Macrophage-specific expression of human lipoprotein lipase accelerates atherosclerosis in transgenic apolipoprotein e knockout mice but not in C57BL/6 mice

Transgenic mice with macrophage-specific expression of human (hu) lipoprotein lipase (LPL) were generated to determine the contribution of macrophage LPL to atherogenesis. Macrophage specificity was accomplished with the scavenger receptor A promoter. Complete characterization demonstrated that macrophages from these mice expressed huLPL mRNA and secreted enzymatically active huLPL protein. Expression of huLPL was macrophage specific, because total RNA isolated from heart, thymus, lung, liver, muscle, and adipose tissues was devoid of huLPL mRNA. Macrophage-specific expression of huLPL did not exacerbate lesions in aortas of C57BL/6 mice even after 32 weeks on an atherosclerotic diet. However, when expressed in apolipoprotein E knockout background, the extent of occlusion in the aortic sinus region of male huLPL+ mice increased 51% (n=9 to 11, P<0.002) compared with huLPL- mice after they had been fed a Western diet for 8 weeks. The proatherogenic effect of macrophage LPL was confirmed in serial sections of the aorta obtained after mice had been fed a Western diet for 3 weeks. By immunohistochemical analysis, huLPL protein was detected in the lesions of huLPL+ mice but not in huLPL- mice. Our results establish that macrophage LPL accelerates atherosclerosis in male apolipoprotein E knockout mice.

Calcium-activated potassium channels mask vascular dysfunction associated with oxidized LDL exposure in rabbit aorta

Endothelium-dependent vasodilation is impaired in atherosclerosis. Oxidized low density lipoprotein (ox-LDL) plays an important role, possibly through alterations in G-protein activation. We examined the effect of acute exposure to ox-LDL on the dilator responses of isolated rabbit aorta segments. We sought also to evaluate the specificity of this dysfunction for dilator stimuli that traditionally operate through a Gi-protein mechanism. Aortic segments were prepared for measurement of isometric tension. After contraction with prostaglandin F2alpha, relaxation to thrombin, adenosine diphosphate (ADP), or the endothelium-independent agonists, sodium nitroprusside (SNP) or papaverine was examined. Maximal relaxation to thrombin was impaired in the presence of ox-LDL (17.7+/-3.7% p<0.05) compared to control (no LDL) (52.6+/-4.0%). Ox-LDL did not affect maximal relaxation to ADP or SNP. However, in the presence of charybdotoxin (CHTX: calcium-activated potassium channel inhibitor) ox-LDL impaired relaxation to ADP (17.4+/-3.2%). CHTX did not affect control (no LDL) responses to ADP (69.6+/-5.0%) or relaxation to thrombin or papaverine. In conclusion, ox-LDL impairs relaxation to thrombin, but in the case of ADP, calcium-activated potassium channels compensate to maintain this relaxation.

Lipoprotein lipase- and hepatic triglyceride lipase- promoted very low density lipoprotein degradation proceeds via an apolipoprotein E-dependent mechanism

Apolipoprotein E (apoE) is the primary recognition signal on triglyceride-rich lipoproteins responsible for interacting with low density lipoprotein (LDL) receptors and LDL receptor-related protein (LRP). It has been shown that lipoprotein lipase (LPL) and hepatic triglyceride lipase (HTGL) promote receptor-mediated uptake and degradation of very low density lipoproteins (VLDL) and remnant particles, possibly by directly binding to lipoprotein receptors. In this study we have investigated the requirement for apoE in lipase-stimulated VLDL degradation. We compared binding and degradation of normal and apoE-depleted human VLDL and apoE knockout mouse VLDL in human foreskin fibroblasts. Surface binding at 37 degrees C of apoE knockout VLDL was greater than that of normal VLDL by 3- and 40-fold, respectively, in the presence of LPL and HTGL. In spite of the greater stimulation of surface binding, lipase-stimulated degradation of apoE knockout mouse VLDL was significantly lower than that of normal VLDL (30, 30, and 80%, respectively, for control, LPL, and HTGL treatments). In the presence of LPL and HTGL, surface binding of apoE-depleted human VLDL was, respectively, 40 and 200% of normal VLDL whereas degradation was, respectively, 25 and 50% of normal VLDL. LPL and HTGL stimulated degradation of normal VLDL in a dose-dependent manner and by a LDL receptor-mediated pathway. Maximum stimulation (4-fold) was seen in the presence LPL (1 microgram/ml) or HTGL (3 microgram/ml) in lovastatin-treated cells. On the other hand, degradation of apoE-depleted VLDL was not significantly increased by the presence of lipases even in lovastatin-treated cells. Surface binding of apoE-depleted VLDL to metabolically inactive cells at 4 degrees C was higher in control and HTGL-treated cells, but unchanged in the presence of LPL. Degradation of prebound apoE-depleted VLDL was only 35% as efficient as that of normal VLDL. Surface binding of apoE knockout or apoE-depleted VLDL was to heparin sulfate proteoglycans because it was completely abolished by heparinase treatment. However, apoE appears to be a primary determinant for receptor-mediated VLDL degradation. Our studies suggest that overexpression of LPL or HTGL may not protect against lipoprotein accumulation seen in apoE deficiency.

