Epithelial Mesenchymal Transition and Periodontitis

Overview

Periodontitis is a chronic inflammatory disease results is destruction of the attachment apparatus of the teeth and ultimately tooth loss. Epithelial-mesenchymal transition (EMT) is a process comprises of series of events that influence a polarized epithelial cell to undergo molecular/morphological changes leading to acquisition of mesenchymal cell phenotype. This process is responsible for suppressing epithelial-phenotype and it is known to be triggered by chronic exposure to inflammatory cytokines, Gram-negative bacteria, hypoxia, smoking, and hyperglycemia. Both periodontitis and EMT share common risk factors/promoters; however, the role of EMT in the pathogenesis of periodontitis is not fully elucidated yet. Potential induction of EMT within periodontal pockets may disrupt epithelial barrier thus facilitating invasion of pathogenic periodontal pathogens to deeper tissues resulting in further tissue breakdown and non-resolving periodontal lesion.

Periodontitis is a highly prevalent inflammatory disease affecting the attachment apparatus of the teeth, leading to progressive destruction of periodontal ligament and resorption of alveolar bone which if not treated, at early stages, it will lead to tooth loss. It is characterised by presence of a wide diversity of pathogenic bacteria, specifically Gram-negative anaerobes, that possess range of virulence factors responsible for triggering intense inflammatory response. Although this response is protective in nature; however, it leads to undesirable collateral damage to the surrounding tissues that is further aggravated by the aberrant immune response of the host. Epithelial-mesenchymal transition (EMT) is a process comprises of series of events that influence a polarized epithelial cell to undergo molecular/morphological changes leading to acquisition of mesenchymal cell phenotype. EMT is modulated by range of regulatory pathways; mainly, downstream of TGF-β signaling activity which is evident in many developmental and pathological situations in which EMT is reported, including embryogenesis, inflammation and tumor metastasis. The hallmark of TGFβ signaling is up-regulation of Snail, an E-cadherin repressor. Overexpression of Snail has been found in various fibrotic diseases, including liver fibrosis and renal fibrosis. E-cadherin is a calcium-dependent homophilic cell adhesion molecule expressed on the cell surfaces of epithelium and is a critical structure for stratification of squamous epithelia. The significant reduction in E-cadherin expression observed in the gingival epithelium during pocket formation and is believed to contribute to the pathogenesis of periodontal disease. Vimentin is a type III intermediate filament (IF) protein that is expressed in mesenchymal cells. IF, along with tubulin-based microtubules and actin-based microfilaments, comprises the cytoskeleton. All IF proteins are expressed in a highly developmentally-regulated fashion; vimentin is the major cytoskeletal component of mesenchymal cells. Because of this, vimentin is often used as a marker of mesenchymal-derived cells or cells undergoing EMT during both normal development and metastatic progression. β-catenin is a dual function protein, involved in regulation and coordination of cell-cell adhesion and gene transcription. β-catenin also acts as a morphogen in later stages of embryonic development. Together with TGF-β, an important role of β-catenin is to induce a morphogenic change in epithelial cells. It induces them to abandon their tight adhesion and assume a more mobile and loosely associated mesenchymal phenotype. Role of EMT on compromising epithelial barrier function of periodontal pocket lining is not fully elucidated yet. Reported risk factors of EMT including prolonged exposure to cytokines, Gram-negative bacteria, tobacco and hypoxia are also relevant to periodontitis. Potential induction of EMT within periodontal pockets may disrupt epithelial barrier thus facilitating invasion of pathogenic periodontal pathogens to deeper tissues resulting in further tissue breakdown and non-resolving periodontal lesion.