Electrophilic Chemicals but not UV Irradiation or Reactive Oxygen Species Activate Nrf2 in Keratinocytes In Vitro and In Vivo

Abstract

The NF-E2-related factor 2 (Nrf2) transcription factor is a potent inducer of cytoprotective genes, which encode--among others--enzymes that detoxify reactive oxygen species (ROS). As we demonstrated a crucial role of Nrf2 in the prevention of skin carcinogenesis, it is of interest to identify Nrf2-activating factors in keratinocytes. For this purpose, keratinocytes from mice transgenic for an Nrf2-responsive reporter gene were analyzed. Electrophilic compounds activated the reporter in keratinocytes, and induced nuclear translocation of Nrf2 and the expression of known Nrf2 target genes. This is biologically relevant, as we show that Nrf2-mediated gene expression protects keratinocytes from the toxicity of these substances. By contrast, hydrogen peroxide, glucose oxidase, UVA, and UVB irradiation had no effect, although these treatments strongly increased the levels of intracellular ROS. To verify these results in vivo, transgenic reporter mice with and without functional Nrf2 alleles were topically treated with electrophilic chemicals or irradiated with UVB. Electrophiles but not UVB activated the reporter in an Nrf2-dependent manner. These results provide the basis for the identification of novel Nrf2 activators in keratinocytes with therapeutic potential for skin tumor prevention.

Hepatitis B Virus Induces Expression of Antioxidant Response Element-regulated Genes by Activation of Nrf2

“…The latter one represents a process based on multiple factors besides the induction of this signaling cascade by the two regulatory proteins. Our data argue against a prominent role of ROS as mediators of the HBV-dependent induction of Nrf2/ARE-regulated genes (51). However, there are many reports discussing ROS as mediators of the regulatory protein function or as important factors for HBV-dependent induction of HCCs (52).…”

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

“…Measurement of ROS and GSH levels, and of gGT activity Intracellular levels of ROS were determined in primary and immortalized keratinocytes using H 2 DCF-DA (Invitrogen) as described previously (Durchdewald et al 2007). For measurement of GSH levels, the epidermis was isolated from adult murine back skin.…”

“…In response to low doses of UVB or long-wave UVA irradiation of a mouse keratinocyte cancer cell line and of human fibroblasts and keratinocytes, increased Nrf2 activity was detected (Kannan and Jaiswal 2006;Gruber et al 2010). 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).…”

Cinnamoyl-based Nrf2-activators targeting human skin cell photo-oxidative stress

“…Indeed, germ line Nrf2-null mice are highly susceptible to electrophilic and oxidative stress [35]. Moreover, recent studies strongly suggest a role of Nrf2-mediated gene expression in the suppression of acute photo-oxidative damage in skin cells and also in the prevention of epidermal chemical (TPA/DMBA-induced) and UVinduced carcinogenesis [37][38][39]. In the skin of Nrf2 knockout mice, UVB-irradiation induced a more pronounced sunburn reaction and oxidative DNA damage as compared to wildtype mice, but no alterations in UVB-induced carcinogenesis were observed [40].…”