The Vitamin D Receptor As A Therapeutic Target In Oxidative Stress And Nrf‐2 Signaling: Role Of Vitamin D And Urolithin A

Abstract

Vitamin D is an essential nutrient, commonly acquired via dietary intake and/or from endogenous cutaneous synthesis in response to ultraviolet radiation. The biologically active form of vitamin D, 1,25-dihydroxyvitamin D (1,25D), binds to the vitamin D receptor (VDR) and stimulates formation of an active hetero-complex with the retinoid X receptor (RXR). This VDR-RXR heterodimer controls vitamin D-regulated genes in such target tissues as kidney and colon, modulates immune defenses, and controls cellular proliferation. VDR may also play a significant role in preventing oxidative damage, potentially delaying the aging process, and serving as an anti-carcinogenic mediator. We hypothesize that VDR may target genes encoding antioxidant enzymes which contain antioxidant-responsive elements (AREs) that act as binding sites for transcriptional regulators such as nuclear factor (erythroid-derived 2)-like 2 (Nrf-2). This study aims to investigate the influence of vitamin D-VDR signaling on Nrf-2 activity. In order to probe a possible molecular mechanism, an ARE-luciferase reporter plasmid was employed to measure Nrf-2 activity in human embryonic kidney cells (HEK-293) in the presence of 1,25D/VDR. Results indicate cells transfected with both Nrf-2 and VDR displayed Nrf-2 activity that was modulated in a 1,25D- and VDR-dependent manner; with low 1,25D enhancing Nrf-2 activity while higher concentrations inhibited Nrf-2. When treating cells with 1,25D and/or urolithin-A (UA), a nutraceutical hypothesized to cooperate with vitamin D, Nrf-2 activity was instead consistently upregulated. Moreover, in qPCR studies with Nrf-2 target genes GCLC and HMOX1, UA and 1,25D treatment resulted in similar enhancement and/or suppression of Nrf-2, consistent with the luciferase-based assays. Collectively, these results imply that VDR likely targets Nrf-2 genes indirectly perhaps by influencing the activity of Nrf-2 transfactors and/or by post-translational modification of Nrf-2 to either activate or suppress Nrf-2-directed gene regulation. The modulation of Nrf-2 activity by a VDR-mediated pathway identifies a possible regulatory role for vitamin D in anti-oxidation and establishes the significance of vitamin D to human senescence and aging.