Degradation Of Low Density Lipoprotein Research Articles

Binding, degradation, and utilization of plasma high density and low density lipoproteins for progesterone production in cultured rat luteal cells.

These studies were intended to examine the binding and degradation of plasma lipoprotein fractions and the utilization of lipoprotein-bound cholesterol for progesterone production in cultured rat luteal cells. These cells bound [125I]human low density lipoprotein ([125I]iodo-hLDL),[125I]human high density lipoprotein ([125I]iodo-hHDL), and [125I]iodorat HDL ([125I] iodo-rHDL) with high affinity. The equilibrium dissociation constants of the binding of labeled rHDL, hHDL, and hLDL were 90.5, 78.3, and 36.8 micrograms/ml, respectively. All three lipoproteins were also degraded in a concentration-dependent manner, with apparent Km values of 18.3, 17.5, and 22.4 micrograms/ml for rHDL, hHDL, and hLDL, respectively. The degradation of the lipoproteins was inhibited by lysosomotropic agents, transglutaminase inhibitors, and metabolic inhibitors, suggesting that these lipoproteins undergo internalization and lysosomal degradation. In addition, all three lipoproteins also augmented the hCG-stimulated steroidogenesis. When cells were incubated with reconstituted LDL (the cholesterol ester in the LDL was replaced with [3H]cholesteryl linoleate) and the steroids produced identified, incorporation of tritium label in the progesterone fraction was observed in a time- and concentration-dependent manner. The incorporation of tritium into progesterone was increased by hCG and inhibited by an excess of unlabeled LDL in the incubation medium. These results show that the ovarian cells use lipoproteins as a source of cholesterol for steroidogenesis through receptor-mediated uptake and internalization, and the evidence suggests that the lipoproteins are intracellularly degraded.

Hydrocortisone decreases the internalization of low density lipoprotein in cultured human fibroblasts and arterial smooth muscle cells

The effect of hydrocortisone on the cellular low density lipoprotein (LDL) pathway was studied in cultured human skin fibroblasts and arterial smooth muscle cells. Hydrocortisone decreased both uptake and degradation of 125I-LDL, LDL binding, measured at 4 C, was not affected by the hormone. Physiological concentrations of hydrocortisone (4 . 1 X 10(-8) mol/l) resulted in a 30% reduction of LDL uptake and degradation which could not be accounted for by an effect of the hormone on macromolecular synthesis, cell protein or cell number. To test whether the decrease in uptake and degradation of 125I-LDL was due to reduced internalization, the effect of hydrocortisone on the internalization of prebound LDL was determined and found to be decreased. Also, preincubation with unlabelled LDL in the presence of hydrocortisone resulted in less down regulation of LDL receptor activity than when no hydrocortisone was present. An effect of the hormone on bulk phase endocytosis has been excluded, since hydrocortisone did not affect either LDL degradation by receptor negative cells or endocytosis of 14C-sucrose by normal skin fibroblasts. Thus, hydrocortisone impairs LDL catabolism by decreasing the internalization of LDL normally bound to its cell surface receptor. These results may be relevant to the pathogenesis of atherosclerosis in conditions associated with reduced cellular LDL catabolism.

Regulation of the uptake and degradation of beta-very low density lipoprotein in human monocyte macrophages.

In normal human monocyte macrophages 125I-labeled beta-migrating very low density lipoproteins (125I-beta-VLDL), isolated from the plasma of cholesterol-fed rabbits, and 125I-human low density lipoprotein (LDL) were degraded at similar rates at protein concentrations up to 50 micrograms/ml. The high affinity degradation of 125I-labeled human LDL saturated at approximately 50 micrograms/ml; however, 125I-labeled rabbit beta-VLDL high affinity degradation saturated at 100-120 micrograms/ml. The activity of the beta-VLDL receptor was 3-fold higher than LDL receptor activity on freshly isolated normal monocyte macrophages, but with time-in-culture both receptor activities decreased and were similar after several days. The degradations of both beta-VLDL and LDL were Ca2+ sensitive, were markedly down regulated by sterols, and were up regulated by preincubation of the cells in a lipoprotein-free medium. The beta-VLDL receptor is genetically distinct from the LDL receptor as indicated by its presence on monocyte macrophages from a familial hypercholesterolemic homozygote. Human thoracic duct lymph chylomicrons as well as lipoproteins of Sf 20-5000 from fat-fed normal subjects inhibited the degradation of 125I-labeled rabbit beta-VLDL as effectively as nonradioactive rabbit beta-VLDL. We conclude: 1) the beta-VLDL receptor is genetically distinct from the LDL receptor, and 2) intestinally derived human lipoproteins are recognized by the beta-VLDL receptor on macrophages.

