Figure: lipoprotein metabolism

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Introduction

VLDL* is synthesized & secreted by hepatocytes.
VLDL synthesis is stimulated by caloric intake without regard to caloric source.
IDL* & LDL* are derived from VLDL through the action of tissue lipoprotein lipase.
the VLDL particles transport hepatic-derived cholesterol & triglycerides to peripheral tissues.

apo C2 is a cofactor of tissue lipoprotein lipase which removes most of the triglycerides from VLDL
as VLDL is metabolized, apo C2 & apo C3 are transferred to HDL*.

IDL is formed from VLDL.
it is a short-lived particle containing approximately equal amounts of triglyceride & cholesterol & apo B100 & apo E

the mature LDL particle is formed from IDL by the continued action of lipoprotein lipase
apo E-mediated hepatic lipase activity may also participate in formation of the mature LDL particle (alternate pathway)
LDL catabolism takes place in the liver & the periphery by receptor-mediated endocytosis.

both the liver & the intestine are involved in production of HDL
HDL is produced as a discoid-shaped particle containing apo A1, +/- apo A2, lecithin & cholesterol (nascent HDL)
LCAT* activity is necessary for transformation of this nascent HDL into HDL3, a spherical lipid-transporting moiety containing apo A1, apo A2, apo C1, apo C2, apo C3, apo D & apo E
apo A1 stimulates LCAT activity
apo A2 may associate with esterfied cholesterol & facilitate entry of esterfied cholesterol into the core of the HDL particle
apo A2 inhibits LCAT activity
thus is formed HDL3

formation of HDL2 from HDL3 occurs in tissues in association with VLDL & chylomicron metabolism
thus conversion of HDL3 to HDL2 results in an uptake of cholesterol & triglyceride
HDL3 may be regenerated from HDL2 by the action of hepatic lipase
thus HDL facilitates return of cholesterol & triglycerides from the periphery to the liver (reverse cholesterol transport)
HDL2 metabolism presumably occurs in peripheral tissues (liver, kidney, muscle etc)

chylomicrons are synthesized & secreted by intestinal epithelium into the lymphatics
triglyceride constitutes more than 80% of the chylomicron particle by weight
apo-B48, apo A1, apo A2, apo A4 collectively constitute < 2% of chylomicrons by weight
during traversal of the lymphatics enroute to entry of the venous circulation through the thoracic duct, chylomicrons accumulate apo C from HDL filtered into the lymph from the plasma
after entry into the systemic circulation, chylomicrons are metabolized in tissues through the action of lipoprotein lipase