The technical centerpiece of the work is a family of fluorescent biosensors called iNap, which fluoresce in proportion to NADPH concentration and can be targeted to specific cellular compartments. When the researchers expressed these sensors in adult mouse cardiomyocytes, they observed that swimming and running exercise elevated the NADPH signal in the cytosol while leaving the mitochondrial pool untouched. This compartmental specificity matters, because cytosolic and mitochondrial NADPH fluxes are known to be independently regulated, and it points the finger away from mitochondrial transhydrogenase and toward cytosolic sources. Among those sources, the pentose phosphate pathway, or PPP, stood out: a glucose-shunting branch of metabolism whose rate-limiting enzyme, glucose-6-phosphate dehydrogenase (G6PD), strips electrons from glucose-6-phosphate and deposits them onto NADP+ to make NADPH.

The causal chain was established through a series of loss-of-function experiments. When the team inhibited PPP activity or depleted cytosolic NADPH in mice, the heart’s response to exercise was blunted: the characteristic enlargement of cardiomyocytes that normally accompanies training failed to materialize. Conversely, the study showed that NADPH itself is a driver of cardiomyocyte growth, acting by inhibiting the HDAC3/C/EBPβ pathway—a signaling axis previously implicated in the control of exercise-induced cardiac growth. In other words, the reducing equivalents manufactured by the PPP do more than keep reactive oxygen species in check; they directly tune an epigenetic and transcriptional program that tells heart cells to grow in a coordinated, physiological manner rather than in the maladaptive fashion seen in disease.

The protective side of the story emerged from mouse models of acute ischemia–reperfusion injury, a scenario that unfolds in millions of patients each year when a blocked coronary artery is reopened and the sudden return of oxygen floods the tissue with reactive oxygen species. Mice with an exercise-activated PPP/NADPH pathway suffered measurably less injury from a subsequent ischemia–reperfusion challenge, and this protection persisted: four weeks after the insult, heart function remained significantly preserved compared with sedentary controls. The mechanism appears to be twofold. First, abundant NADPH sustains the glutathione and thioredoxin antioxidant systems that neutralize the oxidative burst of reperfusion. Second, by suppressing HDAC3 activity, NADPH restrains the transcriptional changes that push stressed cardiomyocytes toward death and maladaptive remodeling.

What makes the study more than an elegant piece of mechanistic cardiology is its drug-discovery angle. The researchers screened 310 Tibetan medicinal compounds for the ability to raise intracellular NADPH, and one molecule rose to the top: lyciumspermidine-0527, a spermidine derivative derived from a plant used in traditional Tibetan medicine. Biochemical and structural analyses showed that this compound directly activates G6PD, the rate-limiting enzyme of the pentose phosphate pathway, stabilizing the enzyme’s active dimeric form. In cultured cardiomyocytes and in living mice, lyciumspermidine-0527 elevated cytosolic NADPH, promoted cardiomyocyte growth, and—most strikingly—alleviated ischemia–reperfusion injury to a degree that rivaled exercise itself.

Safety data accompanying the compound were encouraging. Across a range of doses in vitro and in vivo, lyciumspermidine-0527 did not significantly induce cell death in the heart, and histological examination of heart, liver, lung, kidney, and brain tissue showed no overt toxicity. The compound also maintained redox homeostasis in cardiomyocytes, consistent with its proposed mechanism of boosting, rather than overwhelming, the cell’s antioxidant buffering capacity. Genetic confirmation followed: when G6PD was knocked down, the protective effects of the compound vanished, placing G6PD squarely upstream of the observed benefit and ruling out major off-target explanations for the phenotype.

Conceptually, the findings reframe physiological cardiac hypertrophy as a metabolic checkpoint phenomenon. The healthy heart’s enlargement in response to exercise has long been distinguished from the pathological hypertrophy of hypertension or heart failure by its reversibility and preserved function, but the molecular gatekeepers separating the two states have been only partially mapped. By demonstrating that PPP-derived NADPH is required for exercise-induced growth and simultaneously protective against injury, the study positions a single metabolite as a checkpoint that integrates the demand for biosynthesis (NADPH feeds fatty acid and nucleotide production), antioxidant defense (NADPH powers glutathione reductase), and epigenetic regulation (NADPH inhibits the HDAC3–Ncor complex). Exercise, in this view, is a metabolic intervention, and its cardiac benefits can be recapitulated—at least in mice—by pharmacologically opening a single metabolic valve.

The translational horizon is tantalizing but cautious. Lyciumspermidine-0527 is not yet a drug, and the leap from mouse models of surgically induced ischemia–reperfusion to human myocardial infarction is considerable. G6PD activity is a double-edged sword: the same pathway that supports antioxidant defense in cardiomyocytes also supports nucleotide synthesis and growth in cancer cells, raising questions about systemic effects that would need careful evaluation in longer-term studies. Nonetheless, the identification of a small molecule that directly activates G6PD offers a starting point for medicinal chemistry, and the iNap biosensor toolkit the team deployed provides a template for screening compounds by their ability to raise NADPH in specific subcellular compartments rather than by crude proxies.