Hepatic triglyceride lipase promotes low density lipoprotein receptor-mediated catabolism of very low density lipoproteins in vitro

We demonstrate here that hepatic triglyceride lipase (HTGL) enhances VLDL degradation in cultured cells by a LDL receptor-mediated mechanism. VLDL binding at 4 degrees C and degradation at 37 degrees C by normal fibroblasts was stimulated by HTGL in a dose-dependent manner. A maximum increase of up to 7-fold was seen at 10 microg/ml HTGL. Both VLDL binding and degradation were significantly increased (4-fold) when LDL receptors were up-regulated by treatment with lovastatin. HTGL also stimulated VLDL degradation by LDL receptor-deficient FH fibroblasts but the level of maximal degradation was 40-fold lower than in lovastatin-treated normal fibroblasts. A prominent role for LDL receptors was confirmed by demonstration of similar HTGL-promoted VLDL degradation by normal and LRP-deficient murine embryonic fibroblasts. HTGL enhanced binding and internalization of apoprotein-free triglyceride emulsions, however, this was LDL receptor-independent. HTGL-stimulated binding and internalization of apoprotein-free emulsions was totally abolished by heparinase indicating that it was mediated by HSPG. In a cell-free assay HTGL competitively inhibited the binding of VLDL to immobilized LDL receptors at 4 degrees C suggesting that it may directly bind to LDL receptors but may not bind VLDL particles at the same time. We conclude that the ability of HTGL to enhance VLDL degradation is due to its ability to concentrate lipoprotein particles on HSPG sites on the cell surface leading to LDL receptor-mediated endocytosis and degradation.

Agonist-specific impairment of coronary vascular function in genetically altered, hyperlipidemic mice

The objectives of the present study were to 1) examine mechanisms involved in endothelium-dependent responses of coronary arteries from normal mice and 2) determine whether vascular responses of coronary arteries are altered in two genetic models of hypercholesterolemia [apolipoprotein E (apoE)-deficient mice (apoE -/-) and combined apoE and low-density lipoprotein receptor (LDLR)-deficient mice (apoE + LDLR -/-)]. Plasma cholesterol levels were higher in both apoE -/- and apoE + LDLR -/- compared with normal mice on normal and high-cholesterol diets (normal chow: normal 110 +/- 5 mg/dl, apoE -/- 680 +/- 40 mg/dl, apoE + LDLR -/- 810 +/- 40 mg/dl; high-cholesterol chow: normal 280 +/- 60 mg/dl, apoE -/- 2,490 +/- 310 mg/dl, apoE + LDLR -/- 3,660 +/- 290 mg/dl). Coronary arteries from normal (C57BL/6J), apoE -/-, and apoE + LDLR -/- mice were isolated and cannulated, and diameters were measured using videomicroscopy. In normal mice, vasodilation in response to ACh and serotonin was markedly reduced by 10 microM Nomega-nitro-L-arginine (an inhibitor of nitric oxide synthase) or 20 microM 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ; an inhibitor of soluble guanylate cyclase). Vasodilation to nitroprusside, but not papaverine, was also inhibited by ODQ. Dilation of arteries from apoE -/- and apoE + LDLR -/- mice on normal diet in response to ACh was similar to that observed in normal mice. In contrast, dilation of arteries in response to serotonin from apoE -/- and apoE + LDLR -/- mice was impaired compared with normal. In arteries from both apoE -/- and apoE + LDLR -/- mice on high-cholesterol diet, dilation to ACh was decreased. In apoE + LDLR -/- mice on high-cholesterol diet, dilation of coronary arteries to nitroprusside was increased. These findings suggest that dilation of coronary arteries from normal mice in response to ACh and serotonin is dependent on production of nitric oxide and activation of soluble guanylate cyclase. Hypercholesterolemia selectively impairs dilator responses of mouse coronary arteries to serotonin. In the absence of both apoE and the LDL receptor, high levels of cholesterol result in a greater impairment in coronary endothelial function.

Phenotypic consequences of a deletion of exons 2 and 3 of the LDL receptor gene

Screening for structural alterations of the low density lipoprotein (LDL) receptor gene by Southern blot analysis revealed an abnormal band pattern in one subject with a clinical diagnosis of homozygous familial hypercholesterolemia (FH). The molecular defect was further characterized by polymerase chain reaction and cDNA sequencing. These analyses identified a 4.8 kb in-frame deletion of exons 2 and 3, where exon 1 was spliced to exon 4. This deletion is expected to produce a receptor that has lost the two first cysteine-rich repeats of the ligand-binding domain. Previously published data of in vitro site-directed mutagenesis has shown that binding of LDL to such a receptor is reduced to 70% of normal. A mild phenotype in our FH homozygote is consistent with that observation. In contrast, heterozygotes carrying this deletion have a relatively more severe phenotype that is comparable to that of heterozygotes carrying a null-allele. A severe phenotype was also found in a compound heterozygote carrying this deletion. Possible mechanisms for this phenotypic variability are discussed.-Rødningen, O. K., S. Tonstad, J. D. Medh, D. A. Chappell, L. Ose, and T. P. Leren. Phenotypic consequences of a deletion of exons 2 and 3 of the LDL receptor gene.

