Receptor-independent low-density lipoprotein catabolism. Evaluation of 2-hydroxyacetaldehyde-treated lipoprotein as a probe for its measurement

Effects of combined postischemic hypothermia and delayed N-tert-butyl-alpha-pheylnitrone (PBN) administration on histopathologicaland behavioral deficits associated with transient global ischemia in rats

Previous cerebral ischemia studies have reported the limitations of restricted periods of postischemic hypothermia in producing long-term neuroprotection. The present experiment attempts to determine whether delayed treatment with the free radical scavenger N-tert-butyl-a-phenylnitrone (PBN) is protective at 2 months following transient global forebrain ischemia, and whether additive effects can be observed when PBN is administered in combination with moderate hypothermia. For this aim rats were subjected to 10 min of two-vessel forebrain ischemia followed by (a) 3 h of postischemic normothermia (37 degrees C); (b) 3 h of postischemic hypothermia (30 degrees C); (c) normothermic procedures combined with delayed injections of PBN (100 mg/kg) on days 3, 5 and 7 post-insult; (d) postischemic hypothermia combined with delayed PBN treatment; or (e) sham procedures. Outcome measures included cognitive behavioral testing and quantitative histopathological analysis at 2 months. Postischemic PBN injections induced a systemic hypothermia (1.5 degrees C-2.0 degrees C) that lasted for 2-2.5 h. Water maze testing revealed significant performance deficits relative to shams in the normothermic ischemic group, with the postischemic hypothermia and PBN groups showing intermediate values. A significant attenuation of cognitive deficits was observed in the animal group receiving the combination postischemic hypothermia and delayed PBN treatment. Quantitative CA1 hippocampal cell counts indicated that each of the ischemia groups exhibited significantly fewer viable CA1 neurons compared to sham controls. However, in rats receiving either delayed PBN treatment or 3 h of postischemic hypothermia, significant sparing of CA1 neurons relative to the normothermic ischemia group was observed. These data indicate that hypothermia combined with PBN treatment provides long-term cognitive improvement compared to nontreatment groups. PBN-induced mild hypothermia could contribute to the neuroprotective effects of this pharmacological strategy.

Visualization of the interaction of native and modified lipoproteins with parenchymal, endothelial and Kupffer cells from human liver

The interaction of low density lipoprotein, acetylated low density lipoprotein and apolipoprotein E-free high density lipoprotein with parenchymal, endothelial and Kupffer cells of human liver was visualized. For this purpose, the fluorescent phospholipid analog 1,1'-dioctadecyl-3,3,3',3'-tetramethyl indocarbocyanine perchlorate was used to label the lipoproteins. The involvement of both parenchymal and nonparenchymal cells in the uptake of 1,1'-dioctadecyl-3,3,3',3'-tetramethyl indocarbocyanine perchlorate-labeled low density lipoprotein and acetylated low density lipoprotein was studied using in vitro perfusion of human liver tissue blocks. In addition, primary hepatocyte cultures were used to visualize the interaction with 1,1'-dioctadecyl-3,3,3',3'-tetramethyl indocarbocyanine perchlorate-labeled apolipoprotein E-free high density lipoprotein and (modified) low density lipoprotein. 1,1'-dioctadecyl-3,3,3',3'-tetramethyl indocarbocyanine perchlorate-low density lipoprotein showed a time-dependent and concentration-dependent interaction with both hepatocytes and Kupffer cells, although the intensity of the interaction with parenchymal cells varied strongly among the liver donors. Uptake of 1,1'-dioctadecyl-3,3,3',3'-tetramethyl indocarbocyanine perchlorate-low density lipoprotein by both cell types was strongly inhibited by the presence of excess unlabeled low density lipoprotein in the (perfusion) medium. Methylation and hydroxyacetaldehyde treatment of low density lipoprotein prevented the uptake of low density lipoprotein. This indicated that the uptake of low density lipoprotein by Kupffer and parenchymal cells was mediated by the low density lipoprotein receptor. 1,1'-dioctadecyl-3,3,3',3'-tetramethyl indocarbocyanine perchlorate-acetylated low density lipoprotein was mainly taken up in situ by liver endothelial cells and by a minor population of Kupffer cells. Polyinosinic acid, a known inhibitor of the scavenger receptor, prevented the uptake by liver endothelial cells. Therefore human liver endothelial cells express active scavenger receptors on their surface. Apolipoprotein E-free 1,1'-dioctadecyl-3,3,3',3'-tetramethyl indocarbocyanine perchlorate-high density lipoprotein was found to be associated with the membrane of cultured liver parenchymal cells but was not taken up intracellularly, indicating a cholesterol exchange process occurring extracellularly at the plasma membrane. The cellular localization of lipoprotein receptors and uptake of the various classes of lipoproteins are comparable with the situation in rats.

