supporting aspartate biosynthesis

Title: Supporting Aspartate Biosynthesis Is an Essential Function of Respiration in Proliferating Cells
Authors: Lucas B. Sullivan, Dan Y. Gui, Aaron M. Hosios, Lauren N. Bush, Elizaveta Freinkman, Matthew G. Vander Heiden
Published in: Cell, Volume 162, Pages 552-563, July 30, 2015
Corresponding Author: mvh@mit.edu
DOI: http://dx.doi.org/10.1016/j.cell.2015.07.017

Electron Acceptors & Cell Proliferation:
- Electron acceptor deficiency limits proliferation in respiration-deficient cells.
- Alternative electron acceptor, alpha-ketobutyrate (AKB), supports proliferation under respiration deficiency.
- Aspartate enables proliferation in the absence of electron acceptors.
- Respiration plays a crucial role in aspartate synthesis for proliferating cells.

Mitochondrial Respiration Basics:
- Couples nutrient oxidation to ATP production through redox reactions, transferring electrons ultimately to molecular oxygen, reducing it to water.
- Generates free energy > electrochemical gradient > drives ATP synthesis, membrane transport, and thermogenesis.
Impacts of Respiration on Proliferation:
- Many proliferating cells resort to fermentation, primarily glycolysis for ATP production rather than relying solely on respiration.
- Cancer cells exhibit increased glucose consumption, utilizing glycolysis to produce lactate despite the presence of oxygen. This phenomenon is known as the Warburg effect.
- There is an apparent need for mitochondria to enable proliferation despite reliance on aerobic glycolysis.
- mitochondrial respiration and aerobic glycolysis occur parallel in proliferating cells
Experimentation Context:
- ρ⁰ cells (mtDNA deficient) can't proliferate without exogenous uridine and pyruvate because have non-functional ETC
- This research shows lack in mitochondrial respiration = reduced electron acceptors > impaired aspartate synthesis, inhibits cell proliferation,
- proliferation restored = high [aspartate]; exogenous electron acceptors from other cells

Study of Pyruvate:
- Indicates that pyruvate serves dual roles:
  - Acts as a carbon substrate for biosynthetic intermediates (e.g. p > oxaloacetate via pyruvate carboxylate; p > acetyl coa via pyruvate dehydrogenase complex)
  - Is an exogenous electron acceptor + regenerates NAD+ via lactate dehydrogenase (LDH) reaction, critical for glycolysis and NAD+ balance.
AKB as an Electron Acceptor:
- AKB = alt substrate, 143B CytB cell proliferation in no pyruvate
- AKB can prevent proliferation blocks in pyruvate-deficient conditions.
- Mechanistically, it supports the regeneration of NAD+ but contributes little as a carbon substrate compared to pyruvate - doesn’t restore o2 consumption
- Experimental Findings:
  - Proliferation rates improved with AKB application in response to respiration inhibitors.
  - increased NAD+/NADH ratios suggests both pyruvate and AKB sustain NAD+ pools crucial for metabolic functions.
  - AKB and pyruvate = different metabolic pathways > AKB = e- acceptor, not carbon substrate

Nucleotide Pool Analysis:
- The research indicates respiration deficiency leads to the limits on nucleotide synthesis due to aspartate availability.
- reduced respiration > reduced adenine synthesis > less adenine made > less electron acceptors available > reduced cell proliferation
- The study quantified purine nucleotides shows greatly reduced AMP/IMP and GMP/IMP ratios with no AKB > can’t convert IMP to AMP/GMPP.
- reaction uses NAD+, so inhibited by reduced NAD+
- IMP > AMP uses aspartate; reduced AMP from aspartate deficiency
- adenine only restores growth when in cell phase of limiting adenine levels, growth is only role of aspartate that adenine restores

Critical Role of Aspartate:
- aspartate not electron acceptor, but is in demand in biosynthesis, but becomes limiting for proliferation in cells without electron acceptor
- Exogenous supplementation of aspartate alleviates cell growth impediments, restores nucleotide levels particularly AMP, while GMP remains limited due to limited NAD+ availability > both GMP and NAD+ aren’t restored by aspartate treatment.
- respiration = O2 as electron acceptors > supports aspartate biosynthesis - no mitochondrial respiration = use AKB or pyruvate as electron acceptors > aspartate demand met exogenously, no need for electron acceptors > therefore major function of proliferation cell respiration = aspartate synthesis.

Proliferation Studies with Inhibitors:
- The study tests several respiration inhibitors on diverse cell lines, revealing dependency on electron acceptors for proliferation—even under conditions limiting standard energetic outputs.
- Specific inhibition of ATP synthase was shown to maintain viability and proliferative rates in the presence of electron acceptors like AKB but was not directly linked to ATP generation.

This research proposes a paradigm shift where the primary role of mitochondrial respiration in proliferating cells is to supply electron acceptors for aspartate biosynthesis rather than focusing solely on ATP production.
Highlights the metabolic adaptations of cancer cells and their reliance on aspartate amidst respiration deficiencies, suggesting potential therapeutic avenues targeting these metabolic pathways.

Cell Culture Methods:
- Uses adherent cells cultured in DMEM with specific supplements including uridine and glucose.
Proliferation Calculation:
- Doubling rates calculated based on final and initial cell counts following specific time intervals post-treatment.
NAD+/NADH Measurements:
- Modified protocols according to NAD/NADH Glo Assay guidelines for accurate metabolite quantification.