Cooperative regulation of mouse aldose reductase (AKR1B3) gene transcription by Nrf2, TonEBP, and c-jun

Abstract

Aldose reductase (AR), a member of aldo-keto reductase family, is the rate-limiting enzyme in the polyol pathway, and is known to play a key role in the pathogenesis of diabetic complications. AR also catalyzes the reduction of reactive aldehydes, thereby detoxifying endogenous as well as xenobiotic aldehydes in various tissues. The transcription of the AR gene was previously shown to be augmented by various stimuli including osmotic and oxidative stresses. A highly conserved region composed of an antioxidant response element (ARE), AP-1 site, and tonicity responsive enhancer (TonE) has been identified within the 5'-flanking region of the AR genes of humans, rats, and mice, which we designated as the multiple stress response region (MSRR). We previously showed that the transcription factor Nrf2 activated AR transcription via ARE within MSRR. In the present study, we examined the interactions among Nrf2, c-Jun, and the TonE-binding protein (TonEBP) in the regulation of AR gene transcription. In gene reporter assays using luciferase reporter constructs containing the MSRR of the mouse AR (AKR1B3) gene with HepG2 cells, the forced expression of Nrf2 or TonEBP significantly increased promoter activity. The synergistic augmentation of promoter activity was observed when Nrf2 and TonEBP were co-introduced. On the other hand, the co-expression of c-Jun repressed promoter activation by Nrf2 and TonEBP. The mutation of the AP-1 site within MSRR did not affect the repressive effects of c-Jun, while the introduction of truncated c-Jun proteins lacking the leucine zipper domain no longer suppressed Nrf2-or TonEBP-mediated transactivation, suggesting that c-Jun repressed promoter activity independently of the AP-1 site and that interactions with protein factors via the leucine zipper domain were necessary for its negative effects on Nrf2 and TonEBP. These results indicate that AR promoter activity is cooperatively regulated by multiple transcription factors via MSRR.

Keywords: AP-1; Aldo-keto reductase; HepG2; Tonicity responsive enhancer; antioxidant response element; c-Fos.