PTGS2 and periodontitis: Experimental rat models [20] of periodontitis have indicated that PE exerts its effects by decreasing the levels of inflammatory cytokines, including tumor necrosis factor-α (TNF-α), interleukin-β1 (IL-β1) and interleukin-6 (IL-6), by down-regulating the expression of cyclooxygenase-2 (COX-2), metal matrix proteinase-9 (MMP-9) and inducible nitric oxide synthase (iNOS) proteins [21].