Abstract 4310: Regular intake of high-fat diet exacerbates oxidative stress-mediated DNA hypermethylation and activation of Nrf2-Keap1 signaling in UVB-induced skin tumors in SKH-1 hairless mice
Authors: Singh Tripti , Prasad Ram , Katiyar Santosh K. 
Published: July 2016

Abstract
Generation of reactive oxygen species (ROS) is an essential metabolic process for maintaining normal cellular homeostasis; however elevated and sustained levels of ROS contribute in many human diseases, including cancer. Sustained and excessive cellular ROS result in genetic as well as epigenetic alterations, leading to deregulation of oncogenes and tumor suppressor genes. Nuclear factor erythroid 2-related factor 2 (Nrf2) is a ubiquitous transcription factor, capable of eliciting the coordinated induction of cytoprotective genes in response to oxidative stress. Upon oxidative insult, Nrf2 rapidly translocate to nucleus, activates genes encoding anti-oxidative enzymes such as heme oxygenase 1 (HO-1) and superoxide dismutase (SOD). Earlier, we have shown that regular intake of high-fat diet (HF-diet) enhances tumor development in UV-exposed SKH-1 hairless mouse skin compared to control-diet (C-diet) fed mice. Administration of HF-diet along with exposure of UVB radiation (180 mJ/cm2, 3x/week for 24 weeks) resulted in sustained oxidative stress as evident by depletion in antioxidant defense capabilities. The levels of reduced glutathione (40%, p&lt;0.001), glutathione peroxidase (49%, p&lt;0.001) and SOD (47%, p&lt;0.001) were noted, while the level of lipid peroxidation was significantly increased (160%, p&lt;0.001) in skin tumors of HF-diet fed mice compared with C-diet fed mice. Epigenetic alterations in association with oxidative stress were analyzed by estimation of global DNA methylation, DNA methyltransferase (Dnmt) activity and histone modifications. We observed significant increase of Dnmt activity (3 fold, p&lt;0.001) and global DNA methylation level (2 folds, p&lt;0.001) along with Dnmt proteins and methylated histone in tumors of HF-diet fed mice compared with tumors of C-diet fed mice. Oxidative stress disrupts Nrf2-Keap-1 binding by DNA hypermethylation and epigenetic silencing of Keap1 gene leading to its inactivation, release and translocation of Nrf2 to nucleus. Our RT-PCR data showed elevated levels of methylated Keap-1 and lower levels of un-methylated Keap-1 gene in skin tumors from HF-diet fed mice. Disruption of Keap-Nrf2 binding resulted in higher nuclear translocation of Nrf2 and activation of its target genes NQO-1 and HO-1 as confirmed by western blot analyses. Taken together, this study shows that chronic exposure to UVB along with administration of HF-diet induces oxidative stress, causes epigenetic modifications and loss of Keap1 function leading to nuclear accumulation of Nrf2 which creates a susceptible environment for skin tumor growth.
Citation Format: Tripti Singh, Ram Prasad, Santosh K. Katiyar. Regular intake of high-fat diet exacerbates oxidative stress-mediated DNA hypermethylation and activation of Nrf2-Keap1 signaling in UVB-induced skin tumors in SKH-1 hairless mice. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 4310.