Pathway: HDL remodeling
Reactions in pathway: HDL remodeling :
HDL remodeling
HDL (high-density lipoprotein) particles play a central role in the reverse transport of cholesterol, the process by which cholesterol in tissues other than the liver is returned to the liver for conversion to bile salts and excretion from the body and provided to tissues such as the adrenals and gonads for steroid hormone synthesis (Tall et al. 2008).
ABCG1 mediates the movement of intracellular cholesterol to the extracellular face of the plasma membrane where it is accessible to circulating HDL (Vaughan & Oram 2005). Spherical (mature) HDL particles can acquire additional molecules of free cholesterol (CHOL) and phospholipid (PL) from cell membranes.
At the HDL surface, LCAT (lecithin-cholesterol acyltransferase) associates strongly with HDL particles and, activated by apoA-I, catalyzes the reaction of cholesterol and phosphatidylcholine to yield cholesterol esterified with a long-chain fatty acid and 2-lysophosphatidylcholine. The hydrophobic cholesterol ester reaction product is strongly associated with the HDL particle while the 2-lysophosphatidylcholine product is released. Torcetrapib associates with a molecule of CETP and a spherical HDL particle to form a stable complex, thus trapping CETP and inhibiting CETP-mediated lipid transfer between HDL and LDL (Clark et al. 2006).
Spherical HDL particles can bind apoC-II, apoC-III and and apoE proteins.
ABCG1 mediates the movement of intracellular cholesterol to the extracellular face of the plasma membrane where it is accessible to circulating HDL (Vaughan & Oram 2005). Spherical (mature) HDL particles can acquire additional molecules of free cholesterol (CHOL) and phospholipid (PL) from cell membranes.
At the HDL surface, LCAT (lecithin-cholesterol acyltransferase) associates strongly with HDL particles and, activated by apoA-I, catalyzes the reaction of cholesterol and phosphatidylcholine to yield cholesterol esterified with a long-chain fatty acid and 2-lysophosphatidylcholine. The hydrophobic cholesterol ester reaction product is strongly associated with the HDL particle while the 2-lysophosphatidylcholine product is released. Torcetrapib associates with a molecule of CETP and a spherical HDL particle to form a stable complex, thus trapping CETP and inhibiting CETP-mediated lipid transfer between HDL and LDL (Clark et al. 2006).
Spherical HDL particles can bind apoC-II, apoC-III and and apoE proteins.
Because of their hydrophobicity, lipids are found in the extracellular spaces of the human body primarily in the form of lipoprotein complexes. Chylomicrons form in the small intestine and transport dietary lipids to other tissues in the body. Very low density lipoproteins (VLDL) form in the liver and transport triacylglycerol synthesized there to other tissues of the body. As they circulate, VLDL are acted on by lipoprotein lipases on the endothelial surfaces of blood vessels, liberating fatty acids and glycerol to be taken up by tissues and converting the VLDL first to intermediate density lipoproteins (IDL) and then to low density lipoproteins (LDL). IDL and LDL are cleared from the circulation via a specific cell surface receptor, found in the body primarily on the surfaces of liver cells. High density lipoprotein (HDL) particles, initially formed primarily by the liver, shuttle several kinds of lipids between tissues and other lipoproteins. Notably, they are responsible for the so-called reverse transport of cholesterol from peripheral tissues to LDL for return to the liver.
Three aspects of lipoprotein function are currently annotated in Reactome: chylomicron-mediated lipid transport, LDL endocytosis and degradation, and HDL-mediated lipid transport, each divided into assembly, remodeling, and clearance subpathways.
By definition cells have a critical separation between inner (cytoplasmic) and outer (extracellular) compartments. This separation provides for protection, gradient assembly, and environmental control but at the same time isolates the interior compartments of the cell from energy resources, oxygen, and raw materials. Cells have evolved a myriad of mechanisms to regulate, and enable transportation of small molecules ascross plasma membranes and between cellular organelle compartments within cells.