Mutant Chinese hamster ovary cells pleiotropically defective in receptor-mediated endocytosis.

Populations of Chinese hamster ovary cells selected for resistance to diphtheria toxin were found to be highly enriched for mutants deficient in the uptake of lysosomal hydrolases via the mannose 6-phosphate receptor. One doubly defective mutant, DTF 1-5-1, exhibited increased resistance to Sindbis virus, although it was able to bind and internalize virus normally. Normal production of virus was obtained when, subsequent to virus binding, the mutant was exposed for 2 min to acidic pH. Similarly, a shift to acidic pH increased the sensitivity of DTF 1-5-1 to diphtheria toxin 12-fold. Decreased uptake of lysosomal hydrolases by the mutant correlated with decreased mannose 6-phosphate receptor activity at the cell surface; results of lactoperoxidase-catalyzed iodination indicated that the surface-associated receptor was present but inactive on DTF 1-5-1. Total mannose 6-phosphate receptor activity was also decreased in the mutant and this decrease was reflected by increased secretion of lysosomal hydrolases. The phenotype of DTF 1-5-1 resembles in many ways that of cells treated with ammonia. We suggest that the defect in DTF 1-5-1 stems from an inability to deliver virus, diphtheria toxin, and lysosomal hydrolases to an acidic compartment. Other ligands may be endocytosed through a different pathway since the defect of DTF 1-5-1 did not decrease the endocytosis of ricin, modeccin, or Pseudomonas toxin and had minimal effects on uptake and degradation of low density lipoprotein.

Enhanced macrophage degradation of biologically modified low density lipoprotein.

Low density lipoprotein (LDL) conditioned by incubation in the presence of rabbit aortic or human umbilical vein endothelial cells (endothelial cell-modified LDL) was degraded by macrophages three to five times more rapidly than LDL incubated in the absence of cells (control LDL). This enhanced degradation occurred mostly via a high affinity, saturable pathway related to the pathway for macrophage uptake of acetylated LDL. Conditioning LDL with cultured aortic smooth muscle cells had a qualitatively similar but smaller effect; conditioning with fibroblasts had no effect. Conditioning very low density lipoproteins or high density lipoproteins with endothelial cells did not affect subsequent metabolism of these lipoproteins by macrophages. Endothelial cell-modified LDL, while degraded more rapidly than control LDL by macrophages, was degraded more slowly by cultured smooth muscle cells and by human skin fibroblasts. Degradation of endothelial cell-modified LDL by macrophages was accompanied by stimulation of cholesterol esterification, inhibition of cholesterol synthesis, and a net increment in total cellular cholesterol content. Thus, a biologically generated modification of LDL is described that markedly alters cholesterol metabolism of macrophages and, consequently, may play a role in foam cell formation during atherogenesis.

Interaction of serum lipoproteins with the intestine. Evidence for specific high density lipoprotein-binding sites on isolated rat intestinal mucosal cells

