Dietary tryptophan restriction selectively suppresses lipogenesis, lowers cholesterol, and reduces adiposity without altering food consumption

Technical Summary: Dietary Tryptophan Restriction Selectively Suppresses Lipogenesis, Lowers Cholesterol, and Reduces Adiposity Without Altering Food Consumption

Problem Statement
Current nutritional interventions for obesity and metabolic syndrome often rely on caloric restriction or broad macronutrient modulation, which can lead to reduced lean body mass, altered feeding behavior, and metabolic adaptation. While individual amino acid restrictions (e.g., methionine, leucine, cysteine) have shown promise, their effects are frequently confounded by reduced food intake or specific genetic dependencies (e.g., cystathionine γ-lyase deficiency). There is a critical need to identify a dietary intervention that selectively targets adiposity and lipid dysregulation while preserving lean mass, normalizing feeding behavior, and functioning independently of caloric restriction.

Methodology
The study employed a systematic screening approach using defined, isocaloric synthetic diets in mice to interrogate the physiological consequences of removing each of the nine essential amino acids individually.

• Animal Models: Experiments utilized male and female C57BL/6J (wild-type), FVB, leptin-deficient (ob/ob), and Fgf21 knockout mice. Models included wild-type, diet-induced obesity (DIO), and genetic obesity (ob/ob).
• Dietary Interventions: Mice were fed control diets or diets lacking a single essential amino acid (including tryptophan, termed "Trypt-out") for periods ranging from 3 days to 18 weeks. High-fat diet (HFD) and high-sucrose variations were used to test robustness against obesogenic challenges.
• Mechanistic Probes:
  • Translation Analysis: Global protein synthesis was assessed via puromycylation (OPP incorporation) in vivo and in vitro (hepatocytes, adipocytes).
  • Microbiome Depletion: Antibiotic treatment (penicillin/streptomycin) was used to determine if microbiome-derived metabolites mediated the phenotype.
  • Metabolite Rescue: Exogenous administration of tryptophan metabolites (kynurenine, indole-3-pyruvate, 5-HTP) and sucrose supplementation were tested to rule out metabolite deficiency or carbon source limitation.
  • Cell Death Assays: Ferroptosis inhibitors (liproxstatin-1, ferrostatin-1) and cell viability assays were used to distinguish between adipocyte death and lipid depletion.
  • Omics: RNA-seq and proteomics were performed on liver, inguinal white adipose tissue (iWAT), and skeletal muscle to identify transcriptional and translational adaptations.
  • Comparative Pharmacology: The effects of Trypt-out were compared against atorvastatin (ATO) treatment in ob/ob mice.
• Physiological Metrics: Body composition (MRI), respiratory exchange ratio (RER), glucose tolerance, hormone levels (FGF21, insulin, leptin), liver enzymes (AST/ALT), bile acid profiling, and fecal caloric content were measured.

Key Contributions and Results

1. Selective Fat Loss Without Caloric Restriction:
   Among all essential amino acid depletions, only tryptophan (Trp) restriction ("Trypt-out") induced rapid, profound fat loss while preserving lean body mass. Crucially, this occurred without a reduction in food consumption. In contrast, other amino acid restrictions (e.g., isoleucine, phenylalanine) reduced food intake and caused loss of both fat and lean mass.

2. Reversal of Obesity and Metabolic Dysfunction:
   Trypt-out prevented and reversed obesity in wild-type, ob/ob, and DIO mice. It improved glucose tolerance, reduced circulating insulin and leptin, and resolved hepatic steatosis without causing liver injury (normal AST/ALT levels). The intervention remained effective even when mice were fed continuous high-fat or high-sucrose diets.

3. Mechanism: Selective Suppression of Lipogenic Enzymes:

   • Protein Synthesis: Global protein synthesis was preserved (and even slightly upregulated in the liver via ribosome biogenesis), ruling out translational collapse.
   • Enzyme Specificity: Proteomic analysis revealed a selective depletion of key lipogenic and cholesterol biosynthetic enzymes in the liver, including FASN, ACLY, ACACA, SCD1, HMGCS1, MVD, and DHCR7. These proteins are notably rich in tryptophan residues.
   • Transcriptional vs. Translational: While mRNA levels for some enzymes were modestly altered, the protein levels were significantly reduced, suggesting a post-transcriptional mechanism linked to Trp availability.
   • FGF21 Independence: Although Trypt-out increased hepatic FGF21, the weight loss and metabolic benefits persisted in Fgf21 knockout mice, indicating the mechanism is FGF21-independent.

4. Uncoupling Lipid Absorption from Synthesis:
   Trypt-out did not impair intestinal lipid absorption. Bile acid production (specifically cholic acid) was maintained or increased, and fecal caloric content remained unchanged. Instead, the diet uncoupled dietary lipid uptake from endogenous lipid storage. Dietary lipids were utilized for essential functions (e.g., bile acid synthesis, membrane homeostasis) rather than stored as triglycerides.

5. Cholesterol Reduction Compared to Statins:
   Trypt-out caused a profound reduction in circulating and hepatic cholesterol. In ob/ob mice, the Trypt-out diet resulted in significantly lower serum cholesterol levels than ATO-treated mice fed a control diet. Unlike statins, which trigger a compensatory feedback loop (upregulation of HMG-CoA reductase via SREBP-2), Trypt-out uniformly reduced the expression of enzymes throughout the cholesterol and lipid biosynthetic pathways without inducing compensatory upregulation.

6. Adipose Remodeling via Lipid Depletion:
   Histological and metabolomic analyses confirmed that fat loss resulted from the depletion of intracellular lipid stores within adipocytes rather than adipocyte death (no increase in cleaved caspase-3 or ferroptosis markers). Adipocyte viability and identity markers were preserved.

Significance and Claims
The paper establishes dietary tryptophan restriction as a unique, well-tolerated strategy to selectively eliminate adiposity and normalize metabolic dysfunction without reducing food intake or lean mass. The authors identify tryptophan as a master regulator of systemic lipid homeostasis.

Key claims regarding the significance of these findings include:

• Metabolic Rewiring: Trypt-out reprograms metabolism by suppressing de novo lipogenesis and cholesterol biosynthesis while preserving nutrient absorption and protein synthesis.
• Therapeutic Potential: The intervention offers a potential therapeutic strategy for metabolic syndrome, obesity, diabetes, and cardiovascular disease, particularly for patients who cannot tolerate caloric restriction or statin therapy.
• Robustness: The effects are resistant to obesogenic diets (high-fat/high-sugar) and persist even with intermittent "binge" feeding patterns, suggesting a durable mechanism for weight control.
• Safety Profile: The intervention preserves liver integrity and does not induce hepatotoxicity, distinguishing it from some other metabolic stressors.

The study concludes that tryptophan restriction acts through a non-canonical pathway to decouple dietary lipid uptake from endogenous lipid production, creating a metabolically expensive state that drives selective adipose tissue loss.