Our study demonstrated that CXCL16-CXCR6 mediates CD8 T-cell skin trafficking under oxidative stress in patients with vitiligo. The CXCL16 expression in human keratinocytes induced by ROS is, at least in part, caused by unfolded protein response activation.

Oxidative stress caused by hydrogen peroxide (H(2)O(2)) leads to cell death and has been implicated in the pathogenesis of vitiligo. The nuclear factor E2-related factor 2 (Nrf2)-antioxidant response element (ARE), a major antioxidant pathway, regulates oxidative stress-related cytoprotective genes. We hypothesized that the Nrf2-ARE pathway protects human melanocytes from H(2)O(2)-induced oxidative damage through the induction of downstream antioxidative genes. Thus, we used Nrf2 short interfering RNA (siRNA) and pCMV6-XL5-Nrf2 to downregulate or upregulate Nrf2 expression in immortalized human melanocyte cell line PIG1. The melanocytes were then analyzed under different oxidative stress conditions for cell viability and apoptosis. Our study demonstrated that heme oxygenase-1 (HO-1) was the most induced antioxidant gene in PIG1 cells after treatment with H(2)O(2). Knockdown of Nrf2 or zinc protoporphyrin IX (ZnPP) treatment increased cell death caused by H(2)O(2) in melanocytes, but upregulation of Nrf2 or hemin treatment reduced cell death caused by H(2)O(2) in melanocytes. In addition, the H(2)O(2)-induced Nrf2-ARE/HO-1 pathway was confirmed in primary cultured human melanocytes by examining the expression and translocation of Nrf2 and HO-1. These data suggested that regulation of the Nrf2/HO-1 pathway can reduce H(2)O(2)-induced oxidative damage in human melanocytes. Our data demonstrate that HO-1 protects human melanocytes from oxidative damage via the Nrf2-ARE pathway.

Vitiligo melanocytes possess higher susceptibility to oxidative insults. Consistent with this, impairment of the antioxidant defense system has been reported to be involved in the onset and progression of vitiligo. Our previous study showed that the nuclear factor E2-related factor 2-antioxidant response element (Nrf2-ARE) pathway and its downstream antioxidant enzyme heme oxygenase-1 (HO-1) are crucial for melanocytes to cope with H2O2-induced oxidative damage. Here, we sought to determine whether the diminished Nrf2-ARE activity that contributes to reduced downstream antioxidant enzymes and increased oxidative stress could be the reason why melanocytes are more vulnerable to vitiligo. We found that vitiligo melanocytes exhibited hypersensitivity to H2O2-induced oxidative injury because of reduced Nrf2 nuclear translocation and transcriptional activity, which led to decreased HO-1 expression and aberrant redox balance. Moreover, we also found that the level of serum HO-1 was significantly decreased and that of IL-2 was markedly increased in 113 vitiligo patients when compared with healthy controls. These data demonstrate that impaired activation of Nrf2 under oxidative stress could result in decreased expression of antioxidant enzymes and increased death of vitiligo melanocytes.