To determine if plasma lipoproteins interact and therefore possibly regulate intestinal lipoprotein metabolism, we investigated the binding, internalization, and degradation of 125I-labeled low density lipoprotein (LDL) and high density lipoprotein (HDL) by enzyme-dispersed rat intestinal mucosal cells. Both human and rat LDL and HDL were bound, internalized, and degraded in a concentration-dependent manner with calculated half-saturation occurring at approximately 30, 35, 35, and 15 micrograms/ml for human LDL, rat LDL, human HDL, and rat HDL, respectively. Isolated brush border membranes had no saturable or specific binding sites for 125I-labeled HDL or LDL, suggesting that lipoproteins may be bound to receptors on lateral or basal membranes of mucosal cells. Compared with HDL, LDL binding was characterized by a large non-specific component. LDL of human and the rat were not only displaced by excess LDL but at least as effectively by excess HDL of their own species. Labeled HDL was displaced by corresponding unlabeled lipoproteins, but human LDL could produce only minor displacement of human HDL3. ApoE-deficient rat HDL, separated by heparin-Sepharose affinity chromatography also showed highly specific saturable binding to intestinal cells. Thus, apparently two different lipoprotein binding sites exist in intestinal plasma membranes, one recognizing B and/or E apoproteins present in human and rat LDL and rat HDL while another binds human HDL3 and apoE-deficient rat HDL which contain A apoproteins as major components.

Specific high-affinity binding of high density lipoproteins to cultured human skin fibroblasts and arterial smooth muscle cells.

Binding of human high density lipoproteins (HDL, d = 1.063-1.21) to cultured human fibroblasts and human arterial smooth muscle cells was studied using HDL subjected to heparin-agarose affinity chromatography to remove apoprotein (apo) E and B. Saturation curves for binding of apo E-free 125I-HDL showed at least two components: low-affinity nonsaturable binding and high-affinity binding that saturated at approximately 20 micrograms HDL protein/ml. Scatchard analysis of high-affinity binding of apo E-free 125I-HDL to normal fibroblasts yielded plots that were significantly linear, indicative of a single class of binding sites. Saturation curves for binding of both 125I-HDL3 (d = 1.125-1.21) and apo E-free 125I-HDL to low density lipoprotein (LDL) receptor-negative fibroblasts also showed high-affinity binding that yielded linear Scatchard plots. On a total protein basis, HDL2 (d = 1.063-1.10), HDL3 and very high density lipoproteins (VHDL, d = 1.21-1.25) competed as effectively as apo E-free HDL for binding of apo E-free 125I-HDL to normal fibroblasts. Also, HDL2, HDL3, and VHDL competed similarly for binding of 125I-HDL3 to LDL receptor-negative fibroblasts. In contrast, LDL was a weak competitor for HDL binding. These results indicate that both human fibroblasts and arterial smooth muscle cells possess specific high affinity HDL binding sites. As indicated by enhanced LDL binding and degradation and increased sterol synthesis, apo E-free HDL3 promoted cholesterol efflux from fibroblasts. These effects also saturated at HDL3 concentrations of 20 micrograms/ml, suggesting that promotion of cholesterol efflux by HDL is mediated by binding to the high-affinity cell surface sites.

The regulation of 3-hydroxy-3-methylglutaryl-CoA reductase activity, cholesterol esterification and the expression of low-density lipoprotein receptors in cultured monocyte-derived macrophages.

Human blood monocytes cultured in medium containing 20% whole serum showed the greatest activity of 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA) reductase and [14C]acetate incorporation into non-saponifiable lipids around the 7th day after seeding, the period of greatest growth. Although there was enough low-density lipoprotein (LDL) in the medium to saturate the LDL receptors that were expressed by normal cells at that time, HMG-CoA reductase activity and acetate incorporation were as high in normal cells as in cells from familial-hypercholesterolaemic (FH) patients. Both the addition of extra LDL, which interacted with the cells by non-saturable processes, and receptor-mediated uptake of acetylated LDL significantly reduced reductase activity and increased incorporation of [14C]oleate into cholesteryl esters in normal cells and cells from FH patients ('FH cells'), and reduced the expression of LDL receptors in normal cells. Pre-incubation for 20h in lipoprotein-deficient medium apparently increased the number of LDL receptors expressed by normal cells but reduced the activity of HMG-CoA reductase in both normal and FH cells. During subsequent incubations the same rate of degradation of acetylated LDL and of non-saturable degradation of LDL by FH cells was associated with the same reduction in HMG-CoA reductase activity, although LDL produced a much smaller stimulation of oleate incorporation into cholesteryl esters. In normal cells pre-incubated without lipoproteins, receptor-mediated uptake of LDL could abolish reductase activity and the expression of LDL receptors. The results suggested that in these cells, receptor-mediated uptake of LDL might have a greater effect on reductase activity and LDL receptors than the equivalent uptake of acetylated LDL. It is proposed that endogenous synthesis is an important source of cholesterol for growth of normal cells, and that the site at which cholesterol is deposited in the cells may determine the nature and extent of the metabolic events that follow.

