pregnant women should be provided with oral-health education and preventive personal plaque control measures that should become healthy habits throughout life. Non-surgical periodontal therapy is safe during pregnancy, and especially during the second trimester of gestation.

The epigenetic alterations in periodontal disease such as histone acetylation and DNA methylation, and the subsequently altered gene expression might partially explain the role of periodontal inflammation in cancer development and tumor growth.
