Elevated G6PD expression coexisted with higher NADP⁺/NADPH and GSSG/GSH ratios under prolonged injury. The result shows why metabolic compensation is not necessarily the same as redox recovery.
The liver increased a central component of its NADPH-generating machinery—and still became more oxidized.
That is the striking pattern in a study by Ou-Yang et al.
In mice with hepatocyte-specific loss of nuclear receptor corepressor 1, NCoR1, G6PD protein expression was elevated. Through the oxidative pentose phosphate pathway, G6PD connects glucose metabolism to cytosolic NADPH generation.
Yet four weeks after a single injection of the genotoxic hepatocarcinogen diethylnitrosamine, DEN, administered at 15 days of age, the NADP⁺/NADPH and GSSG/GSH ratios were also higher. Both redox systems shifted toward oxidation, while mitochondrial reactive oxygen species increased, hepatic ATP declined, and oxidative damage and apoptosis rose.
The liver responded.
But response was not the same as recovery.
The same genetic model shows how context and persistence can change the outcome. Loss of NCoR1 increased Fasn and Acc2 expression, hepatic lipid accumulation and the broader de novo fatty-acid synthesis program.
After partial hepatectomy, this lipogenic program supported regeneration. Hepatocyte proliferation increased, liver mass recovered earlier, and ACC2 knockdown or treatment with orlistat reduced the regenerative advantage.
The acute response carried a measurable redox cost. During early regeneration, the NADP⁺/NADPH ratio and mitochondrial reactive oxygen species were elevated. By 48 hours, however, the differences between NCoR1-deficient and control livers had largely narrowed.
The disturbance was real, but temporally contained.
Four weeks after DEN, the pattern no longer resolved in the same way. Lipogenic proteins and G6PD remained elevated, while the NADP⁺/NADPH and GSSG/GSH ratios remained shifted toward oxidation. Mitochondrial reactive oxygen species, energy loss and cellular damage accompanied the persistent metabolic response.
Orlistat attenuated several of these lipid, oxidative and apoptotic changes, supporting a contribution of enhanced fatty-acid synthesis to the phenotype.
This is more informative than a simple model of NADPH depletion.
The data define a systems-level pattern rather than a direct map of NADPH flux. G6PD expression documents induction of a central component of the oxidative pentose phosphate pathway but does not quantify pathway flux or NADPH production. Whole-liver NADP⁺/NADPH measurements capture the integrated redox outcome rather than the distribution of reducing equivalents among individual enzymes or intracellular compartments.
A central component of a major NADPH-generating pathway was induced. Lipogenic demand persisted. The measured NADP and glutathione redox states nevertheless became more oxidized.
This pattern is consistent with NADPH allocation pressure.
Here, NADPH allocation pressure describes a state in which a generating response is induced, yet simultaneous reductive demands persist beyond the capacity of compensation to restore the previous redox state.
Production, availability and recovery are not interchangeable concepts.
Independent work in PXR-null mice adds a complementary perspective. Following partial hepatectomy, loss of the xenobiotic and endobiotic sensor PXR blunted transient hepatic lipid accumulation and impaired normal liver regeneration, with changes extending to genes involved in lipid biosynthesis. The NCoR1 phenotype was not shown to be PXR-mediated; the convergence lies at the level of regulated lipid remodeling as part of the adaptive response to tissue loss.
In the DEN model, increased oxidative damage and apoptosis were ultimately associated with fewer tumors, consistent with enhanced elimination of damaged or preneoplastic cells. This model-specific outcome should not be transferred directly to human metabolic liver disease.
The broader principle is more fundamental.
Transient lipogenic activity supported regeneration, and the associated redox disturbance receded.
Under prolonged injury, a persistent lipogenic program was accompanied by an NADPH-generating response, but redox balance was no longer restored.
The liver did not fail to respond.
Its response failed to bring the liver back.
References
Ou-Yang, Q. et al. Distinct role of nuclear receptor corepressor 1 regulated de novo fatty acids synthesis in liver regeneration and hepatocarcinogenesis in mice. Hepatology 67, 1071-1087 (2018). https://doi.org/10.1002/hep.29562
Dai, G. et al. Pregnane X receptor is essential for normal progression of liver regeneration. Hepatology 47, 1277-1287 (2008). https://doi.org/10.1002/hep.22129