Acetaldehyde modification of low density lipoprotein accelerates its catabolism in man

Acetaldehyde (AcA), the first metabolite in ethanol oxidation, is chemically highly reactive and binds covalently to the free amino groups of various proteins. In this study, we examined the metabolism of acetaldehyde-modified LDL (AcA-LDL) in man. LDL was isolated from human volunteers, radiolabelled with either 125I or 131I, incubated in various AcA concentrations (Aca-LDL) and injected back into the donors simultaneously with LDL incubated in identical conditions but omitting AcA (C-LDL). Acetaldehyde treatment did not change the chemical composition, electrophoretic mobility or the flotation characteristics of LDL. The proportion of free amino groups of AcA-LDL, ranging from 97 to 54.5%, was negatively correlated with the final concentration of AcA used in the incubation medium (r = -0.99, P less than 0.001). AcA modification of LDL accelerated its in vivo catabolism in man in such a way that the fractional catabolic rate (FCR) for AcA-LDL was negatively correlated with the percentage of free amino groups in AcA-LDL (r = -0.87, P less than 0.01). The clearance of AcA-LDL modified in 0.4, 2.0, 4.0 and 8.0 mM AcA was 0.9, 1.4, 2.5 and 3.7 times faster than the clearance of C-LDL, respectively. If AcA-LDL is formed in man after ethanol ingestion, its rapid clearance may be one possible mechanism for the low LDL levels observed in chronic alcohol users.

Receptor-independent low-density lipoprotein catabolism

Cultured cells, animal models, and man all possess mechanisms for LDL degradation that do not require the agency of the high-affinity receptor. In healthy individuals their functional significance can be determined using LDL which has been modified by specific chemical reactions designed to inhibit it receptor binding. Although the precise nature of the pathways has not been defined, there is evidence to implicate the monocyte-macrophage system in the process.

Limited nonenzymatic glucosylation of low-density lipoprotein does not alter its catabolism in tissue culture

This study examines the effects of various degrees of chemical modification of low-density lipoprotein (LDL) on its catabolism by various cell types. Moderate glucosylation of LDL does not alter its interaction with the high-affinity receptor present on human fibroblasts at concentration of 5-2000 micrograms LDL-cholesterol/ml. Only heavily glucosylated LDL (more than 12 lysine residues glucosylated per apolipoprotein B) or LDL glucosylated in the presence of Na(CN)BH3, i.e., conditions not expected to occur in diabetes, inhibit receptor-mediated internalisation and degradation. Moderately glucosylated LDL is also readily recognized by cultured rat hepatocytes and porcine endothelial cells. Human monocyte-derived macrophages accumulate cholesteryl ester when incubated with acetylated LDL for 12 days but no enhanced cholesteryl ester formation was found when native or glucosylated LDL (3.3 lysines glucosylated per apolipoprotein B) were used.

Contribution of the receptor pathway to low density lipoprotein catabolism in humans. New methods for quantitation

Receptor-mediated catabolism of low density lipoprotein (LDL) by cultured cells depends on the presence of functionally significant arginine and lysine residues on the lipoprotein apoprotein. When these are blocked, the recognition process is abolished, and catabolism of the modified lipoprotein is restricted to other mechanisms. Accurate discrimination between the activities of the receptor and nonreceptor pathways in vivo depends critically on the metabolic properties of this chemically modified lipoprotein. Here we report our experiences with two lysine-modified LDL tracers, glucosylated LDL (GLC-LDL) and 2-hydroxyacetaldehyde-treated LDL (HOET-LDL). The fractional clearance rate of GLC-LDL (0.25 +/- 0.05 pools/day, n = 5) was 50% of that of control material (0.51 +/- 0.09 pools/day) injected simultaneously into normal subjects. The HOET-LDL was also retarded in its clearance. Here, however, the fractional clearances of the control (0.37 +/- 0.06 pools/day, n = 6) and modified lipoprotein (0.19 +/- 0.03 pools/day) were lower than those obtained by the glucosylation procedure. We suspect that the prolonged incubation required for glucosylation of LDL artifactually accelerated its catabolism. The HOET-LDL does not suffer from this defect and seems to be a better tracer of the receptor-independent pathway. In a group of 10 subjects, HOET-LDL was metabolically indistinguishable from 1,2 cyclohexanedione-treated, arginine-modified LDL.