Modulation of Lipoprotein B Binding to the LDL Receptor by Exogenous Lipids and Apolipoproteins CI, CII, CIII, and E

Abstract

We have recently shown that apo B-containing lipoproteins isolated by immunoaffinity chromatography bind to the LDL receptor with an affinity dependent on their apo E or apo CIII content. However, these lipoproteins--LpB:E, LpB:CIII, and LpB:CIII:E--isolated from whole plasma have variable lipid and apolipoprotein contents, and it is difficult to consider each parameter separately, particularly because an increase in the apo CIII content is always associated with an increase in the content of other C apolipoproteins. Therefore, we used affinity-purified LpB free of other apolipoproteins. Lipid content of LpB was increased by incubation with a lipid emulsion, and this triglyceride-enriched LpB was named TG-LpB. Free apo CI, apo CII, apo CIII, and apo E were added to LpB and TG-LpB and their associations to the lipoprotein were assessed by gel filtration, nondenaturing electrophoresis, and immunoblotting. Molar ratios of 6 (apo E), 30 (apo CII), 20 (apo CIII), and 30 (apo CI) for 1 apo B were obtained. The association of apo CII to LpB and TG-LpB induced modifications to the LpB structure and a redistribution of lipids and apolipoproteins on the lipoprotein particles. The binding of these LpBs and TG-LpBs with and without added apo CI, CII, CIII, and E was tested at 4 degrees C on the LDL receptors of HeLa cells. The increased content of lipids reduced TG-LpB binding to the LDL receptor.(ABSTRACT TRUNCATED AT 250 WORDS)

Secretory Sphingomyelinase, a Product of the Acid Sphingomyelinase Gene, Can Hydrolyze Atherogenic Lipoproteins at Neutral pH

“…Tissue culture media and reagents were purchased from Life Technologies, Inc., and fetal bovine serum was from Gemini Bioproducts (Calabasas, CA). Human native apoCIII (29), human recombinant nonpancreatic soluble PLA2 (sPLA 2 ) (30), and partially purified phospholipid transfer protein (31) were prepared as described previously. Soybean lipoxygenase and all other reagents were from Sigma.…”

“…Briefly, [ 3 H]SM-emulsions (0.4 ml; 182 nmol of SM) were incubated with 200 g of apoCIII (22.5 nmol) for 2 h at 40°C. A portion of the emulsions was re-isolated from free apoCIII using ultrafiltration (29) as follows: the crude emulsion/apoCIII mixture was diluted to 2 ml with buffer containing 150 mM NaCl, 0.3 mM EDTA, pH 7.4, and ultrafiltered and concentrated to 0.2 ml using a Centricon 30 (molecular weight cut-off ϭ 30,000); the concentrated emulsions were then diluted to 2 ml, and the process was repeated 5 times. [ 3 H]SM-emulsions run through the same enrichment and re-isolation protocols in the absence of apoCIII served as the control for the experiments in Fig.…”

Impact of bariatric surgery on apolipoprotein C-III levels and lipoprotein distribution in obese human subjects

“…19 Third, several studies have shown that apoC-III inhibits receptor-mediated uptake of TRL by the liver. 21,22 Kinetic studies using tracer methodology have confirmed that VLDL-apoC-III is overproduced in overweight insulin-resistant subjects, 15 in hypertriglyceridemic subjects compared with controls, 53 and in centrally, obese men compared with controls. 54 Increased plasma concentrations of apoC-III are associated with impaired VLDL1 clearance in obese insulin-resistant men 55 and VLDLapoC-III clearance is reduced in centrally obese men compared with controls.…”

“…Elevated apoC-III has been postulated to contribute to the atherogenic dyslipidemia by the impairment of TRL and HDL metabolism including increased VLDL production and secretion, 18 disturbance of TRL clearance by inhibiting LPL 19 or hepatic lipase, 20 and interfering with TRL and their remnant binding to hepatic lipoprotein receptors. 21,22 Beyond the effects of apoC-III on TRL and HDL metabolism, this apolipoprotein has also been shown to promote inflammation and endothelial cell dysfunction, potentially contributing to atherosclerosis and cardiovascular disease. 23,24 In several clinical studies, elevated apoC-III in HDL and non-HDL fractions was a significant predictor of coronary events and progression of coronary artery disease.…”

Polymorphisms in the insulin response element of APOC-III gene promoter influence the correlation between insulin and triglycerides or triglyceride-rich lipoproteins in humans

“…The presence of apoC-III at the surface of LpC-III:B also reduces the ability of this particle to bind cellular receptors in vitro. 28,29 Plasma lipoprotein particle levels are sensitive to changes in their constituent apolipoprotein. For instance the concentration of LpC-III:B is increased after ingestion of a fatty meal 25 and in patients with hypertriglyceridemia.…”