Introduction
All organisms have various enzymatic and nonenzymatic antioxidant defense systems to protect themselves from the harmful effects of oxidative stress (OS). Under normal physiological conditions, there is a dynamic balance between reactive oxygen species (ROS) and antioxidant system (AO). When the balance disappears in favor of ROS, OS occurs with a decrease in antioxidant defense and/or an increase in ROS activity and causes varying degrees of damage to tissues [
1,
2], such as depolymerization of extracellular matrix components, lipid peroxidation, oxidation of enzymes such as antiproteases, induction of proinflammatory cytokines and DNA damage [
3,
4].
8-hydroxydeoxyguanosine (8-OHdG) is an oxidized nucleoside released by DNA damage. Studies have shown that 8-OHdG in body fluids acts as an oxidative stress marker and is used to evaluate oxidative damage in disorders, including chronic inflammatory diseases [
5‐
8].
FOXOs are members of the O (other) class of the Forkhead superfamily called FKHR (forkhead rhabdomyosarcoma) [
9]. Four members of this class are known as FOXO1, FOXO3, FOXO4, and FOXO6. While FOXO1, FOXO3, and FOXO4 play a role in apoptosis signaling [
10,
11], FOXO1 and FOXO6 antagonize oxidative stress through the transcription of manganese superoxide dismutase, which catalyzes the conversion of O
2− to H
2O
2 [
12,
13]. It helps cells maintain and counteract the effect of ROS by stimulating cell cycle inhibition [
11,
14]. It has been observed that FOXO1 overexpression protects periodontal ligament cells against oxidative damage and increases their osteogenic capacity in the inflammatory setting [
15].
As a result of studies examining the role of FOXO1 in antioxidant defense, it has been revealed that it is associated with some systemic inflammatory diseases such as cardiovascular diseases [
16], brain diseases [
17], and skeletal muscle diseases [
18], but there is not enough data on its relationship with periodontitis. In this study, we tested the hypothesis that the FOXO1 may regulate antioxidant mechanisms in periodontal inflammation and aimed to evaluate the salivary and serum FOXO1 and 8-OHdG levels in advanced periodontitis patients.
Discussion
This study aimed to evaluate the salivary and serum FOXO1 and 8-OHdG levels in Stage III periodontitis patients. Based on the hypothesis that FOXO1 may regulate antioxidant mechanisms in periodontal inflammation, we evaluated and compared saliva and serum samples obtained from SIIIGB and SIIIGC with non-periodontitis healthy controls. While salivary 8-OHdG level was statistically significantly higher in periodontitis groups, salivary FOXO-1 level was significantly lower.
Recent studies have shown a significant relationship between periodontal disease and oxidative stress [
23‐
29], and 8-OHdG is accepted as a sensitive indicator of DNA damage and a marker that defines oxidative stress [
30,
31]. As a result of this study, salivary 8-OHdG levels were significantly higher in periodontitis groups than in the control group, consistent with the literature [
32‐
34]. On the other hand, salivary 8-OHdG levels did not show significant difference between the periodontitis groups. Considering the literature data, 8-0HdG levels are accepted as a marker of periodontal inflammation [
35,
36]. According to the 2017 World Workshop on the Classification of Periodontal and Peri-Implant Diseases and Conditions [
19], Stage III periodontitis is graded according to bone loss/age, and subclassification in our periodontitis group is made accordingly. It was evaluated whether there was a difference between these grades regarding oxidative stress and FOXO. As a result of this evaluation, no difference was found between Grade B and Grade C.
Although there is a difference in salivary 8-OHdG levels between the periodontitis and control groups, serum 8-OHdG levels were similar in periodontitis and control groups. Similarly, Konopka et al. observed no difference in serum 8-OHDg levels between periodontitis and control groups [
37]. The researchers concluded that the oxidative burst observed in periodontitis is extensive enough to cause significant DNA damage at the local level. The level of 8-OHdG can be higher than the plasma concentration in the microcirculation and effectively change the redox status locally.
FOXO1, a transcription factor, plays a crucial role in regulating various cellular processes such as cell survival, differentiation, the reduction of ROS, and apoptosis [
38,
39]. In the current study, salivary FOXO1 levels were statistically significantly lower in periodontitis groups than in the control group. Antioxidant systems are expected to be activated to prevent or reduce tissue damage caused by oxidative stress in periodontal disease. Accordingly, FOXO1 levels, whose antioxidant effect is known, could be expected to be higher in periodontitis groups. However, it can be thought that the high level of oxidative stress (8-OHdG) may prevent the functioning of antioxidant systems [
40]. However, it is also known that ROS regulates the localization and activation of FOXO1. In case of an increase in OS, it causes FOXO1 to be inactivated by its acetylation by being transported out of the nucleus and reduces target gene expression [
41]. On the other hand, it may be another interpretation to assume that the destruction is severe due to the insufficient functioning of the antioxidant mechanism in the periodontal destruction process. On the contrary, when activated, FOXO1 functions as a vital signal integrator, contributing to maintaining homeostasis and adapting to environmental shifts [
42], which are crucial for periodontal health. Consequently, Ren et al. highlighted that disturbances in the normal signaling of FOXO1 could have implications for periodontal dysbiosis [
43]. From this point of view, we can also consider that FOXO1 levels were lower than the control group in our study as the deterioration of physiological FOXO1 signal in periodontal inflammation. Interestingly, despite growing evidence indicating that periodontal pathogens can induce FOXO1 activity [
43,
44] to maintain its intracellular existence by inhibiting apoptosis [
44], the specific roles of FOXO1 in periodontal homeostasis and disease are not extensively documented.
FOXO1 plays a crucial role in regulating different aspects of mucosal immunity by influencing the migration and activation of dendritic cells, macrophages, and neutrophils. It also impacts the development and function of T-helper cells and B-lymphocytes [
45‐
47]. Moreover, FOXO1 controls cytokine production, protects hematopoietic stem cells from oxidative stress, and regulates vital functions of keratinocytes, potentially contributing to the maintenance or restoration of the epithelial barrier [
48,
49]. However, the effects and role of FOXO1 can vary depending on the specific conditions, such as diabetes [
50,
51]. Consequently, predicting the impact of FOXO1 on various diseases can be challenging. In subsequent studies, examining the behavior of FOXO1 in different stages of periodontitis will highlight the intricate nature of FOXO. These findings indicate that epigenetic factors like high glucose levels or elevated oxidative stress highly regulate it.
The cross-sectional nature of this study is its main limitation; due to the study design, randomization in the selection of patients was not possible.
Our results showed a negative correlation between salivary 8-OHdG and FOXO1 levels. It is known that oxidative stress is one of the most critical factors in the tissue destruction mechanism in periodontal breakdown. In contrast, a deficiency in all antioxidant systems in the disease process is mentioned [
52]. Considering all these, the negative correlation of FOXO1 saliva level with 8-OHdG may also be related to the nature of the disease.
Gaining a deeper understanding of the mechanisms involved in protection against oxidative stress, which is an essential factor in the development of periodontal disease, will be valuable for a more comprehensive understanding of the pathology of periodontitis. Literature data indicate that FOXO-1 may be a new therapeutic agent in some inflammatory diseases such as diabetes [
53], obesity [
54], and hypertension [
55]. However, no study confirms FOXO1 as a therapeutic agent in periodontal disease. More in vitro and in vivo studies are needed to ensure the role of FOXO1 in promoting tissue regeneration in periodontitis, the immune system, and osteogenesis.
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