Cap-independent Nrf2 translation is part of a lipoic acid-stimulated detoxification stress response

Abstract

Little is known about either the basal or stimulated homeostatic mechanisms regulating nuclear tenure of Nf-e2-related factor 2 (Nrf2), a transcription factor that mediates expression of over 200 detoxification genes. Our data show that stress-induced nuclear Nrf2 accumulation is largely from de novo protein synthesis, rather than translocation from a pre-existing cytoplasmic pool. HepG2 cells were used to monitor nuclear Nrf2 24 hrs following treatment with the dithiol micronutrient (R)-α-lipoic acid (LA; 50 μM), or vehicle. LA caused a ≥2.5-fold increase in nuclear Nrf2 within 1 hr. However, pretreating cells with cycloheximide (50 μg/ml) inhibited LA-induced Nrf2 nuclear accumulation by 94%. Providing cells with the mTOR inhibitor, rapamycin, decreased basal Nrf2 levels by 84% after 4 hrs, but LA overcame this inhibition. LA-mediated de novo protein translation was confirmed using HepG2 cells transfected with a bicistronic construct containing an internal ribosome entry sequence (IRES) for Nrf2, with significant (P<0.05) increase in IRES use under LA treatment. These results suggest that a dithiol stimulus mediates Nrf2 nuclear tenure via cap-independent protein translation. Thus, translational control of Nrf2 synthesis, rather than reliance solely on pre-existing protein, may mediate the rapid burst of Nrf2 nuclear accumulation following stress stimuli.

Nutritive values, flavor amino acids, healthcare fatty acids and flesh quality improved by manganese referring to up-regulating the antioxidant capacity and signaling molecules TOR and Nrf2 in the muscle of fish

“…Meanwhile, Mn deficiency induced the high-abundance of Keap1 mRNA level in grass carp muscle, whereas optimal Mn supplementation resulted in the low-abundance of Keap1 gene expression in grass carp muscle. Moreover, it was reported that signaling molecule TOR up-regulated the Nrf2 expression in human hepatic carcinoma cells (Shay, Michels, Li, Kong, & Hagen, 2012). The results from the present study showed that Mn deficiency and excess both induced the low-abundance mRNA levels of TOR, whereas optimal dietary Mn supplementation up-regulated the TOR gene expression in grass carp muscle.…”

Dietary leucine improves flesh quality and alters mRNA expressions of Nrf2-mediated antioxidant enzymes in the muscle of grass carp ( Ctenopharyngodon idella )

“…Our results indicated that 13.3 g kg −1 leucine supplementation down-regulated the expression of Keap1 gene in the muscle, suggesting that leucine may facilitate Nrf2 nuclear translocation to induce mRNA transcripts for SOD1, CAT and GPx, likely through down-regulating Keap1 gene expression in fish. In addition to stimulation of Nrf2 nuclear translocation, newly synthesized Nrf2 plays important role in Nrf2 nuclear accumulation (Shay et al, 2012). This author found that TOR was involved in the synthesis of Nrf2 protein in HepG2 cells.…”

“…Accordingly, there may be a close relationship between leucine and Nrf2, and Nrf2 may be involved in modulating gene expression of antioxidant enzymes regulated by leucine in fish muscle, which has not been investigated. Furthermore, mammalian target of rapamycin (mTOR) has been shown to regulate Nrf2 levels in HepG2 cells (Shay et al, 2012). Ribosomal protein S6 kinase 1 (S6K1), an effector of TOR, is sensitive to nutrients (Um et al, 2006).…”

An increased need for dietary cysteine in support of glutathione synthesis may underlie the increased risk for mortality associated with low protein intake in the elderly

“…Some of these-such as lipoic acid-also boost translation of Nrf2 messenger RNA via an internal ribosome entry sequence (IRES) (Shay et al 2012). Hence, phase 2 inducers typically increase intracellular glutathione synthesis by increasing GCL transcription and expression.…”