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NRF2 as a determinant of cellular resistance in retinoic acid cytotoxicity
Kah Poh Tan1, Kazuhiro Kosuge, Mingdong Yang
1Division of Clinical Pharmacology and Toxicology, Physiology and Experimental Medicine Program, The Research Institute, The Hospital for Sick Children, Department of Pediatrics and Department of Pharmacology, University of Toronto, Toronto, ON, Canada.
Abstract:
Clinical use of retinoic acids (RA) is hindered by toxicity possibly related to oxidative stress. Recently, RA at relatively low concentrations was shown to inhibit NRF2 and the expression of its target antioxidative genes. This raises the possibility that RA toxicity may result from cellular inability to cope with resultant oxidative stress. Using in vitro cell and in vivo mouse models, we report that RA, specifically all-trans-RA (atRA) at concentrations implicated in toxicity, can activate NRF2 and induce NRF2 target genes, particularly the subunits of the rate-limiting enzyme of glutathione biosynthesis, glutamate cysteine ligase (GCLM/GCLC). RNA interference-mediated silencing of NRF2, but not of retinoid X receptor-alpha and -beta, reduced basal and atRA-induced GCLM/GCLC gene expression. Moreover, RA increased nuclear accumulation of NRF2, antioxidant response element (ARE) reporter activity, and NRF2 occupancy at AREs. 4-Hydroxynonenal, a lipid peroxidation product, was increased by RA. Inhibition of MEK1/ERK mitogen-activated protein kinases significantly suppressed atRA-induced NRF2 activation and ARE-regulated gene expression, reducing cell resistance against toxic concentrations of RA. NRF2-silenced cells were vulnerable to atRA-induced mitochondrial toxicity and apoptosis. In conclusion, toxic RA activates NRF2, thereby triggering an adaptive response against the resultant oxidative stress. NRF2 enhancement as a therapeutic target of retinoid toxicity awaits further investigation.
Insights
Toxic retinoic acids (RA) paradoxically activate the NRF2 antioxidant pathway, triggering a protective response against oxidative stress. This NRF2 activation may be a key factor in understanding and potentially mitigating retinoid toxicity.
Area of Science:
- Molecular Biology
- Toxicology
- Cell Biology
Background:
- Clinical use of retinoic acids (RA) is limited by toxicity, potentially linked to oxidative stress.
- Previous studies suggested RA inhibits the NRF2 antioxidant pathway, raising concerns about cellular defense mechanisms.
- This study investigates the complex relationship between RA, oxidative stress, and the NRF2 pathway.
Purpose of the Study:
- To determine if retinoic acids, at toxic concentrations, activate or inhibit the NRF2 pathway.
- To elucidate the role of NRF2 in cellular response to all-trans-retinoic acid (atRA)-induced toxicity.
- To explore the signaling pathways involved in atRA-mediated NRF2 activation.
Main Methods:
- Utilized in vitro cell and in vivo mouse models.
- Employed RNA interference to silence NRF2 and retinoid X receptor genes.
- Assessed NRF2 activation, target gene expression (glutathione biosynthesis enzymes), nuclear translocation, DNA binding, and cell viability.
- Investigated the role of MEK1/ERK signaling pathway.
Main Results:
- All-trans-retinoic acid (atRA) at toxic concentrations activates NRF2 and induces its target genes, including glutamate cysteine ligase (GCLM/GCLC).
- NRF2 silencing diminished basal and atRA-induced GCLM/GCLC expression, increasing cell vulnerability to atRA toxicity.
- atRA increased NRF2 nuclear accumulation, antioxidant response element (ARE) activity, and NRF2 binding to AREs, while also increasing a lipid peroxidation product.
- MEK1/ERK pathway inhibition suppressed atRA-induced NRF2 activation and gene expression, reducing cellular resistance to toxic RA concentrations.
Conclusions:
- Toxic concentrations of RA activate the NRF2 pathway, initiating an adaptive response to counteract associated oxidative stress.
- NRF2 plays a crucial role in protecting cells against RA-induced mitochondrial toxicity and apoptosis.
- Targeting NRF2 enhancement warrants further investigation as a potential strategy to manage retinoid toxicity.
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