Identification and partial characterization of discrete apolipoprotein B containing lipoprotein particles produced by...

The purpose of this study was to test the use of human hepatocarcinoma HepG2 cells as a model for studying the formation and secretion of human hepatic lipoproteins. To this end, we determined the rate of accumulation and percent composition of neutral lipids and apolipoproteins in the culture medium of HepG2 cells and isolated and partially characterized the apolipoprotein B (ApoB) containing lipoprotein particles. The rates of accumulation in the medium of HepG2 cells, grown in minimum essential medium during a 24-h incubation, of triglycerides, cholesterol, and cholesterol esters expressed as microgram/(g of cell protein X h) were 373 +/- 55, 167 +/- 14, and 79 +/- 10, respectively; the secretion rates for apolipoproteins B, A-I, E, A-II, and C-III were 372 +/- 36, 149 +/- 14, 104 +/- 13, 48 +/- 4, and 13 +/- 1 microgram/(g of cell protein X h), respectively. The major portion of ApoB was present in very low density lipoproteins (VLDL) and low-density lipoproteins (LDL) (84%), with the remainder occurring in high-density lipoproteins (HDL) (16%). Approximately 10-13% of ApoA-I and ApoA-II were present in VLDL and LDL, while 60% of ApoE occurred in HDL and 40% in VLDL and LDL. To separate ApoB-containing lipoproteins, secreted lipoproteins were fractionated by either sequential immunoprecipitation or immunoaffinity chromatography with antibodies to ApoB and ApoE. Results showed that 60-70% of ApoB occurred in the culture medium as lipoprotein B (LP-B) and 30-40% as lipoprotein B:E (LP-B:E). Both ApoB-containing lipoproteins represent polydisperse systems of spherical particles ranging in size from 100 to 350 A for LP-B and from 200 to 500 A for LP-B:E. LP-B particles were identified in VLDL, LDL, and HDL, while LP-B:E particles were only present in VLDL and LDL. The major neutral lipid of both ApoB-containing lipoproteins was triglyceride (50-70% of the total neutral lipid content); cholesterol and cholesterol esters were present in equal amounts. The LP-B:E particles contained 70-90% ApoB and 10-30% ApoE. The ApoB was identified in both types of particles as B-100. A time study on the accumulation of ApoB-containing lipoproteins showed that LP-B particles were secreted independently of LP-B:E particles.