For now, the study’s most immediate contribution is conceptual clarity. It explains, in mechanistic terms, why the exercising heart grows well and withstands injury better, connecting the dots from a transcription factor (SP1-driven G6PD expression), through a metabolic flux (the pentose phosphate pathway), to a redox cofactor (cytosolic NADPH), and finally to an epigenetic effector (HDAC3/C/EBPβ) and a clinical phenotype (preserved function after reperfusion). It also validates a natural-product-inspired approach to finding exercise mimetics for the heart. If the PPP/NADPH axis holds up in larger preclinical models, the prospect of a pill that trains the heart’s metabolism—conferring some of exercise’s cardioprotection on patients who cannot exercise—moves from metaphor to plausible pipeline.

Subject of Research: Pentose phosphate pathway-derived NADPH as a metabolic checkpoint regulating exercise-induced physiological cardiac hypertrophy and protection against ischemia–reperfusion injury

Article Title: Pentose phosphate pathway-derived NADPH facilitates physiological hypertrophy and alleviates ischemia–reperfusion injury in the heart

Article References: Wu, D., Chen, T., Dong, X., Li, H., Cheng, Z., Li, H., Chao, Y., Li, F., Yin, Y., Guo, F., Xu, X., Zhang, Y., Ning, K., Fu, X., Bian, Y., Ma, F., Ritterhoff, J., Wang, W., & Hu, Q. (2026). Pentose phosphate pathway-derived NADPH facilitates physiological hypertrophy and alleviates ischemia–reperfusion injury in the heart. Nature Metabolism. https://doi.org/10.1038/s42255-026-01587-9

Image Credits: AI Generated

DOI: 10.1038/s42255-026-01587-9

Keywords: NADPH, pentose phosphate pathway, cardiac hypertrophy, exercise, ischemia-reperfusion injury, glucose-6-phosphate dehydrogenase, cardiomyocytes, HDAC3, lyciumspermidine-0527, redox homeostasis, cardiac metabolism, Nature Metabolism

Nuclear receptors are ligand-activated transcription factors that bind specific DNA sequences and switch target genes on or off in response to small molecules. Several members of this superfamily are central players in bile acid biology. The farnesoid X receptor, or FXR, is the canonical bile acid sensor, activated by chenodeoxycholic acid and related species, and it feeds back to suppress the enzyme CYP7A1, the rate-limiting step in classical bile acid synthesis, largely through induction of the repressive nuclear receptor SHP. The pregnane X receptor and the constitutive androstane receptor act as xenobiotic and bile acid sensors that induce detoxification and export pathways, protecting hepatocytes from bile acid overload. The liver X receptor links oxysterol sensing to bile acid synthesis via CYP7A1 induction, while the retinoid X receptor serves as a common dimerization partner for many of these receptors. Each of these receptors, however, does not act alone: their output depends on the coregulatory proteins they recruit to DNA.

This is where SMRT enters the picture. SMRT, short for silencing mediator of retinoid and thyroid hormone receptors and also known as NCOR2, is one of the two major corepressors of the nuclear receptor superfamily, the other being its paralog NCoR1. In the absence of activating ligand, many nuclear receptors bind SMRT, which in turn recruits large multiprotein complexes containing histone deacetylases such as HDAC3, along with transducin beta-like proteins and other chromatin-modifying enzymes. These complexes deacetylate histones and maintain local chromatin in a transcriptionally repressive state. When ligand binds, the receptor undergoes a conformational change, releases the corepressor, and recruits coactivators instead. This ligand-dependent corepressor exchange is the fundamental switch that converts receptor binding into transcriptional activation, and it means that the abundance, localization, and post-translational modification state of SMRT directly shapes how strongly and how quickly target genes respond to their ligands.

For bile acid homeostasis specifically, SMRT has been implicated in the regulation of nearly every major nuclear receptor pathway in the hepatocyte and the enterocyte. In the FXR pathway, corepressor exchange at FXR target promoters such as SHP, BSEP, and the intestinal FGF15/FGF19 locus determines the strength of the enterohepatic feedback signal that restrains CYP7A1 expression. In the CAR and PXR pathways, SMRT occupancy at promoters of detoxification genes such as CYP2B and CYP3A, and at bile acid exporters, governs the inducible defense against cholestatic injury. In the LXR pathway, corepressor association with LXR/RXR heterodimers at the CYP7A1 promoter influences the balance between bile acid synthesis and cholesterol efflux. The review emphasizes that these are not independent circuits but an integrated network in which SMRT acts as a shared rheostat, tuning gene expression across pathways in response to nutritional state, circadian cues, and inflammatory signals.