Lipoproteins of the extravascular space: alterations in low density lipoproteins of interstitial inflammatory fluid

Although extrahepatic degradation of low density lipoproteins (LDL) by peripheral cells is considered to be a significant component of daily cholesterol homeostasis, the nature of lipoproteins in the extravascular space has not been well described. Using a sponge implantation model in the rabbit, we examined lipoproteins prepared from interstitial inflammatory fluid. Inflammatory fluid cholesterol is correlated with plasma values, (r = 0.735, P less than 0.01), but triglyceride values are not. Examination of inflammatory fluid lipoproteins by agarose gel electrophoresis, column chromatography, and density gradient centrifugation revealed a marked reduction in LDL concentration as compared to plasma LDL. Inflammatory fluid low density lipoproteins prepared by sequential density flotation had a larger mean diameter, they were erratic in shape, and contained more triglyceride and less cholesterol and cholesteryl ester than plasma LDL. Total cholesterol to protein ratio was significantly reduced in inflammatory fluid LDL (0.73 vs. 1.10, P less than 0.05). Inflammatory fluid LDL migrated further than plasma LDL on agarose electrophoresis, despite similar apoprotein patterns. These data concur with findings of altered composition and electrophoretic mobility of plasma LDL modified in vitro by exposure to acetylating agents, malondialdehyde, or aortic cells in culture, and they may represent the actual form of LDL in the extravascular space.

Bile acid metabolism in hypothyroid subjects: response to substitution therapy.

Bile acid kinetics and biliary lipid composition were determined in ten hypothyroid patients before and after treatment with L-thyroxine. Hypothyroid patients had normal synthesis rates of cholic acid and chenodeoxycholic acid. Hormone treatment, which lowered plasma cholesterol by about 35%, stimulated the formation of chenodeoxycholic acid by about 40% but did not significantly change the synthesis of cholic acid or total primary bile acids. The mean relative biliary concentration of deoxycholic acid was decreased from 30% to 19% and that of chenodeoxycholic acid was concomitantly increased. Cholesterol saturation of bile was decreased by treatment in six of the patients, but the mean value before treatment (135 +/- 13%) was not significantly different from that obtained after treatment (108 +/- 9%). It is suggested that the hypocholesterolaemic effect of thyroid hormones is not primarily due to an increased degradation of cholesterol to bile acids. Similar to what is observed in heterozygous familial hypercholesterolaemia, the defective receptor mediated degradation of plasma low density lipoproteins in hypothyroidism is thus apparently associated with a quantitatively normal catabolic rate of cholesterol to bile acids.

Inhibition of thymidine transport by low density lipoprotein and its lipid components.

We previously showed that low density lipoprotein (LDL) induces an early decrease in the thymidine-transport rate and subsequently inhibits cell proliferation. We now have demonstrated that a dose-dependent decrease in the rate of thymidine transport was found in all the cell lines examined irrespective of their sensitivity to the inhibitory effect of LDL on cell growth. Thus, inhibition of both transport and cell growth by LDL are not necessarily coupled. In contrast, 3-O-methyl-D-glucose transport was not affected by an addition of LDL. The specific inhibition of transport was not suppressed by an addition of chloroquine. Thus, degradation of LDL within the lysosome was not required for LDL to inhibit thymidine transport. A mixture of lipids extracted from LDL was as inhibitory as LDL in the various types of cells examined. Calf serum also prevented the inhibitory effect of both LDL and its lipid constituents equally. Phosphatidylcholine produced no decrease in thymidine transport, but cholesterol inhibited it. We concluded that LDL-associated lipids, particularly cholesterol, play an essential role in the LDL-induced decrease of transport.

