When the Cell Cannot Build: Mitochondrial Congestion and the Hidden Biology of Anabolic Resistance

Anabolism is the biology of building.

Cells build proteins, membranes, nucleotides, organelles and extracellular matrix. Tissues rebuild muscle after exercise, replace damaged cells, expand immune populations during infection and regenerate after injury. These processes require nutrients, hormonal signals and growth pathways—but they also require something more fundamental: sufficient metabolic throughput to convert those signals and raw materials into functional new structure.

This distinction matters because anabolic resistance is often explained mainly as a signalling problem. Insulin does not work properly. Muscle responds poorly to amino acids. Growth-factor signalling becomes blunted. The mTOR pathway is less responsive.

Those mechanisms are important, but they may not tell the whole story.

A recent study in Nature Metabolism provides a striking example of another possibility: a cell may receive an anabolic signal and possess some of the required substrates, yet still be unable to complete the anabolic programme because mitochondrial congestion has restricted both energy production and biosynthetic flow.

A new consequence of succinate dehydrogenase impairment

The study, led by Madeleine Hart and colleagues, examined what happens when succinate dehydrogenase, or SDH, becomes impaired. SDH has a unique position in metabolism because it participates in both the tricarboxylic acid cycle and the mitochondrial electron transport chain, where it functions as complex II.

When SDH activity was inhibited, intracellular aspartate initially fell. This was expected because mitochondrial metabolism is a major source of oxaloacetate, which can be converted into aspartate.

What happened next was more surprising.

Aspartate levels eventually rebounded, yet cell proliferation remained impaired. The cells appeared to recover the metabolite, but they did not recover normal function.

The researchers found that the rebound occurred partly because aspartate was no longer being used efficiently. Succinate accumulated to very high concentrations and directly inhibited aspartate transcarbamylase, or ATCase, the enzyme that commits aspartate to pyrimidine nucleotide synthesis.

As a result, the cells developed pyrimidine insufficiency, stalled during DNA replication, activated replication-stress signalling and became dependent on the ATR–CHK1 checkpoint pathway. Genetic SDH loss produced similar abnormalities in cultured cells and in a mouse tumour model.

The important lesson is that a metabolite can accumulate not because the pathway has recovered, but because its downstream use has become obstructed.

Mitochondrial congestion is more than low ATP

Mitochondrial dysfunction is often reduced to the phrase “low energy production.” That is important, but incomplete.

A better model is bioenergetic impedance: resistance to the movement and transformation of energy, electrons, carbon and reducing equivalents through the metabolic network.

When mitochondrial throughput slows, several things can occur simultaneously:

• NADH oxidation becomes constrained.

• The ubiquinone pool becomes more reduced.

• TCA-cycle reactions slow or reverse.

• Intermediates accumulate upstream of bottlenecks.

• Downstream products become scarce.

• Carbon is rerouted into lactate, lipids or other alternative pathways.

• Stress-response systems suppress growth and prioritize survival.

Steady-state ATP concentration may not initially collapse. Cells can preserve ATP by reducing expensive activities such as protein synthesis, proliferation, transport and repair. ATP measurements can therefore appear relatively stable while the cell has already lost much of its ATP-generating reserve.

The cell remains alive, but its ability to build has declined.

How unresolved impedance produces anabolic resistance

The SDH study helps reveal several distinct routes through which chronic mitochondrial impedance can create anabolic resistance.

1. Insufficient energy to execute anabolism

Protein synthesis, nucleotide production, membrane formation, ion transport, protein folding and organelle biogenesis are energetically expensive.

When mitochondrial ATP-generating capacity is constrained, the cell must triage its energy use. Essential functions such as membrane potential, ion balance and basic survival receive priority. Growth and renewal become conditional.

AMPK and other energy-sensing pathways can suppress mTOR activity and protein translation. The integrated stress response can reduce global protein synthesis while selectively producing stress-response proteins.

In this situation, anabolic resistance is protective in the short term. It prevents the cell from committing to construction that it cannot energetically complete.

When the constraint becomes chronic, however, the same response contributes to tissue wasting, poor recovery and declining functional reserve.

2. Inadequate precursor production

Anabolism requires more than ATP. Cells also need carbon skeletons, nitrogen donors, nucleotides, amino acids, lipids and one-carbon units.

Mitochondrial congestion can reduce the production of oxaloacetate and aspartate. Aspartate is especially important because it contributes to:

• pyrimidine synthesis;

• purine synthesis;

• asparagine production;

• protein synthesis;

• metabolic exchange between mitochondrial and cytosolic compartments.

The Hart study showed that SDH inhibition created a hierarchy of biosynthetic limitations. Pyrimidine deficiency emerged first. When pyrimidines were supplied externally, purine limitation became apparent. When both nucleotide branches were supported, additional aspartate-dependent needs became limiting.

This means that anabolic resistance may arise from several distributed bottlenecks rather than one missing nutrient.

Removing one bottleneck may simply reveal the next one.

3. Accumulated metabolites become active inhibitors

Congestion does not merely reduce downstream supply. It also causes upstream metabolites to accumulate.

The new study demonstrates that accumulated succinate is not simply a passive marker of SDH impairment. It becomes an active source of downstream resistance by inhibiting ATCase.

This creates a two-sided constraint:

reduced mitochondrial throughput → less aspartate production

and simultaneously:

succinate accumulation → less aspartate utilization

The first limits substrate supply. The second blocks the pathway that would use the substrate.

