The significance of Nrf2 pathway in (photo)-oxidative stress response in melanocytes and keratinocytes of the human epidermis

The expression of genes encoding antioxidant and/or phase 2 detoxifying enzymes can be enhanced in response to various environmental stresses. The main transcription factor involved in this response is nuclear factor erythroid 2-related factor 2 (Nrf2). Nrf2 activity is negatively regulated by the protein Kelch-like-Ech-associated-protein 1 (Keap1). While the roles of Nrf2 and phase 2 genes in chemoprevention of carcinogenesis have been well described; only few studies have dealt with their role in skin cancer. Normal human keratinocytes (NHK) and melanocytes (NHM) were treated by chemical inducers of the Nrf2 pathway or by small interfering RNAs (siRNA) used to knock down Keap1 mRNA. The above treatments resulted in significant stimulation of NQO-1 (NADPH-Quinone-Oxidoreductase 1) gene expression. GCL (gamma-Glutamyl-cysteinyl-ligase) gene was also induced but interestingly increased mRNA encoding the catalytic, heavy subunit GCLC was mainly stimulated in NHK, whereas the mRNA encoding the modifier, light subunit GCLM was mostly induced in NHM. HO-1 (Heme Oxygenase 1) gene induction was relatively strong in NHM, but generally absent in NHK, except when the cells were subjected to cytotoxic doses of the above chemicals. Exposure to solar UV (UVB + UVA, 300-400 nm) or to UVA alone (320 400 nm) confirmed this trend, but interestingly, at doses where cell growth reduction was comparable, UVA was generally more efficient than solar UV in inducing phase 2 genes. When siRNAs directed against Nrf2 were used, a strong down-regulation of NQO-1 expression was observed in both, NHM and NHK, whereas reduction of HO-1 expression was mainly detected in NHM. To our knowledge, this is the first study comparing phase 2 gene modulation in NHK and NHM. The results hereby presented should contribute to a better understanding of the molecular mechanisms involved in skin adaptation to environmental stress.