The physiological consequences of disrupting this corepressor function are substantial. Animal studies over the past two decades have shown that perturbing corepressor complexes, particularly the SMRT- and NCoR-associated HDAC3 complex, produces profound metabolic phenotypes. Liver-specific deletion of HDAC3 in mice leads to marked alterations in lipid and bile acid metabolism, reflecting the loss of repressive chromatin marks at metabolic gene promoters. Similarly, mice with impaired corepressor recruitment to specific nuclear receptors display altered bile acid pool sizes, changes in the hydrophobicity of the bile acid pool, and altered sensitivity to cholestatic liver injury. Because bile acids themselves are cytotoxic at high concentrations and pro-carcinogenic when they accumulate in the intestine, the fidelity of this corepressor-mediated feedback is a matter of liver and gut health, not merely transcriptional housekeeping.

The review also highlights the emerging role of SMRT in the enterohepatic circulation itself. Bile acids are secreted into bile, stored in the gallbladder, released into the duodenum after meals, reabsorbed in the terminal ileum, and returned to the liver through portal blood. At each step, nuclear receptors in enterocytes and hepatocytes sense the local bile acid concentration and adjust transporter expression accordingly. SMRT-dependent repression and derepression at these transporter genes, including ASBT in the ileum and NTCP and BSEP in the liver, helps set the flux rate of the entire circulation. Disruption of this tuning can shift the bile acid pool toward more hydrophobic species, which are stronger FXR agonists but also more damaging to membranes, thereby creating a feedback loop that links corepressor function to both signaling intensity and cytotoxic risk.

Beyond classical bile acid metabolism, the review connects SMRT-regulated nuclear receptor signaling to systemic metabolic disease. Bile acids activate FXR and the membrane receptor TGR5 to influence glucose homeostasis, insulin sensitivity, thermogenesis in brown adipose tissue, and energy expenditure. The bile acid pool is also a major determinant of gut microbiome composition, and the microbiome in turn modifies bile acids through deconjugation and dehydroxylation, generating secondary bile acids with distinct receptor specificities. Because SMRT shapes the expression of the enzymes and transporters that determine which bile acids circulate and in what amounts, corepressor function sits upstream of this entire host-microbe metabolic axis. The authors argue that this positions SMRT as a potential node at which diet, microbiome, and nuclear receptor pharmacology converge, with implications for nonalcoholic fatty liver disease, cholestasis, metabolic syndrome, and gastrointestinal cancers.

Therapeutically, the framework has clear implications. FXR agonists such as obeticholic acid are already approved for certain cholestatic conditions and are being evaluated for metabolic liver disease, while dual and selective modulators of FXR, TGR5 agonists, and CAR activators are in various stages of development. The review suggests that the efficacy of such ligands depends not only on receptor binding but on the corepressor and coactivator landscape of the target tissue. A ligand that promotes efficient corepressor release will produce a stronger transcriptional response than one that does not, and tissue-specific differences in SMRT abundance could explain variable drug responses between patients and between liver and intestine. This perspective points toward a second generation of nuclear receptor therapeutics designed with coregulator exchange in mind, and toward biomarkers based on coregulator occupancy that could predict which patients will benefit from bile acid-directed therapies.

The review closes by identifying open questions that will define the next phase of the field. These include mapping the genome-wide occupancy of SMRT at bile acid-related loci in a cell-type-specific manner, determining how post-translational modifications of SMRT such as phosphorylation, ubiquitination, and sumoylation respond to nutritional and inflammatory signals, and clarifying how SMRT and NCoR1 divide labor between hepatocytes, cholangiocytes, and intestinal epithelial cells. Single-cell and chromatin profiling technologies are expected to accelerate this work, as are degrader and PROTAC approaches that can selectively manipulate corepressor complexes. What is already clear, the authors contend, is that bile acid homeostasis cannot be understood as a simple ligand-receptor-feedback circuit. It is a coregulator-dependent system in which SMRT provides the repressive counterweight that gives nuclear receptor signaling its dynamic range, its tissue specificity, and its resilience against metabolic and toxicological stress.

Subject of Research: SMRT corepressor regulation of nuclear receptor signaling in bile acid homeostasis

Article Title: SMRT regulation of nuclear receptors orchestrates bile acid homeostasis

Article References: Kim, K., Fang, S., Hong, S.-H., Cho, H., Leblanc, M., Jacinto, S., Schnabl, B., Atkins, A. R., Yu, R. T., Liddle, C., Truitt, M. L., Fan, W., Lee, C. M., Downes, M., & Evans, R. M. (2026). SMRT regulation of nuclear receptors orchestrates bile acid homeostasis. Experimental & Molecular Medicine. https://doi.org/10.1038/s12276-026-01823-y

Image Credits: AI Generated

DOI: 10.1038/s12276-026-01823-y

Keywords: SMRT, NCOR2, nuclear receptors, bile acids, FXR, CYP7A1, cholestasis, HDAC3, liver metabolism, enterohepatic circulation, corepressors, metabolic disease