This is a general principle of unresolved congestion: accumulated intermediates can inhibit enzymes, alter protein modifications, disturb redox balance, activate inflammatory signalling and reshape gene expression. The metabolic backlog begins to create additional impedance of its own.

4. Replication stress prevents cellular renewal

Pyrimidines are required for DNA synthesis. When they become insufficient or imbalanced, replication forks slow or stall.

The cell activates ATR and CHK1 to stabilize DNA replication and prevent catastrophic damage. Proliferation slows, cells accumulate in S phase and cell-cycle progression becomes conditional on resolving the metabolic problem.

This is a direct form of anabolic resistance. The cell may continue taking up nutrients and even increase in size, but it cannot safely complete DNA replication and divide.

This distinction is important:

Growth signalling may remain active while regenerative completion fails.

Replication stress is likely to be especially relevant in tissues that depend on continuous cell renewal, including:

• intestinal and epithelial tissues;

• bone marrow;

• immune-cell populations;

• skin;

• wound-healing tissues;

• stem and progenitor compartments.

In skeletal muscle, mature fibres are largely postmitotic, so replication stress is unlikely to explain all forms of muscle protein anabolic resistance directly. However, it could impair satellite-cell expansion and therefore limit muscle regeneration after injury, illness or training.

5. Stress checkpoints override anabolic signals

Cells do not evaluate anabolic signals in isolation. Insulin, amino acids and growth factors are interpreted alongside information about energy status, redox balance, DNA integrity, oxygen availability and proteostatic load.

If those systems indicate unresolved danger, growth can be actively suppressed even when nutrients are abundant.

Relevant checkpoints include:

• AMPK during energetic stress;

• the integrated stress response during amino-acid, mitochondrial or proteostatic stress;

• ATR–CHK1 during replication stress;

• p53 and cell-cycle checkpoints during DNA damage;

• HIF signalling during oxygen limitation;

• inflammatory pathways during mitochondrial stress and metabolite accumulation.

Anabolic resistance may therefore represent an integrated safety decision:

Do not build while the metabolic system cannot reliably support completion.

6. More substrate can increase pressure without increasing flow

This framework also explains why simply providing more calories, protein, amino acids or anabolic stimulation may sometimes produce limited benefit.

When metabolic throughput is intact, additional substrate can support growth.

When throughput is constrained, additional substrate may instead increase:

• redox pressure;

• substrate accumulation;

• lactate production;

• lipid storage;

• incomplete oxidation;

• oxidative or reductive stress;

• inflammatory signalling.

The problem is not always inadequate input. It may be inadequate capacity to process the input.

Increasing anabolic stimulation without resolving the bottleneck can be compared to sending more vehicles into an already congested road network. Traffic pressure rises, but effective movement does not.

Anabolic resistance as failed anabolic completion

This suggests a broader definition of anabolic resistance.

Anabolic resistance is not only reduced sensitivity to insulin, amino acids or mTOR signalling. It can also arise when cells lack the energetic, metabolic and checkpoint capacity to complete growth and renewal.

A useful conceptual sequence is:

unresolved bioenergetic impedance

↓

reduced ATP generating reserve

reduced precursor production

metabolite accumulationredox and proteostatic stress

replication stress

growth-checkpoint activation

↓

failure to convert anabolic signals into functional repair

This can produce several apparently different outcomes:

• poor muscle response to protein or exercise;

• impaired immune-cell expansion;

• delayed wound healing;

• reduced stem-cell regeneration;

• incomplete tissue repair;

• progressive frailty;

• cellular enlargement without effective division;

• storage and hypertrophy without restored function.

Resolution must restore flow, not merely increase input

The practical implication is not that anabolic support is unimportant. Protein, amino acids, hormones, mechanical loading and adequate nutrition remain necessary.

But they may not be sufficient when the deeper problem is unresolved metabolic impedance.

Effective recovery may require restoration of the conditions that permit flow:

• adequate oxygen delivery;

• electron-transport capacity;

• redox balance;

• mitochondrial quality control;

• substrate flexibility;

• micronutrient and cofactor availability;

• removal of inflammatory or toxicant burden;

• sufficient sleep and recovery;

• appropriately dosed physical activity;

• resolution of infection, injury or chronic stress demand.

Anabolism depends not only on having building materials. It depends on whether the entire network can move energy and matter through the required pathways without becoming congested.

The larger lesson

The Hart study focuses on SDH loss, succinate, aspartate and pyrimidine synthesis. Its direct evidence comes largely from engineered cells and tumour models, so it does not by itself prove the same mechanism explains every form of human anabolic resistance.

Yet it provides a powerful mechanistic example of a broader principle:

Mitochondrial congestion can restrict anabolism not only by reducing ATP output, but also by limiting precursor availability, accumulating inhibitory metabolites and activating checkpoints that prevent unsafe growth.

A cell can have nutrients.

It can receive growth signals.

It can even preserve its ATP concentration.

But when bioenergetic impedance remains unresolved, the cell may still be unable to build, divide, repair or recover.

That may be one of the hidden biological foundations of anabolic resistance.

Reference

Hart, M. L., Sokolov, D., Danquah, S., Zheng, E., Doan, A. D., Davidsen, K., MacPherson, D., & Sullivan, L. B. (2026). Succinate dehydrogenase loss suppresses pyrimidine biosynthesis via succinate-mediated inhibition of aspartate transcarbamylase. Nature Metabolism, 8, 1390–1409. https://doi.org/10.1038/s42255-026-01524-w