Nutrient-dependent phosphorylation channels lipid synthesis to regulate PPARα.

Area of Science:

• Biochemistry
• Molecular Biology
• Cell Biology

Background:

• Peroxisome proliferator-activated receptor (PPAR)α is a nuclear receptor crucial for coordinating liver metabolism, particularly during fasting.
• Fatty acid synthase (FAS) plays a dual role in energy storage and activating hepatic PPARα through endogenous ligand synthesis.

Purpose of the Study:

• To elucidate the paradoxical relationship between FAS and hepatic PPARα activation.
• To investigate the role of subcellular compartmentalization and covalent modification of FAS in regulating liver metabolism and gene expression.

Main Methods:

• Analysis of cytoplasmic versus membrane-associated FAS activity in mouse liver and hepatoma cells under varying nutritional conditions.
• Site-directed mutagenesis of key phosphorylation sites on FAS (Thr-1029 and Thr-1033).
• Pharmacological inhibition of mammalian/mechanistic target of rapamycin complex 1 (mTORC1) using rapamycin and assessment of FAS activity and PPARα target gene expression.
• Gene silencing of FAS to confirm its role in rapamycin-mediated effects.

Main Results:

• Fasting increases the ratio of cytoplasmic to membrane FAS activity, indicating enhanced cytoplasmic FAS activity during PPARα activation.
• Nutrient-dependent and compartment-selective phosphorylation of FAS at Thr-1029 and Thr-1033 occurs during feeding or insulin treatment.
• Mutating these phosphorylation sites enhances PPARα target gene expression.
• mTORC1 inhibition reduces FAS phosphorylation, boosts cytoplasmic FAS activity, and increases PPARα target gene expression, an effect dependent on FAS presence.

Conclusions:

• Hepatic FAS activity is differentially regulated in cytoplasmic and membrane-associated compartments based on nutritional status.
• Compartment-specific covalent modification of FAS, particularly phosphorylation, is a key mechanism linking nutrient availability to PPARα-mediated gene expression.
• FAS acts as a crucial sensor, channeling lipid synthesis through distinct subcellular locations to modulate gene expression according to metabolic needs.