Pleiotrophin and midkine, a family of mitogenic and angiogenic heparin-binding growth and differentiation factors

The lipid emulsions used in parenteral nutrition are constituted of particles rich in triacylglycerols (TAG) called artificial chylomicrons (200-500 nm in diameter; monolayer of phospholipids PL] enveloping a TAG core) and PL-rich particles called liposomes (diameter inferior to 80 nm; bilayer of PL around an aqueous phase), which represent the excess emulsifier. Introduced into the circulation, the two populations of particles come into contact with circulating lipoproteins and cell membranes and experience the same overall fate: exchanges and transfers of lipids and apolipoproteins, enzymatic hydrolysis of TAG and PL, and internalization by different tissues. The relative importance of these different metabolic processes varies depending on the type of particle. The artificial chylomicrons undergo a hydrolysis of their TAG by lipoprotein lipase, with a release of fatty acids and formation of smaller particles of remnants, which are rapidly removed by the liver. In delivering fatty acids to the tissue, artificial chylomicrons fulfill an energy transport function similar to the natural chylomicrons. The liposomes hold little energy interest, and they also have deleterious effects when infused in excess. They inhibit the lipolysis of artificial chylomicrons and, by actively capturing endogenous cholesterol, they stimulate tissue cholesterogenesis and accumulate in the blood as lipoprotein-X, a long-lived abnormal lipoprotein. To limit as much as possible the metabolic perturbations due to the intravenous administration of exogenous PL, the emulsion has to be infused at a low rate, and should contain the minimal amount of excess PL.