Dibutyryl cyclic amp inhibits ldl binding in cultured fibroblasts and arterial smooth muscle cells

The interaction of low density lipoproteins (LDL) with cell membrane receptors is regulated by certain hormones. To test whether LDL receptor activity is influenced by cyclic AMP, the effect of dibutyryl cyclic AMP on LDL binding uptake and degradation was studied in cultured human skin fibroblasts and in rat and human arterial smooth muscle cells. Dibutyryl cyclic AMP in concentrations of 10 −3 to 10 −7 M inhibited LDL binding and degradation in fibroblasts and rat arterial smooth muscle cells, and degradation in human arterial smooth muscle cells. Inhibition of LDL receptor activity by dibutyryl cyclic AMP did not appear to be due to a non-specific toxic effect of the nucleotide. It appeared to be mediated on the affinity of the receptor for LDL rather than receptor number.

多発性骨髄腫患者における低脂血症の検討

Serum lipids were studied in 17 patients with multiple myeloma. Total cholesterol, low density lipoprotein (LDL) and high density lipoprotein (HDL) cholesterol levels (mean±SD) were significantly reduced in 6 patients with plasma cells over 30 per cent in bone marrow compared to 28 normal subjects (103±34mg/dl versus 172±24mg/dl, 46±19mg/dl versus 94±17mg/dl and 39±21mg/dl versus 53±11mg/dl respectively). Otherwise, hypocholesterolemia were not observed in 11 patients with small amount of plasma cells (less than 29%). There was a negative correlation between serum cholesterol and M-protein in multiple myeloma and also a rise towards a normal cholesterol level as a result of the effective treatment.LDL catabolism is mainly mediated through the LDL receptor pathway in the parenchymal cells. The effect of M-protein on LDL receptor was studied in cultured human skin fibroblasts. No increase of LDL uptake and degradation was observed with M-protein.Low serum cholesterol levels seems to be mediated by the high serum M-protein level or the great amount of plasma cells in bone marrow. Levels of serum total and lipoprotein cholesterol provide valuable informations in assessment of the disease activity.

Autoregulation of the modified low density lipoprotein receptor in human monocyte-derived macrophages.

Regulation of the macrophage receptor for modified low density lipoprotein (LDL) was evaluated using human monocyte-derived macrophages and acetyl LDL. Factors that regulate native LDL receptor activity in other cell types, such as the cholesterol content of the incubation medium, insulin, and platelet-derived growth factor had no effect on acetyl LDL degradation. Conditioned medium from mature macrophages significantly stimulated acetyl LDL degradation and enhanced cholesterol esterification by freshly isolated monocytes. Time course studies indicated that increasing time in culture was associated with increasing potency of the stimulating activity of macrophage-conditioned medium. These data suggest that a macrophage secretory product may be a prime modulator of modified LDL receptor activity on monocyte-macrophages, thus exerting an autocrine regulatory effect. The positive autoregulation of the human macrophage receptor for modified LDL could accelerate cellular cholesteryl ester accumulation and macrophage-derived foam cell formation.

Receptor-mediated uptake of hypertriglyceridemic very low density lipoproteins by normal human fibroblasts

