[Physiology and pathophysiology of the metabolism of lipoproteins]

PHYSIOLOGY: Lipoproteins (LP) are generally classified according to their density. Triglycerides are mainly transported in chylomicrons and very low density LP (VLDL), cholesterol is mainly transported in low density LP (LDL) and high density LP (HDL). The metabolism of LP is controlled by their apolipoproteins, by specific receptors, enzymes, and transfer proteins. Triglycerides and cholesterol from the diet are transported in chylomicrons. The triglycerides are rapidly hydrolyzed by LP-lipase to yield chylomicron remnants. The released free fatty acids are used either for storage in adipose tissue or for oxidation in other tissues. Dietary cholesterol is transported in the chylomicron remnants to the liver. Cholesterol and triglyceride are also synthesized in the liver and then secreted into the blood in the form of VLDL. VLDL triglycerides are metabolized by LP-lipase to intermediate density LP (IDL), which are either taken up by the liver or further catabolized to LDL. LDL are bound and taken up by specific receptors (LDL receptors) in the liver and many other tissues. By this pathway, cholesterol is transported from the liver to peripheral tissues. LDL can be modified by oxidation and then taken up by macrophages in the arterial intima resulting in the formation of foam cells, an important step in atherogenesis. HDL play an important role in reverse cholesterol transport (transport of cholesterol back to the liver, the only site of cholesterol excretion).

Various mutations in the LP-lipase gene or in the apo C-II gene result in LP-lipase deficiency. Homozygous carriers of the mutated gene show defective metabolism of chylomicrons and VLDL with extreme hypertriglyceridemia, eruptive xanthomas, hepatosplenomegaly and recurrent bouts of acute pancreatitis. Many mutations in the LDL receptor gene have been described as the primary cause of familial hypercholesterolemia due to LDL receptor deficiency. LDL receptor deficiency results in the accumulation of LDL in the plasma and deposition of LDL cholesterol in tendons and skin (xanthomas) and arteries (atheromas). In homozygotes, coronary heart disease begins in childhood. Familial defective apo B-100 is caused by a mutation in codon 3500 of the apo B gene. LDL with the mutated apo B is not recognized by the LDL receptor and LDL accumulates in the blood. Mutant forms of apo E (apo E-2 and others) are not bound to the LDL(B,E)-receptor resulting in accumulation of chylomicrons and VLDL remnants (beta-VLDL) and IDL. For the manifestation of type III hyperlipemia, additional genetic, hormonal or environmental factors are involved. Cholesterol deposition in macrophages of the arterial intima and skin gives rise to atherosclerosis of coronary and peripheral arteries and xanthomas. The pathogenesis of familial combined hyperlipemia, the most frequent form of primary hyperlipemias, is multifactorial and has not been clarified in detail.