From Cellular Genotype to Cigarette Smoke-Induced Phenotype: The Case of Nrf2

Cigarette smoke (CS) is a complex chemical mixture estimated to be composed of up to 5000 different chemicals including several class I carcinogens (according to IARC classification), a plethora of reactive oxygen and nitrogen species (ROS, RNS), and numerous free radicals. It is long-standing common scientific opinion that these compounds, which interact either directly or indirectly with target molecules in the O2-containing extra-cellular and intra-cellular milieu of the respiratory tract or elsewhere in the organism, are the drivers of CS-dependent chronic disease, mainly lung cancer, chronic obstructive pulmonary disease (COPD), and cardiovascular disease. Consequent to acute and chronic CS exposure, cells continuously attempt to cope with deteriorated micro-environmental conditions (e.g., pro-oxidant, mutagenic), which results in global changes to their transcriptomes. Research over the last decade has identified the transcription factor Nrf2 as being responsible for orchestrating cellular defense against any kind of oxidant stress on a large scale (1), though recent studies challenge this exclusive protective image by showing that the trans-activating potential of Nrf2 is abused by tumor cells, especially during lung tumorigenesis (2). Here, we demonstrate, starting with in vitro research, that CS exposure results in the stabilization, nuclear translocation, and DNA binding of Nrf2. Promoter deletion studies using the heme oxygenase-1 (HMOX1) gene as an inducible antioxidant paradigm revealed that this gene is activated mainly by CS via several cis antioxidant responsive elements (AREs) recognized by Nrf2 in trans (3). The conclusion from these in vitro studies, i.e., that the pathway underlying the activation of Nrf2 is a major target of CS exposure, was confirmed in vivo by subjecting Nrf2-/- and Nrf2+/+ mice to smoke inhalation over 5 months (3 different CS doses). Results show that CS-exposed Nrf2-/- mice, in contrast to their WT littermates, are strongly impaired regarding the expression of antioxidant and phase II-related genes, although the effect appears to be compensated for to a minor extent by other transcription factors. Comparison of the gene expression data set with that of age-matched KEAP1 conditional KOS confirmed known Nrf2 target genes, identified new Nrf2 target genes, and provided evidence for an age-related adaptation effect of Nrf2 activity (Drs. K. Taguchi and m. Yamamoto, pers. Comm.). Regarding the CS-induced phenotype in relation to the Nrf2 genotype, somewhat enhanced pathological effects were observed for CS-exposed Nrf2-/- mice in terms of a significant attenuation of bodyweight gain, increased scores for ‘mean cord length’ (an indicator for emphysema), and several lung function parameters. While these results underline the relevance of Nrf2 activation as general defense mechanism against CS-induced cellular stress, their quantitative dimension will be discussed in the context of published data (4). 1) Cho et al. Antioxid. Redox signal. (2006) 8: 76-87 2) Lau et al. Pharmacol. Res. (2008) in press 3) Knörr-Wittmann et al. Free Radic. Biol. Med. (2005) 39: 1438-1448 4) Rangasamy et al. J.Clin.Invest. (2004) 114: 1248-1259