Lipoproteins and Lipid Transport

The plasma lipoproteins are the primary means of transport of cholesterol among tissues. In particular, the apo B-containing lipoproteins (VLDL, IDL and LDL) are important for the delivery of cholesterol from the liver to peripheral tissues, while HDL appear to mediate the reverse process of movement of cholesterol from tissues back to the liver. Both of these transport processes are necessary for efficient whole body cholesterol homeostasis, because the liver is the major site of both the production and excretion of cholesterol. However, deviations from a proper balance of transport of cholesterol, either increases in LDL levels or decreases in HDL cholesterol flux, may result in accumulation of cholesterol in extra hepatic tissues. Increased risk of atherosclerosis and CHD may be associated with elevation in the number of LDL particles, increase or decrease in LDL particle size, or changes in the composition of plasma LDL.

The plasma lipoproteins are the primary means of transport of cholesterol among tissues. In particular, the apo B-containing lipoproteins (VLDL, IDL and LDL) are important for the delivery of cholesterol from the liver to peripheral tissues, while HDL appear to mediate the reverse process of movement of cholesterol from tissues back to the liver. Both of these transport processes are necessary for efficient whole body cholesterol homeostasis, because the liver is the major site of both the production and excretion of cholesterol. However, deviations from a proper balance of transport of cholesterol, either increases in LDL levels or decreases in HDL cholesterol flux, may result in accumulation of cholesterol in extra hepatic tissues. Increased risk of atherosclerosis and CHD may be associated with elevation in the number of LDL particles, increase or decrease in LDL particle size, or changes in the composition of plasma LDL.

Cholesterol micro embolization – It occurs when cholesterol is released, usually from an atherosclerotic plaque, and travels as an embolus in the bloodstream to lodge causing an obstruction in blood vessels further away. Most commonly this causes skin symptoms, gangrene of the extremities and sometimes renal failure; problems with other organs may arise, depending on the site at which the cholesterol crystals enter the bloodstream.

Lipoprotein receptors: Deposition of cholesterol esters in the arterial intima is a characteristic feature of human atherosclerosis. Very little is known about the mechanisms by which cells normally regulate their cholesterol ester content. Recent studies in cultured human cells demonstrate the existence of a cell surface receptor that binds plasma low density lipoproteins and regulates the sterol content of cells by modulating the rates of uptake, esterification, and synthesis of cholesterol. A possible role for this lipoprotein receptor in the pathogenesis of atherosclerosis is discussed.

Lipid oxidation & Peroxidation - The oxidation of LDL is a free radical driven lipid peroxidation process and the aldehyde products of lipid hydroperoxide breakdown are responsible for the modification of the LDL apo protein. Aldehyde-modified apoB protein has altered receptor affinity, causing it to be scavenged by macrophages in an uncontrolled manner with the development of foam cells and the initiation of the atherosclerotic lesion. The aldehydic products of lipid peroxidation may also be involved in other aspects of the development of the lesion.

Lipoprotein Lipase activity - Lipoprotein lipase (LPL) is a rate-limiting enzyme that hydrolyzes circulating triglyceride-rich lipoprotein such as very low density lipoproteins and chylomicrons. A decrease in LPL activity is associated with an increase in plasma triglycerides (TG) and decrease in high density lipoprotein (HDL) cholesterol. The increase in plasma TG and decrease in HDL cholesterol are risk factors of coronary heart disease. However, whether LPL directly or indirectly promotes or protects against atherosclerosis remains unclear as two contrary views exist in this regard: one where LPL promotes atherosclerosis and one where LPL protects against atherosclerosis.

  • Cholesterol Micro Embolization
  • Lipoprotein Receptors
  • Lipid Oxidation & Peroxidation
  • Lipoprotein Lipase Activity

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