Our previous studies showed that very low density lipoproteins, Sf 60-400 (VLDL), from hypertriglyceridemia subjects, but not VLDL from normolipemic subjects, suppress HMG-CoA reductase activity in normal human fibroblasts. To determine if this functional abnormality of hypertriglyceridemic VLDL resulted from differences in uptake of the VLDL by the low density lipoprotein (LDL) receptor pathway, we isolated VLDL subclasses from the d less than 1.006 g/ml fraction of normal and hypertriglyceridemic plasma by flotation through a discontinuous salt gradient for direct and competitive binding studies in cultured human fibroblasts. VLDL from the plasma of subjects with hypertriglyceridemia types 4 and 5 were at least as effective as normal LDL in competing for 125I-labeled LDL binding, uptake, and degradation when compared either on the basis of protein content or on a particle basis. By contrast, normolipemic Sf 60-400 VLDL were ineffective in competing with the degradation of 125I-labeled LDL, and Sf 20-60 VLDL (VLDL3) were less effective in reducing specific 125I-labeled LDL degradation than were LDL, consistent with their effects on HMG-CoA reductase activity. In direct binding studies, radiolabeled VLDL from hypertriglyceridemic but not normolipemic subjects were bound, internalized, and degraded with high affinity and specificity by normal fibroblasts. Uptake and degradation of iodinated hypertriglyceridemic VLDL Sf 100-400 showed a saturable dependence on VLDL concentration. Specific degradation plateaued at approximately 25 micrograms VLDL protein/ml, with a half maximal value at 6 micrograms/ml. The most effective competitor of hypertriglyceridemic VLDL uptake and degradation was hypertriglyceridemic VLDL itself. LDL were effective only at high concentrations. Uptake of normal VLDL by normal cells was a linear rather than saturable function of VLDL concentration. By contrast, cellular uptake of the smaller normal VLDL3 was greater than uptake of larger VLDL and showed saturation dependence. After incubation of normal VLDL with 125I-labeled apoprotein E, reisolated 125I-E-VLDL were as effective as LDL in suppression of HMG-CoA reductase activity, suggesting that apoE is involved in receptor-mediated uptake of large suppressive VLDL. We conclude that 1) hypertriglyceridemic VLDL Sf 60-400 are bound, internalized, and degraded by normal fibroblasts primarily by the high affinity LDL receptor-mediated pathway; 2) by contrast, normal VLDL, Sf 60-400 are bound, internalized, and degraded by normal fibroblasts primarily by nonspecific, nonsaturable routes; and 3) of the normal VLDL subclasses, only the smallest Sf 20-60 fraction is bound and internalized via the LDL pathway.

Evidence that recycling of low density lipoprotein receptors does not depend on delivery of receptors to lysosomes

Summary The rates of endocytosis and degradation of low density lipoproteins by human fibroblasts were determined as a function of temperature. At temperatures below 20°C, degradation of 125 I-labeled low density lipoprotein was totally inhibited while endocytosis still occurred albeit at a slower rate than at 37°C. At 20°C, the amount of low density lipoprotein endocytosed during 5 hours was approximately 8 times greater than the amount of low density lipoprotein bound to cell surface receptors. The carboxylic ionophore monensin caused a significant reduction (30%) of cell-surface low density lipoprotein receptors after 20 minutes at 20°C. These results indicate that receptor recycling occurs at temperatures (⩽20°C) at which low density lipoprotein degradation is blocked. Since the inhibition of degradation is probably due to an inhibition of lysosome-endosome fusion, we conclude that recycling of low density lipoprotein receptors does not depend on the delivery of receptors to lysosomes and that dissociation of the ligand from its receptor occurs at an extralysosomal location.

Modification of low density lipoprotein metabolism by growth factors in cultured vascular cells and human skin fibroblasts Dependence upon duration of exposure

The influences of specific growth factors upon binding, internalization and degradation of low density lipoproteins (LDL) were investigated in cultured vascular smooth muscle cells and skin fibroblasts. Plateletderived growth factor (PDGF) and fibroblast growth factor (FGF) significantly stimulated the binding of LDL to high affinity receptors of smooth muscle cells and fibroblasts. The effects of enhanced binding were reflected in elevated rates of internalization and degradation of LDL. FGF and PDGF elicited a mitogenic response as measured by [ 3H]thymidine autoradiography, indicating that altered LDL metabolism was associated with entry into the cell cycle. When fibroblasts were exposed to mitogen for periods long enough to commit the cells to the growth cycle, after which growth factor was removed before addition of LDL however, enhanced LDL binding to cycling fibroblasts appeared to be dependent upon the length of the period of exposure to growth factors in the early part of the cell cycle. LDL binding was stimulated in the presence of PDGF or FGF but not after their removal within 8 h of entry into the cell cycle. Exposure to the growth factors for 16 h or longer resulted in stimulation of LDL metabolism whether or not mitogens were present at the cell surface. PDGF and FGF, therefore, appear to exert a direct influence upon LDL receptor expression in addition to that mediated via the cell cycle.

