Synthesis of two forms of apolipoprotein B by cultured rat hepatocytes

Isolation and characterization of apolipoprotein B-48 and B-100 very low density lipoproteins from type III hyperlipoproteinemic subjects

Two major species of human apolipoprotein (apo) B have been identified, apo B-48 and apo B-100, which are the predominant forms in chylomicrons and very low density lipoproteins (VLDL), respectively. Due to defective hepatic clearance, apo B-48 containing lipoproteins accumulate in the plasma of subjects with type III hyperlipoproteinemia. In the present study, we have used immunoaffinity chromatography to separate type III VLDL into a nonretained (apo B-48 VLDL) and a retained (apo B-100 VLDL) fraction. To achieve complete separation, as determined by electrophoresis and radioimmunoassay, it was necessary to employ two different insolubilized anti-apo B-100 monoclonal antibodies because of immunochemical heterogeneity within the apo B-100 VLDL fraction. The ability to separate apo B-100 VLDL from apo B-48 VLDL shows that the two apo B species are found on different particles. The apo B-48 VLDL had an electrophoretic mobility similar to chylomicrons, whereas the apo B-100 VLDL migrated similarly to total type III VLDL. Both fractions showed a concentration of particles with diameters approximately 100 nm, with apo B-48 VLDL being somewhat more heterogeneous in particle size. The two fractions were qualitatively similar in apolipoprotein composition but apo B-48 VLDL was enriched in apo E, relative to apo B-100 VLDL. Apo B-48 VLDL was enriched in cholesterol esters and deficient in triglycerides and phospholipids when compared with apo B-100 VLDL. The existence of immunochemical heterogeneity in the apo B-100 VLDL may reflect different functional subpopulations of particles within this fraction.

Isolation and analysis of lipoproteins secreted by rat liver hepatocytes

A procedure has been developed for the small-scale isolation and characterization of lipoproteins secreted by cultured rat liver hepatocytes. The lipoproteins in the culture medium were separated into VLDL, LDL, HDL and a fraction with d greater than 1.21 on single-spin density-gradients. The lipoproteins were removed from the gradients by adsorption onto Cab-O-Sil, a hydrated colloidal silica. The lipid components were extracted from the silica with CHCl3/CH3OH and the apoproteins solubilized in a buffer that contained 2% sodium dodecyl sulfate and 6 M urea. The proteins were analyzed on 3-20% acrylamide electrophoresis gels that contained 1% sodium dodecyl sulfate. The two major rat-plasma lipoproteins, VLDL and HDL, were well separated by the gradients. The Cab-O-Sil was shown to bind 90-95% of the HDL and VLDL in the fractions from the gradient. The recovery of the lipid components was essentially quantitative. The recovery of the apolipoproteins was only about 60% but with very good precision. Over a 20 h period, the lipid phosphorus associated with secreted lipoproteins increased linearly. The secretion of apolipoprotein A1 and apolipoprotein E associated with HDL and apolipoprotein B associated with VLDL also increased as a nearly linear function with time. The secretion of apolipoprotein E associated with VLDL was linear only up to approx. 6 h. The availability of this procedure should greatly facilitate further studies on the characterization of lipoproteins secreted by hepatocytes and mechanisms that regulate lipoprotein synthesis and secretion.

Role of insulin in lipoprotein secretion by cultured rat hepatocytes

To study the effect of insulin on lipoprotein synthesis and secretion by the liver, apoprotein and lipid levels were measured in primary rat liver cell cultures grown on fibronectin-coated dishes. Triglycerides, phospholipids, apoprotein (apo) B, apo-E, and apo-C-III3 all accumulated in culture media linearly for periods up to 20 h. During incubations, cellular triglyceride contents increased slightly, while cellular apoprotein and phospholipid contents remained constant. In the absence of insulin, rates of accumulation in media of triglycerides, apo-B, apo-C-III3, and apo-E were 2.5 +/- 0.3 micrograms/mg and 33 +/- 5, 24 +/- 3, and 162 +/- 32 ng/mg cell protein per h, respectively. On gel permeation chromatography and density gradient ultracentrifugation, the majority of apoproteins in media were found to be associated with very low density lipoproteins (VLDL) and very little eluted or sedimented with albumin. Incubations in the presence of 50-800 microU/ml of insulin resulted in dose-dependent decreases of triglyceride, phospholipid, apo-B, and apo-E accumulation in the media, paralleled by increases in the cellular contents of these lipoprotein components. The inhibitory effects of insulin on secretion were reversible. Levels of apo-C-III3 and albumin were not affected by insulin. In addition to decreasing secretory rates, the proportion of apo-B, apo-E, and apo-C-III3 associated with VLDL also decreased after the addition of insulin. Concomitantly, the proportion of apo-B eluting with LDL and apo-C-III3, and apo-E eluting near albumin increased. Control experiments, in which exogenous 125I-VLDL or endogenously labeled [14C]VLDL were added to cultures, revealed that the insulin-induced differences in VLDL accumulation and the lipid association of media apoproteins were not due to differences in the processing of VLDL by cells cultured in the presence or absence of insulin. Therefore, it appears that insulin may inhibit the secretion of VLDL perhaps by reducing the intracellular association of lipids and apoproteins.

Increased lipoprotein-remnant formation in chronic renal failure

Since accelerated atherosclerosis may be induced by excess circulating remnants of triglyceride-rich lipoprotein catabolism, we looked for evidence of remnant particle accumulation in the lipoproteins of 11 patients on long-term dialysis. We found several abnormalities in lipoprotein protein and lipids: enrichment of intermediate-density lipoproteins (ILD) and low-density lipoproteins (LDL) with triglyceride; the presence of apoprotein B48 (a "marker" for intestinal lipoproteins) in very-low-density lipoproteins (VLDL); an increased concentration of apoprotein AIV (a protein related to chylomicron transport); the presence of AIV in VLDL, IDL, and LDL; and the presence in LDL of apoproteins C and E (proteins not normally found in LDL). These findings strongly suggest accumulation of remnants of triglyceride-rich lipoproteins in patients with chronic renal failure who are undergoing peritoneal dialysis or hemodialysis, and may explain in part the increased incidence of coronary deaths among these patients.

Glycosylation of apolipoproteins by cultured rat hepatocytes. Effect of tunicamycin on lipoprotein secretion

Cultured rat hepatocytes were used to measure hepatic synthesis of rat plasma glycoproteins. [3H]Glucosamine was progressively incorporated into the protein of hepatocyte culture media very-low-density lipoprotein, low-density lipoprotein, high-density lipoprotein and the p greater than 1.21 g/ml fraction after 3.5 and 6.5 h incubation. Apolipoproteins B, E and C, as well as transferrin, were identified as glycoproteins. The association of radioactivity with apolipoprotein C of hepatocyte very-low-density and high-density lipoproteins suggests that apolipoprotein C-III-3, the only C apoglycoprotein in the rat, is synthesized de novo by the hepatocytes. Treatment of hepatocytes with tunicamycin, a specific inhibitor of protein glycosylation, resulted in a substantial decrease in [3H]glucosamine incorporation into hepatocyte very-low-density, low-density and high-density lipoproteins and p greater than 1.21 g/ml protein, but had little or no effect on secretion. In the rat, hepatic secretion of lipoproteins and transferrin does not appear to be dependent on prior protein glycosylation.