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

Abstract

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.

Acute Exposure to Solar Simulated Ultraviolet Radiation Affects Oxidative Stress-Related Biomarkers in Skin, Liver and Blood of Hairless Mice

“…The effect was moderately greater after exposure to UVA than solar light. 53) A previous study showed an induction of HO-1 mRNA expression in hairless mice at several time points following UVA treatment (38.7 J/cm 2 ). No mRNA was observed in animals exposed to UVB (0.550 mJ/cm 2 ).…”

Nrf2 establishes a glutathione-mediated gradient of UVB cytoprotection in the epidermis

“…Physiological doses of UVB, in contrast, did not activate Nrf2 in primary mouse keratinocytes or in murine back skin epidermis (Durchdewald et al 2007). Recent results even indicated repression of Nrf2 activity by high UVB doses in normal human keratinocytes (Marrot et al 2008;Kokot et al 2009). The basal activity of Nrf2 seems to be required for protection of keratinocytes from UV cytotoxicity, since Nrf2 knockout mice showed a higher rate of apoptosis and oxidative damage in ear skin keratinocytes after UVB irradiation (Kawachi et al 2008).…”

The effects of phototherapy and melanocytes on keratinocytes

“…Previous studies have demonstrated that increased Nrf2 mRNA expression occurs due to UVA-induced oxidative stress (37,38). Additionally, Marrot et al (39) elucidated that UVA radiation promotes the expression of phase2 enzymes, particularly heme oxygenase 1, in keratinocytes. These phase 2 enzymes are the main method by which cells inhibit ROS generation.…”