Vitamin D Impacts Nrf‐2 Signaling and Antioxidation Pathways that Modulate Cellular Aging

Abstract

Recent studies suggest that the biologically active form of vitamin D, 1,25‐dihydroxyvitamin D (1,25D), plays a role in potentially delaying the aging process, functions as an anti‐carcinogenic agent, and also prevents oxidative damage. 1,25D binding to the vitamin D receptor (VDR) promotes heterodimerization of VDR with the retinoid X receptor (RXR) to stimulate transcription of numerous vitamin D target genes. The VDR‐RXR complex controls transcriptional activity targeting various tissues such as the kidney and colon, regulates immune defenses, and controls cellular proliferation. The nuclear factor (erythroid‐derived 2)‐like 2 (Nrf‐ 2), is a transcriptional regulator that stimulates expression of genes encoding antioxidant enzymes via a DNA sequence referred to as an antioxidant‐responsive element (ARE). Nutraceuticals such as urolithin‐A have been proposed to play a role in this pathway. The current study was designed to investigate how vitamin D‐VDR signaling influences Nrf‐2 activity. Therefore, we utilized an ARE‐luciferase reporter plasmid to measure Nrf‐2 activity in human embryonic kidney cells (HEK‐293) in the presence of VDR. Our results indicate that VDR does not target genes with an ARE sequence directly. Instead, when cells were transfected with both Nrf‐2 and VDR, Nrf‐2 activity was significantly inhibited in a 1,25D‐ and VDR‐dependent manner. However, when cells were treated with urolithin‐A alone, or with vitamin D in conjunction with urolithin‐A, a nutraceutical that is hypothesized to cooperate with vitamin D, Nrf‐2 activity was upregulated. Additionally, real‐time quantitative polymerase chain reaction (qPCR) was employed to test the expression levels of GCLC and HMOX1 (two Nrf‐2 target genes), in the absence and presence of 1,25D, to assess if the mRNA expression results were consistent with the luciferase reporter assays. Thus, the current study reveals that VDR likely targets Nrf‐2 genes indirectly by influencing the activity of the Nrf‐2 transfactor. Collectively, our results demonstrate, for the first time, that VDR either binds directly to the Nrf‐2 protein to inhibit its activity, or 1,25D‐VDR represses Nrf‐2 gene expression resulting in a diminution of Nrf‐2 protein production. Modulation of Nrf‐2 activity by the vitamin D pathway appears to have a regulatory role in anti‐oxidation and cellular aging pathways.Support or Funding InformationNational Institutes of Health; grant numbers: DK033351 and CA140285.