Recently, many diseases have been shown to be associated with increased intestinal permeability. Zonulin, one of the markers showing intestinal permeability, has been associated with various diseases, such as autoimmune diseases [Type-1 diabetes mellitus (DM), celiac disease, inflammatory bowel diseases, ankylosing spondylitis, systemic lupus erythematosus], some tumors or cancers [brain, breast, ovary, pancreas, lung, cancer], neurodegenerative or psychiatric diseases (multiple sclerosis, schizophrenia) . In the literature, there are papers on the relationship between intestinal permeability and zonulin in some dermatological diseases, such as psoriasis, atopic dermatitis, rosacea, dermatitis herpetiformis [20, 21, 22, 23]. In a study by Richetta et al., consisting of 50 psoriasis patients and 32 healthy controls, serum zonulin and LPS concentrations were analyzed to evaluate intestinal permeability. They found that serum zonulin and LPS levels were significantly higher in the patient group than the control group. In addition, they also found a linear correlation between zonulin and LPS. Based on these data, they concluded that there was increased intestinal barrier permeability in psoriasis and that TJ-mediated passage of antigens across the intestinal barrier may play a role in triggering inflammation in the skin .
In our study, we compared plasma zonulin levels of patients with vitiligo and healthy volunteers to investigate whether there is an increased intestinal permeability due to elevated zonulin expression in vitiligo disease. Plasma zonulin level in the patient group (2.02 ± 0.93 ng/ml) was higher than the zonulin level in the control group (1.61 ± 0.47 ng/ml). This difference was significant (p:0.005; p < 0.05). This finding obtained in our study suggests that the elevation of zonulin, which is a remarkable biomarker indicating the increased intestinal permeability, may be responsible for the increased intestinal permeability in the etiopathogenesis of vitiligo disease.
There are also studies on LPS levels, which can both increase intestinal permeability and trigger systemic inflammation [25, 26, 27]. Jayashree et al. investigated zonulin, LPS, IL-6, and TNF-α levels in their study with 45 patients with Type-2 DM and the same number of healthy control groups. They found that these parameters were significantly higher in the patient group than the control group . Loffredo et al., on the other hand, aimed to assess serum LPS and oxidative stress in patients with peripheral artery disease (PAH) and control group in their study consisting of 40 patients with PAH and 40 healthy controls. They found zonulin, LPS, and oxidative stress markers to be higher in the PAH group than the control group. They found a linear correlation between LPS and zonulin as well as between LPS and oxidative stress markers . Similar to the results of this study, we also found a positive, strong (64.8%) and significant correlation between zonulin level and LPS in our study (p:0.001; p < 0.05). Given these findings, we are of the opinion that LPS may play a role in the increased intestinal permeability in vitiligo and that the increased level of LPS in the circulation may also play a role in the formation of the immune response that causes the destruction of melanocytes in vitiligo disease. Besides, the high level of LPS indicates that there might be a possible intestinal dysbiosis in patients with vitiligo.
Studies have demonstrated that there is an imbalance between pro- and anti-inflammatory cytokines in the skin and serum of patients with vitiligo. Singh et al., in their study with 80 patients with vitiligo and healthy controls, investigated the levels of IL-6, IL-2, TNF-α, and IFNγ in the serum of patients and healthy groups to investigate the role of cytokines in the pathogenesis of vitiligo. The mean serum IL-6 and IL-2 levels in the patient group were significantly higher than the normal controls, whereas the mean serum IFNγ level in patients with vitiligo was significantly lower than the control group, and they did not detect a significant difference in serum TNF-α levels between patients with vitiligo and healthy controls . Yang et al., in their study with patients with vitiligo and healthy volunteers, found that the expression levels of IL-6, IL-17, and TNF-α in the blood of the volunteers in the healthy control group were significantly lower than in the vitiligo group, and the differences were significant . In our study, we investigated IL-6 and TNF-α levels in the patient and control groups. While we did not detect a significant difference between the groups regarding IL-6 levels, we found that the TNF-α level of the patient group was significantly higher than the control group (p:0.006; p < 0.05). We found a positive and significant correlation between zonulin level and IL-6 and TNF-α levels. Similar to the previous studies [29, 30, 31], we found the TNF-α level to be higher in patients with vitiligo. We consider that the positive correlation between zonulin level and IL-6 and TNF-α is an indicator that antigen translocation that occurs with increased intestinal permeability in vitiligo disease triggers the immune response. The improvement in vitiligo disease with TNF-α inhibitors also supports our study. However, varying outcomes have been obtained in the treatment with TNF-α inhibitors in vitiligo disease. In this study , in which studies on the effectiveness of TNF-α inhibitors in vitiligo disease were reviewed, the researchers revealed that TNF-α inhibitors achieved successful outcomes in patients with active generalized vitiligo, but that worsening of vitiligo can be seen in those with autoimmune disease, and de novo vitiligo may develop in those who receive TNF-α inhibitors for other diseases. Despite successful results with TNF-α inhibitors, worsening of vitiligo or development of de novo vitiligo suggest that TNF-α may have other functions that are not yet known, in addition to its proinflammatory and melanogenesis inhibition effects.
There are studies investigating the relationship between oxidative stress markers and vitiligo. In a study conducted with sixty patients with vitiligo and 62 healthy controls, TAS, glutathione peroxidase activity (GPX activity) was compared in plasma. They found lower TAS and GPX activity levels in all patients with vitiligo than the control group . Akoğlu et al. investigated TAS, TOS, and OSI in blood samples taken from patients with vitiligo and healthy controls in their study. Significantly lower TAS and higher TOS and OSI values were observed in patients with vitiligo than controls . Yıldırım et al. reported that serum NO levels were higher in patients with vitiligo . In another study, total thiol and native thiol levels were higher in patients with vitiligo than the control group .
In this study, we examined TAS, TOS, OSI, GSH, NO, MPO, MDA levels in erythrocytes and TAS, TOS, OSI, MDA, Total Thiol, Native Thiol, and Disulfide values in plasma to determine the oxidative stress level in the patient and healthy control group. We did not find a significant difference in the level of oxidative stress markers in the plasma of the patient and control groups. The GSH, NO, TAS levels were lower and MDA levels to be higher in the erythrocytes in the patient group than the control group, whereas we did not detect a significant difference in terms of MPO, TOS, and OSI levels. Based on these results, we consider that oxidative stress has a crucial role in the pathogenesis of vitiligo; compensation is achieved regarding oxidative stress in plasma in vitiligo, which is a chronic disease, whereas the oxidant-antioxidant balance in erythrocytes shifts in favor of oxidative stress and compensation is not achieved. Furthermore, in our study, we could not find a significant correlation between zonulin level and oxidative stress markers in the patient group. We think that these values should be studied at the tissue level to more clearly reveal whether oxidative stress has an impact on intestinal permeability since this balance has shifted in favor of oxidative stress in erythrocytes, albeit the balance in terms of oxidative stress has been achieved in the plasma.
The fact that patients with vitiligo were selected without considering segmental and nonsegmental subgroups in the patient group and the small sample size can be considered as limitations of our study.
In conclusion, our study is important as, to our knowledge, it is the first study in the literature to evaluate intestinal permeability in vitiligo disease. In our study, we revealed that the level of zonulin increased in patients with vitiligo; thus, intestinal permeability might be increased. Besides, the high level of LPS suggests that there might be a potential intestinal dysbiosis in patients with vitiligo.