Does hepatic insulin resistance drive liver fat by increasing DNL and VLDL-TG secretion?

In states of metabolic dysfunction, the liver exhibits a phenomenon known as "selective insulin resistance," where the organ fails to suppress glucose production but remains hypersensitive or hyper-activated regarding lipid synthesis. This metabolic split is a primary driver of non-alcoholic fatty liver disease (NAFLD) and systemic dyslipidemia.

Mechanistic pathways of lipid dysregulation

The increase in hepatic fat and circulating triglycerides is driven by specific molecular escapes from insulin resistance:

• De Novo Lipogenesis (DNL): While insulin fails to suppress gluconeogenesis via the FoxO1 pathway, it continues to stimulate SREBP-1c (Sterol Regulatory Element-Binding Protein 1c) through the mTORC1 signaling branch. This leads to the upregulation of key lipogenic enzymes like fatty acid synthase (FASN) and acetyl-CoA carboxylase (ACC). In insulin-resistant individuals, DNL can contribute up to 38% of the intrahepatic triglyceride pool, compared to only 11% in healthy controls.
• VLDL Secretion: Insulin resistance impairs the normal suppression of Very Low-Density Lipoprotein (VLDL) assembly. Under normal conditions, insulin promotes the degradation of apolipoprotein B-100 (ApoB-100). In resistant states, increased FoxO1 activity upregulates microsomal triglyceride transfer protein (MTTP), which facilitates the loading of triglycerides onto ApoB-100, significantly increasing VLDL-TG secretion rates.

Clinical implications for liver fat

The accumulation of liver fat (steatosis) occurs when the rate of triglyceride synthesis and fatty acid uptake exceeds the liver's capacity for oxidation and VLDL export.

• Imbalance of flux: While the liver increases VLDL secretion to compensate for high lipid loads, this export mechanism is often overwhelmed by the massive influx of fatty acids and the accelerated DNL typical of the metabolic syndrome.
• Steatosis progression: Stable isotope tracer studies confirm that the high flux of DNL is a decisive factor in the transition from simple steatosis to more advanced metabolic dysfunction-associated steatotic liver disease (MASLD).

Bottom line

Hepatic insulin resistance selectively promotes de novo lipogenesis via SREBP-1c and increases VLDL secretion by upregulating MTTP. This dual mechanism creates a cycle of increased hepatic fat synthesis and export, directly contributing to both liver steatosis and the atherogenic dyslipidemia observed in aging and metabolic disease.