Increased demand for NAD+ relative to ATP drives aerobic glycolysis, 2020, Luengo et al

[Hoopoe]

Summary
Aerobic glycolysis, or preferential fermentation of glucose-derived pyruvate to lactate despite available oxygen, is associated with proliferation across many organisms and conditions. To better understand that association, we examined the metabolic consequence of activating the pyruvate dehydrogenase complex (PDH) to increase pyruvate oxidation at the expense of fermentation. We find that increasing PDH activity impairs cell proliferation by reducing the NAD+/NADH ratio. This change in NAD+/NADH is caused by increased mitochondrial membrane potential that impairs mitochondrial electron transport and NAD+ regeneration. Uncoupling respiration from ATP synthesis or increasing ATP hydrolysis restores NAD+/NADH homeostasis and proliferation even when glucose oxidation is increased. These data suggest that when demand for NAD+ to support oxidation reactions exceeds the rate of ATP turnover in cells, NAD+ regeneration by mitochondrial respiration becomes constrained, promoting fermentation, despite available oxygen. This argues that cells engage in aerobic glycolysis when the demand for NAD+ is in excess of the demand for ATP.





I wonder whether this anything to do with ME/CFS.

These data argue that cells engage in aerobic glycolysis when the NAD+ demand for oxidation reactions exceeds the demand for ATP, creating a situation in which mitochondrial respiration is insufficient to support NAD+ regeneration.

My thought is that if in ME/CFS some strange metabolic plan is being followed that involves a reduction in pyruvate going into the citric acid cycle, as well as increase in lactate (evidence that this pyruvate is being fermented), then maybe the cells are doing this because they need more NAD+ than usual.

https://www.cell.com/molecular-cell/fulltext/S1097-2765(20)30904-7

 

[Hoopoe]

NAD+ degradation
NAD+ consumption
As a co-substrate important to various postsynthesis modifications of fundamental macromolecules, NAD+ can be cleaved by NAD+-consuming enzymes including PARPs, sirtuins, CD38 and SARM1 to generate NAM and ADP-ribose (ADPR) (Fig. 1). The sirtuins are NAD+-dependent deacetylases that are distributed in the nucleus (e.g., SIRT1, SIRT6, and SIRT7), the cytoplasm (e.g., SIRT2) and mitochondria (e.g., SIRT3-5), respectively.26 Through mediating the post-translational modification dependent on NAD+, sirtuins modulate the adaptation to the altered cellular energetic status, especially the activation of oxidative metabolism and stress resistance in mitochondria in various physiological or pathological conditions.26 PARPs catalyze reversible ADP-ribosylation of macromolecular targets including proteins, DNA and RNA, utilizing NAD+ as a cofactor to provide monomer or polymers of ADP-ribose nucleotide.27,28 PARP members can be categorized into several groups, the poly-ADP-ribosyl transferases (e.g., PARP1, 2, and 5), the mono-ADP-ribosyl transferases (e.g., including PARP 3, 4, 6–8, and 10–16) and RBPs (e.g., PARP7, 10, and 12–14).27,29 PARPs-mediated ADP-ribosylation (ADPr) plays an essential role in cellular physiological processes in response to stimuli, particularly oxidative stress-induced DNA damage. Sustained PARP activation triggered by intense insults can cause NAD+ depletion and subsequent cell death.30 CD38 consumes NAD+ to make the calcium-releasing second messengers including ADPR (major product), 20-deoxy-ADPR (2dADPR), NAADP and cADPR, contributing to age-related NAD+ decline.31,32 SARM1 is an important NAD+ consumer in neurons. The dimerization of TIR domain cleaves NAD+ into ADP-ribose, cADPR, and nicotinamide.33,34,

https://www.nature.com/articles/s41392-020-00311-7

 

[Creekside]

My ME symptoms rose dramatically when I took extra tryptophan. Even quickly-digested carbs would make me feel worse 20 minutes later, which is expected as insulin boosts TRP transport into the brain, and is blocked by taking BCAAs. Furthermore, dietary niacin made me feel strongly suicidal. I believe that the lack of dietary niacin was forcing my brain cells to convert excess quinolinic acid to NAD+ to meet demands. Boosting niacin, as an NAD+ precursor, would thus result in QUIN piling up rather than being converted.

This paper ( https://www.frontiersin.org/articles/10.3389/fimmu.2020.00031/full ) is Quinolinate as a Marker for Kynurenine Metabolite Formation and the Unresolved Question of NAD+ Synthesis During Inflammation and Infection.

