Oxidative Stress–Driven Alterations in FoxO-1 and MMP-9 Activity in Smoking-Associated Periodontitis

Objectives: Smoking disrupts oxidative balance and is a major risk factor for periodontitis. Forkhead box protein O-1 (FoxO-1) regulates antioxidant defense, yet its contribution to smoking-associated periodontal breakdown is unclear. This cross-sectional case–control study assessed reactive oxygen species (ROS), matrix metalloproteinase-9 (MMP-9), and FoxO-1 to clarify how smoking modulates oxidative stress and tissue destruction. Methods: Four groups were defined: non-smoker controls (NS-C), non-smoker periodontitis (NS-P), smoker controls (S-C), and smoker periodontitis (S-P). Periodontal parameters were recorded, and gingival crevicular fluid ROS, MMP-9, and FoxO-1 were quantified by enzyme-linked immunosorbent assay (ELISA). Results: ROS levels were higher in periodontitis than in controls, regardless of smoking (p < 0.0001). MMP-9 was elevated in S-P versus S-C (p = 0.0456) but did not differ between non-smoker groups. FoxO-1 increased with both smoking (p = 0.0069) and inflammation (p < 0.0001). Conclusion: These findings suggest that smoking amplifies periodontal tissue destruction not only by enhancing oxidative burden but also by upregulating FoxO-1–linked pathways associated with proteolysis. The parallel rise of FoxO-1 with MMP-9 might indicate a possible compensatory yet insufficient antioxidant response, positioning FoxO-1 as a mechanistic bridge between inflammation and matrix degradation and a candidate target for host-modulatory therapy.