Low Density Lipoproteins

Conversion of VLDL remnants to LDL appears to be largely the result of hydrolysis of remaining TAG by hepatic triacylglycerol lipase (HTGL). Normally about two-thirds of cholesterol is carried by LDL, most of this LDL cholesterol existing in the form of esters. The only apolipoprotein in LDL is apo B-100. LDL is removed from the circulation largely by hepatic LDL receptors. The level of expression of LDL receptors is a major determinant of serum LDL cholesterol concentrations. The synthesis of LDL receptors is regulated in large part by the liver's content of cholesterol. An increase in hepatic cholesterol content suppresses LDL receptor synthesis and raises serum LDL cholesterol; conversely, a decrease in hepatic cholesterol stimulates receptor synthesis and lowers serum LDL cholesterol. The mechanism whereby hepatic cholesterol controls LDL receptor synthesis is through a regulatory protein called sterol regulatory element-binding protein (SREBP). When hepatic cholesterol content falls, SREBP is activated and stimulates the synthesis of LDL receptors.

The regulatory form of cholesterol in the liver cell is unesterified cholesterol, not cholesterol ester. The hepatic content of unesterified cholesterol depends on several factors including the amounts of cholesterol derived from chylomicrons and other lipopro-teins, hepatic synthesis of cholesterol, secretion of cholesterol into bile, conversion of cholesterol into bile acids, esterification of cholesterol, and secretion of cholesterol into serum with VLDL. Factors that influence each of these processes can alter serum LDL cholesterol concentrations by modifying the hepatic content of unesterified cholesterol and thereby expression of LDL receptors.

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