Low density lipoprotein receptor activity in freshly isolated human blood monocytes and lymphocytes

Circulating human monocytes and lymphocytes were isolated by counterflow and density gradient centrifugation. Binding and degradation of low density lipoprotein (LDL) occurred predominantly in monocytes and to a much lesser extent in lymphocytes. The findings are consistent with greater LDL receptor activity in freshly isolated monocytes than lymphocytes, in keeping with differences in other cell surface receptors between these two cell types. Therefore, when freshly isolated mixed mononuclear cells are used to study LDL receptor activity in vivo in humans, careful attention needs to be given to the proportions of monocytes and lymphocytes, or alternatively, relatively pure preparations of monocytes should be used.

Are the binding and degradation of low density lipoprotein altered in Type 2 (non-insulin-dependent) diabetes mellitus?

Studies in vitro have shown that glycosylation of low density lipoprotein (LDL) will decrease its ability to bind to its receptor. We have evaluated the possibility that such an event might occur in vivo in diabetes by comparing the binding and degradation by normal fibroblasts and mouse peritoneal macrophages of LDL obtained from normal control subjects and patients with Type 2 (non-insulin-dependent) diabetes mellitus. When compared with control subjects, Type 2 diabetic patients had elevated fasting glucose (increased by 160%), haemoglobin AIc (increased by 75%), triglyceride (increased by 550%), and cholesterol (increased by 48%) levels. LDL from Type 2 diabetic patients displayed populations of particles with more heterogeneous hydrated densities than LDL from control subjects, with enrichment in the triglyceride content of the lighter population. 125I-LDL from normal and Type 2 diabetic subjects bound to fibroblasts with similar binding affinities and binding capacities. The kinetics of degradation of LDL from normal and Type 2 diabetic subjects by fibroblasts were also similar. Furthermore, all populations of LDL particles from Type 2 diabetic patients were bound and degraded by normal fibroblasts in identical fashions. In addition, 125I-LDL from normal and Type 2 diabetic subjects were not bound or degraded by mouse peritoneal macrophages. It is concluded that the LDL of patients with Type 2 diabetes with moderate hyperglycaemia are not modified sufficiently to alter their normal binding and degradation by human fibroblasts or to cause their uptake by mouse peritoneal macrophages.

Receptor-dependent and receptor-independent degradation of low density lipoprotein in normal rabbits and in receptor-deficient mutant rabbits.

Low density lipoprotein (LDL) catabolism was studied using WHHL rabbits, an inbred strain deficient in LDL receptor activity and, thus, an animal model for homozygous familial hypercholesterolemia. WHHL and normal rabbits were injected with [14C]sucrose-LDL and the tissue sites of LDL degradation were determined 24 h later. On degradation of [14C]sucrose-LDL, the [14C]sucrose ligand remains trapped within tissues as a cumulative measure of degradation. The fractional catabolic rate of [14C]sucrose-LDL in Watanabe heritable hyperlipidemic (WHHL) rabbits was reduced (0.024 +/- 0.010 versus 0.063 +/- 0.026 h-1) but, by virtue of the increased plasma pool, total LDL flux was increased (33.5 +/- 9.6 versus 10.6 +/- 4.4 mg of LDL protein/kg/day). Liver was the predominant site of catabolism in both WHHL and normal rabbits (52.7 +/- 6.9 and 56.6 +/- 6.2% of total degradation). About 90% of hepatic catabolism was attributable to parenchymal cells in both cases. Thus, Kupffer cells, a major component of the reticuloendothelial system, do not play a major role in LDL catabolism in WHHL rabbits. Despite receptor deficiency, the relative contribution of various tissues to overall LDL degradation was not greatly altered and the absolute rate of delivery of LDL to all tissues was increased with the exception of the adrenal. Thus, there was no evidence that the increased degradation occurred in any special subset of "scavenger" cells. Nevertheless, local scavenger cell uptake may be critically important, especially in atherogenesis. If it is assumed that receptor-independent degradation occurs at the same rate in the tissues of WHHL and normal rabbits and that catabolism in the absence of receptors is a linear function of concentration, then one can estimate the fraction of uptake in normal tissues mediated by receptors. The difference in the fraction of the plasma LDL pool cleared per unit of time in normal and WHHL rabbits would reflect the contribution of receptors to fractional clearance. By this calculation, receptor-mediated degradation in normal rabbits was 62% overall, 63% in liver, 92% in adrenal, and 83% in gut.