ABSTRACT

The transcription factor Nrf2 is a key regulator of the cellular stress response, and pharmacological Nrf2 activation is a promising strategy for skin protection and cancer prevention. We show here that prolonged Nrf2 activation in keratinocytes causes sebaceous gland enlargement and seborrhea in mice due to upregulation of the growth factor epigen, which we identified as a novel Nrf2 target. This was accompanied by thickening and hyperkeratosis of hair follicle infundibula. These abnormalities caused dilatation of infundibula, hair loss, and cyst development upon aging. Upregulation of epigen, secretory leukocyte peptidase inhibitor (Slpi), and small proline-rich protein 2d (Sprr2d) in hair follicles was identified as the likely cause of infundibular acanthosis, hyperkeratosis, and cyst formation. These alterations were highly reminiscent to the phenotype of chloracne/“metabolizing acquired dioxin-induced skin hamartomas” (MADISH) patients. Indeed, SLPI, SPRR2, and epigen were strongly expressed in cysts of MADISH patients and upregulated by dioxin in human keratinocytes in an NRF2-dependent manner. These results identify novel Nrf2 activities in the pilosebaceous unit and point to a role of NRF2 in MADISH pathogenesis.

Functions of p53 during mitosis reportedly include prevention of polyploidy and transmission of aberrant chromosomes. However, whether p53 plays these roles during genomic surveillance in vivo and, if so, by direct or indirect means remain unknown. The ability of normal, mature hepatocytes to respond to stimuli, reenter cell cycle and regenerate liver mass offers an ideal setting to assess mitosis in vivo. In quiescent liver, normally high ploidy levels in adult mice increased with loss of p53. Following partial hepatectomy, p53−/− hepatocytes exhibited early entry into cell cycle and prolonged proliferation with an increased number of polyploid mitoses. Ploidy levels increased during regeneration of both WT and p53−/− hepatocytes, but only WT hepatocytes were able to dynamically resolve ploidy levels and return to normal by the end of regeneration. We identified multiple cell cycle and mitotic regulators, including Foxm1, Aurka, Lats2, Plk2 and Plk4, as directly regulated by chromatin interactions of p53 in vivo. Over a time course of regeneration, direct and indirect regulation of expression by p53 is mediated in a gene-specific manner. Conclusion Our results show that p53 plays a role in mitotic fidelity and ploidy resolution in hepatocytes of normal and regenerative liver.

The nuclear factor erythroid-derived 2, like 2 (Nrf2) transcription factor is a key regulator of the antioxidant defense system, and pharmacological activation of Nrf2 is a promising strategy for prevention of toxin-induced liver damage. However, the consequences of Nrf2 activation on liver regeneration (LR) have not been determined. To address this question, we generated mice expressing a constitutively active Nrf2 (caNrf2) mutant in hepatocytes. Expression of the transgene did not affect liver homeostasis. Surprisingly, however, there was no beneficial effect of Nrf2 activation on CCl 4 -induced liver injury and fibrosis. Most important, LR after partial hepatectomy was impaired in caNrf2-transgenic mice as a result of delayed hepatocyte proliferation and enhanced apoptosis of these cells after liver injury. Mechanistically, this involved up-regulation of the cyclin-dependent kinase inhibitor p15 and the proapoptotic protein Bcl2l11 (Bim). Using chromatin immunoprecipitation, we show that the p15 and Bcl2l11 genes are direct targets of Nrf2, which are activated under hyperproliferative conditions in the liver. Conclusion: Activated Nrf2 delays proliferation and induces apoptosis of hepatocytes in the regenerating liver. These negative effects of Nrf2 activation on LR should be considered when Nrf2-activating compounds are used for prevention of liver damage. (HEPATOLOGY 2014;60:670-678) See Editorial on Page 461 T he use of oxygen as an electron acceptor confronts aerobic organisms with the danger of reactive oxygen species (ROS) being formed as by-products of the respiratory chain. ROS formation can be further enhanced by nicotinamide adenine dinucleotide phosphate (NADPH) oxidases, which are particularly abundant in inflammatory cells.1 Low levels of ROS are required for intracellular signaling, 2 but excessive levels damage all types of cellular macromolecules. To limit ROS-induced cell damage, aerobes developed strategies for efficient ROS detoxification. Of particular importance is the transcription factor nuclear factor erythroid-derived 2, like 2 (Nrf2), which controls expression of numerous genes encoding antioxidant proteins and ROS-detoxifying enzymes. 3Under homeostatic conditions, Nrf2 is retained in the cytoplasm by binding to Keap1, which also targets Nrf2 for proteasomal degradation. However, some Nrf2 molecules escape this inhibitory mechanism and translocate to the nucleus, where they bind to antioxidant response elements (AREs) in the promoter/ enhancer regions of their target genes. This leads to a basal expression of most Nrf2 target genes. In the

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