TNF and periodontal disorder: According to the hypothesis that epithelial cell dysfunction may affect the gingival tissue and may facilitate the progression of periodontal disease [19], this study was designed to evaluate the influence of nicotine, P. gingivalis lipopolysaccharide (LPS), and tumor necrosis factor (TNF)-α) on epithelial cell proliferation and the expression of proteins involved in the defense function of the epithelium.