Synthesis and function of hepatic very-low-density lipoprotein

Area of Science:

• Hepatocyte lipid metabolism
• Lipoprotein assembly
• Endocrinology

Background:

• Triacylglycerol (TAG) stored in hepatocytes is a major source for very-low-density lipoprotein (VLDL) assembly.
• Lipolysis and re-esterification of cytosolic TAG are key steps in VLDL production.
• Hepatic lipid metabolism plays a crucial role in systemic energy homeostasis and protection against lipotoxicity.

Purpose of the Study:

• To elucidate the mechanisms of triacylglycerol mobilization and incorporation into VLDL.
• To identify the lipases and regulatory factors involved in hepatic TAG metabolism for VLDL secretion.
• To understand the role of hepatic TAG storage and release in protecting tissues from lipotoxicity.

Main Methods:

• Investigated lipolysis of cytosolic TAG pools using specific lipases (arylacetamide deacetylase, triacylglycerol hydrolase).
• Examined the re-esterification of lipolytic products and their incorporation into VLDL precursors.
• Studied the influence of insulin and microsomal triacylglycerol transfer protein (MTP) on TAG cycling.
• Analyzed the role of phospholipase D activation by ADP-ribosylation factor-1 (ARF-1) in VLDL TAG assembly.

Main Results:

• Hepatocyte VLDL assembly primarily uses TAG mobilized by lipolysis and re-esterification.
• Arylacetamide deacetylase and/or triacylglycerol hydrolase are involved in TAG mobilization.
• Insulin stimulates, while MTP inhibits, the return of re-esterified TAG to the cytosolic pool.
• Phospholipids, via ARF-1-mediated phospholipase D activation, also contribute to VLDL TAG.

Conclusions:

• Hepatic TAG mobilization and secretion as VLDL are critical for managing plasma free fatty acids.
• The liver's capacity to store and release TAG serves as a protective mechanism against lipotoxicity.
• Understanding these pathways provides insights into metabolic disorders and VLDL-related cardiovascular risks.