Lipid filling and lipolysis in adipose tissue and cells

Fatty acids are transported from circulating blood lipoproteins to adipocytes during lipid filling of adipose tissue and from adipocytes to the capillary lumen during lipid mobilization. Our studies with chylomicrons and lipid monolayers show that ampipathic fatty acids formed by the action of lipoprotein lipase locate and move in the interface between lipid and the aqueous phase. Our studies on brown and white adipose tissue show that fatty acids formed by the action of lipoprotein lipase on chylomicrons at the capillary endothelial surface and by the action of hormone sensitive lipase on intracellular lipid droplets locate in an interfacial continuum composed of the outer leaflets of the plasma membrane of cells and the lumenal leaflets of intracellular membranes. Ampipathic fatty acids can be visualized with electron microscopy as lamellar structures. In fixed tissue fatty acids which overcrowd the interfacial continuum form lamellar extensions of the external leaflet of membranes at different sites along their transport route. In adipose tissue of fed animals lamellar structures extend from chylomicrons at the capillary endothelial surface through trans-endothelial channels to adipocytes marking the route of transport of fatty acids during lipid filling of adipose tissue. In adipose tissue of fasted animals lamellar structures extend from intracellular lipid droplets through intracellular channels to the plasma membrane of adipocytes marking the route of fatty acid transport during lipid mobilization. In adipocytes from brown adipose tissue of cold-stressed young animals lamellar structures extend from intracellular lipid droplets to the outer mitochondrial chamber marking the route of transport of fatty acids to mitochondria for oxidation and heat production.