Metabolic Dangers of Nrf2 activation
My brief review of the paper “NRF2 activation induces NADH-reductive stress, providing a metabolic vulnerability in lung cancer” (R1).
Simple interpretation
Scientists discovered an interesting chemical imbalance that can occur in cells. When they blocked a protein called KEAP1, it activated another protein (NRF2) that caused too much NADH to build up in certain cells. NADH is like a cellular fuel carrier - normally, cells need to maintain a careful balance between NADH and its partner molecule NAD+.
When too much NADH accumulates, it’s like having a traffic jam in the cell’s energy production system. This happens because NRF2 activates an enzyme (ALDH3A1) that keeps converting NAD+ to NADH, while simultaneously making it harder for cells to use up the excess NADH.
Some cells can handle this imbalance because they have alternative ways to use up NADH, but others can’t cope and stop growing. This discovery is particularly important because some diseases involve similar NADH imbalances, and understanding this process could lead to new treatments.
Highlights of the study
- NRF2 activation significantly decreased respiratory capacity in KEAP1-dependent cells. This led to disruption of TCA cycle metabolites.
- NRF2 activation caused increased NADH/NAD+ ratios specifically in KEAP1-dependent cells. The effect could be rescued by providing NAD+ precursors or expressing NADH-oxidizing enzymes.
- ALDH3A1, an NRF2-responsive gene, was identified as a key mediator of NADH accumulation.
- Depleting ALDH3A1 could rescue cells from NRF2-induced growth inhibition.
Quotes
NRF2 activation significantly decreased respiratory capacity
We next investigated the impact of NRF2 activation on mitochondrial respiration, finding a substantial decrease in maximal respiratory capacity in KEAP1-dependent cells following KI696 treatment.
This defect in respiration was the result of NRF2, as loss of the transcription factor completely rescued OCR following KI696 treatment. Inhibition of mitochondrial function also extended to mitochondrial metabolism, where we found that tricarboxylic acid (TCA) metabolites were largely downregulated following NRF2 activation in KEAP1-dependent cell lines… (R1)
NRF2 activation caused increased NADH/NAD+ ratios
At baseline, we found that KEAP1-dependent and KEAP1-independent cells had a similar ratio of NADH/NAD+ (Figure S4B). However, following KI696 treatment we found a consistently higher NADH/NAD+ ratio generated in KEAP1-dependent cells compared to KEAP1-independent cells. (R1)
To determine whether NADH/NAD+ imbalance was a primary mechanism underlying NRF2 sensitivity, we treated cells with β-nicotinamide mononucleotide (NMN), an NAD+ precursor, which substantially rescued both high NADH/NAD+ ratio upon NRF2 activation and the concomitant proliferation block in KEAP1-dependent cells… Furthermore, KEAP1-dependent cells expressing the NADH-oxidizing enzymes LbNOX or NDI1 were partially protected from NRF2-mediated defects in proliferation and respiration, in comparison to cells expressing a control protein (METAP2) (R1)
ALDH3A1 was a key mediator of NADH accumulation
To explore the mechanisms by which NRF2 activation increases NADH levels in KEAP1-dependent cells, we focused on 105 enzymes that utilize NAD+ and are up-regulated following NRF2 activation.
We further filtered corresponding NAD-utilizing enzymes based on their ability to mediate resistance to NRF2 activation when depleted, identifying ALDH3A1 as a compelling candidate (Figure 5A).
ALDH3A1 is an NRF2-responsive gene (Figures S3B and S3C) that functions in antioxidant defense, specifically by converting reactive aldehydes to their corresponding carboxylic acids.
The dehydrogenase activity of ADLH3A1 results in the conversion of NAD+ to NADH, and depletion of this gene led to a substantial decrease in the NADH/NAD+ ratio following NRF2 activation in KEAP1-dependent cells. (R1)
KEAP1-(in)dependent cells
These are cancer cells that require functional KEAP1 protein to survive and grow normally. When KEAP1 is inhibited or removed in these cells, they stop growing or die.
This is in contrast to KEAP1-independent cells, which can continue to grow normally even when KEAP1 is inhibited. In this study, about 13% of the lung cancer cell lines tested were found to be KEAP1-dependent. This was surprising to researchers because KEAP1 is typically thought of as a tumor suppressor (something that prevents cancer growth), yet these cancer cells actually needed it to survive.