One bit that caught my attention: "It is possible that prolonged dietary tryptophan excess during a strong immune response may turn out to be an exacerbating factor in certain inflammatory conditions, shifting kynurenine pathway flux from a net positive to an overall negative effect. "

 

[Snow Leopard]

NAD+ is (re)generated via the malate-aspartate shuttle and the glycerol-3-phosphate shuttle

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5737637/figure/f5/

The process is reversible and the kinetics depend on the relative concentrations.

Keeping in mind that cell culture experiments can involve manipulations that aren't necessarily feasible in living organisms... I'm still reading the (OP) article.

These data argue that cells engage in aerobic glycolysis when the NAD+ demand for oxidation reactions exceeds the demand for ATP, creating a situation in which mitochondrial respiration is insufficient to support NAD+ regeneration.

I don't understand this argument.

Fermentation of pyruvate into lactic acid (which itself is a buffer that can be turned back into glucose) simply regenerates the NAD+ that was consumed during formation of pyruvate in the first place. This process cannot generate excess NAD+ to feed other oxidation reactions. Meaning what they are suggesting is simply a transient/temporary process, rather than an equilibrium process.

Secondly, oxidation reactions such as during fatty acid metabolism don't preclude regeneration of NAD+ given the shuttles mentioned above.

On further reading, they experimentally induced a condition where the mitochondrial membrane potential built up to high levels, leading to increased function of Carbonyl cyanide-4-(trifluoromethoxy)phenylhydrazone (FCCP). This is basically acting as a relief valve that reduces the membrane potential, but bypasses the electron transport chain, thus limiting ATP production. This also uncouples NAD+ regeneration from the electron transport chain (bypassing the aforementioned shuttles).

A big weakness in their study is they don't actually show how this hypothesised NAD+ deficiency occurs naturally. In this study, they haven't demonstrated increased fatty acid metabolism or other oxidative pathways.
Experimentally they inhibited Pyruvate dehydrogenase kinases (PDK) using pharmacological agents and CRISPR. These kinases inhibit pyruvate dehydrogenase (PDH) through phosphorylation. Normally PDKs play a key role in PDH regulation, since NADH & Acetyl CoA in turn positively regulates PDK activity. This coupling is already known and is key to the whole process.


As such, little in this paper should be considered particularly surprising.

The abstract could read very differently if written by another author - PDK inhibition removes the necessary feedback loop regulating PDH activity, leading to a high membrane potential, leading to activation of FCCP and a loss of ATP and NAD+ generation in the mitochondria. Or more simply put, the disruption of the regulation of mitochondrial respiration leads to increased lactate production. It doesn't sound so surprising when stated like that.

 

[Mij]

merged

Highlights

• •
  PDH activation suppresses cell proliferation by reducing the NAD+/NADH ratio
• •
  Insufficient ATP demand slows mitochondria NAD+ regeneration in proliferating cells
• •
  Uncoupling mitochondrial respiration from ATP synthesis can increase proliferation
• •
  Aerobic glycolysis reflects increased cell demand for NAD+ relative to ATP turnover

Summary
Aerobic glycolysis, or preferential fermentation of glucose-derived pyruvate to lactate despite available oxygen, is associated with proliferation across many organisms and conditions. To better understand that association, we examined the metabolic consequence of activating the pyruvate dehydrogenase complex (PDH) to increase pyruvate oxidation at the expense of fermentation. We find that increasing PDH activity impairs cell proliferation by reducing the NAD+/NADH ratio. This change in NAD+/NADH is caused by increased mitochondrial membrane potential that impairs mitochondrial electron transport and NAD+ regeneration. Uncoupling respiration from ATP synthesis or increasing ATP hydrolysis restores NAD+/NADH homeostasis and proliferation even when glucose oxidation is increased. These data suggest that when demand for NAD+ to support oxidation reactions exceeds the rate of ATP turnover in cells, NAD+ regeneration by mitochondrial respiration becomes constrained, promoting fermentation, despite available oxygen. This argues that cells engage in aerobic glycolysis when the demand for NAD+ is in excess of the demand for ATP

Clarifying why proliferative cells exhibit aerobic glycolysis: ATP hydrolysis surprisingly constrains NAD re-oxidation.
https://www.cell.com/molecular-cell/fulltext/S1097-2765(20)30904-7#disqus_thread