Chapter 25: Lipid Transport & Storage

To facilitate this, lipids like triacylglycerols and cholesterol are packaged into water-miscible lipoproteins, which consist of a nonpolar core and an amphipathic surface layer. The four primary classes—chylomicrons, very-low-density lipoproteins (VLDL), low-density lipoproteins (LDL), and high-density lipoproteins (HDL)—play distinct roles in moving dietary and synthesized lipids between the liver, intestines, and peripheral tissues. A crucial part of this process involves apolipoproteins, which serve as structural elements, ligands for cell receptors, and enzyme activators, such as apo C-II for lipoprotein lipase. Chylomicrons and VLDL undergo rapid catabolism through the action of lipoprotein lipase, delivering fatty acids to tissues like muscle and heart for energy or adipose tissue for storage. The liver’s central role in secreting VLDL and processing remnant particles is highlighted, noting that imbalances in these pathways can lead to fatty liver diseases, including nonalcoholic fatty liver disease (NAFLD) and alcoholic liver disease (ALD). In adipose tissue, the balance between lipid esterification and lipolysis is tightly regulated by hormones; insulin acts as a potent inhibitor of hormone-sensitive lipase to promote storage, while catecholamines like epinephrine stimulate lipid mobilization. Additionally, the text details the mechanism of reverse cholesterol transport, where HDL removes excess cholesterol from peripheral tissues for hepatic excretion, a process vital for cardiovascular health and the prevention of atherosclerosis. Finally, the specialized function of brown adipose tissue is examined, where thermogenin (uncoupling protein 1) allows for heat generation by dissipating the mitochondrial proton gradient, providing a critical mechanism for nonshivering thermogenesis.