Reduced regulatory capacity of low density lipoproteins from patients with coronary artery disease

Human peritoneal monocytic cells: lipoprotein uptake and foam cell formation

Human peritoneal cells isolated from dialysis effluent have in vivo maturated human macrophages that could serve as a model for studying lipoprotein metabolism and foam cell formation. We previously characterized the low density lipoprotein (LDL) and acetylated LDL (acetyl-LDL) receptor activities of human total peritoneal cells. Now, we provide evidence that both LDL and acetyl-LDL stimulate acylCoA cholesterol:acyl transferase (ACAT) activity of peritoneal cells. Prolonged incubation of cells with LDL results in suppression of ACAT activity, while incubation with acetyl-LDL results in elevated and sustained enzyme activity. When human peritoneal cells were analyzed using flow cytometry, the cell population showed reactivity for CD2, CD4, CD8, CD20, CD14 and HLA-DR antigens. Purified human peritoneal mononuclear cells degraded LDL. Human peritoneal macrophages formed foam cells when exposed to LDL or acetyl-LDL in culture, and lipid deposition increased with incubation time. Macrophages incubated in the presence of butylated hydroxy toluene and LDL did not form foam cells.

Cellular processing of apolipoprotein B-containing lipoproteins from young post-infarction patients and healthy controls

3-Hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA) reductase activity was measured in fibroblasts incubated with large (Sf > 60) and small (Sf 20-60) very low density lipoprotein (VLDL), intermediate density lipoprotein (IDL) and low density lipoprotein (LDL) particles, at similar protein concentrations, from young post-infarction patients and healthy controls. The results showed that apolipoprotein (apo) B-containing lipoproteins (VLDL, IDL, LDL) from patients suppressed HMG-CoA reductase activity to a similar extent compared to apo B-containing lipoproteins from controls. When all subjects taken together were grouped according to triglyceride levels, it was found that small VLDL from hypertriglyceridaemic individuals suppressed the HMG-CoA reductase activity more than small VLDL from normotriglyceridaemic individuals. The opposite pattern was seen for LDL. The lipoprotein composition was related to the respective HMG-CoA reductase activity. In addition to a positive association between the cholesterol content of small VLDL and LDL, and the inhibition of HMG-CoA reductase activity, the apo C I and C II content of small VLDL and IDL was inversely related to the suppression of HMG-CoA reductase activity. This study shows that the cellular processing of apo B-containing lipoproteins in young post-infarction patients and healthy controls is heterogeneous and dependent on the composition of the lipoprotein.

Effect of hyperapo B LDL on cholesterol esterification in THP-1 macrophages

Hyperapobetalipoproteinemia (hyperapo B), a common disorder associated with coronary artery disease, is characterized by an increased number of small, dense, low density lipoprotein (LDL) particles. The cellular mechanisms responsible for early atherosclerosis in hyperapo B are unknown. We tested the hypothesis that hyperapo B LDL may be preferentially metabolized through an LDL receptor independent pathway promoting the accumulation of cellular cholesteryl ester (CE). THP-1 macrophages have little inducible LDL receptor activity after differentiation with phorbol esters and are, therefore, suitable for assessing non-LDL receptor mediated uptake of lipoproteins. LDL isolated from hyperapo B donors was found to have significantly lower total cholesterol to protein ratio (P = 0.03), higher average density (P = 0.0001) and smaller particle diameter (P = 0.016) compared with normal (control) LDL. LDL (250 micrograms lipoprotein-protein/ml) from normal (n = 11) and hyperapo B (n = 18) subjects were incubated for 24 h with THP-1 macrophages. The mean (S.D.) CE accumulation was 6.2 (3.6) for the normal and 6.4 (2.6) for the hyperapo B LDL (P = 0.84). CE accumulation in cells incubated with malondialdehyde modified (MDA) LDL, or without added lipoprotein, was 18.2 (2.0) and 0.6 (0.7), respectively. CE mass accumulation was significantly correlated with time (6-48 h) of incubation and concentration (100-500 micrograms/ml) of LDL protein (P < 0.05); no differences were observed between normal and hyperapo B LDL. Similarly, when the major LDL species was isolated by density gradient ultracentrifugation, mean (S.D.) CE was similar for the normal and hyperapo B LDL (8.7 (1.2) vs. 6.9 (1.5)). There were no differences in the mean (S.D.) incorporation of [14C]oleate into CE (nmol/mg cell protein per 6 h) in THP-1 macrophages incubated with normal or hyperapo B LDL (0.238 (0.045) vs. 0.211 (0.046)); results were comparable in human monocyte-derived (HMD) macrophages (0.298 (0.037) vs. 0.258 (0.022)). Also, mean (S.D.) cellular uptake and degradation (ng 125I/mg cell protein per h) in THP macrophages of normal and hyperapo B LDL were similar (uptake: 18 (14) vs. 12 (6.0); degradation: 58 (32) vs. 44 (8)). In summary: (1) hyperapo B LDL did not stimulate the accumulation of cellular CE via LDL receptor independent pathways in THP-1 macrophages, (2) normal and hyperapo B LDL stimulation of CE synthesis is similar in THP-1 and HMD macrophages and (3) no differences in cellular uptake and degradation of normal and hyperapo B LDL were observed in THP macrophages.