Receptor-mediated mechanisms of lipoprotein remnant catabolism

Chylomicron and VLDL are triglyceride-rich lipoprotein particles assembled by the intestine and liver respectively. These particles are not metabolized by the liver in their native form. However, upon entry into the plasma, their triglyceride component is rapidly hydrolyzed by lipoprotein lipase and they are converted to cholesterol-rich remnant particles. The remnant particles are recognized by the liver and rapidly cleared from the plasma. This process is believed to occur in two steps. (i) An initial sequestration of remnant particles on hepatic cell surface proteoglycans, and (ii) receptor-mediated endocytosis of remnants by hepatic parenchymal cells. The initial binding to proteoglycans may be facilitated by lipoprotein lipase and hepatic lipase which possess both lipid- and heparin-binding domains. The subsequent endocytic process may be mediated by LDL receptors and/or LRP. Both receptors have a high affinity for apoE, a major apolipoprotein component of remnant particles. The lipases may also serve as ligands for these receptors. An impairment of any component of this complex process may result in an accumulation of remnant particles in the plasma leading to atherosclerosis and coronary heart disease.

Overexpression of human superoxide dismutase inhibits oxidation of low-density lipoprotein by endothelial cells

Oxidation of LDL in the subendothelial space has been proposed to play a key role in atherosclerosis. Endothelial cells produce superoxide anions (O2.-) and oxidize LDL in vitro; however, the role of O2.- in endothelial cell-induced LDL oxidation is unclear. Incubation of human LDL (200 microg/mL) with bovine aortic endothelial cells (BAECs) for 18 hours resulted in a 4-fold increase in LDL oxidation compared with cell-free incubation (22.5+/-1.1 versus 6.3+/-0.2 [mean+/-SEM] nmol malondialdehyde/mg LDL protein, respectively; P<0.05). Under similar conditions, incubation of LDL with porcine aortic endothelial cells resulted in a 5-fold increase in LDL oxidation. Inclusion of exogenous copper/zinc superoxide dismutase (Cu/ZnSOD, 100 microg/mL) in the medium reduced BAEC-induced LDL oxidation by 79%. To determine whether the intracellular SOD content can have a similar protective effect, BAECs were infected with adenoviral vectors containing cDNA for human Cu/ZnSOD (AdCu/ZnSOD) or manganese SOD (AdMnSOD). Adenoviral infection increased the content and activity of either Cu/ZnSOD or MnSOD in the cells and reduced cellular O2.- release by two thirds. When cells infected with AdCu/ZnSOD or AdMnSOD were incubated with LDL, formation of malondialdehyde was decreased by 77% and 32%, respectively. Two other indices of LDL oxidation, formation of conjugated dienes and increased LDL electrophoretic mobility, were similarly reduced by SOD transduction. These data suggest that production of O2.- contributes to endothelial cell-induced oxidation of LDL in vitro. Furthermore, adenovirus-mediated transfer of cDNA for human SOD, particularly Cu/ZnSOD, effectively reduces oxidation of LDL by endothelial cells.

Atherosclerosis, vascular remodeling, and impairment of endothelium-dependent relaxation in genetically altered hyperlipidemic mice

We examined the vascular structure and endothelium-dependent relaxation in two genetic models of hypercholesterolemia: apolipoprotein E (apoE)-knockout mice and combined apoE/LDL receptor-double-knockout mice. Intimal area was increased markedly in proximal segments of thoracic aortas from apoE/LDL receptor-knockout mice [0.13 +/- 0.03 (mean +/- SE) mm2] compared with normal (C57BL/6J) mice (0.002 +/- 0.002 mm2, P < .05). Despite intimal thickening, the vascular lumen was not smaller in the aortas of apoE/LDL receptor-knockout mice (0.52 +/- 0.03 mm2) than in normal mice (0.50 +/- 0.03 mm2). In apoE-deficient mice, intimal thickening was minimal or absent, even though the concentration of plasma cholesterol was only modestly less than that in the double-knockout mouse (14.9 +/- 1.1 vs 18.0 +/- 1.2 mmol/L, respectively, P < .05). Relaxation of the aorta was examined in vitro in vascular rings precontracted with U46619. In normal mice, acetylcholine produced relaxation, which was markedly attenuated by the nitric oxide synthase inhibitor NG-nitro-L-arginine (100 microM). Relaxation to acetylcholine and the calcium ionophore A23187 was normal in apoE-deficient mice (in which lesions were minimal) but greatly impaired in the proximal segments of thoracic aortas of apoE/LDL receptor-deficient mice, which contained atherosclerotic lesions. Vasorelaxation to nitroprusside was similar in normal and apoE-knockout mice, with modest but statistically significant impairment in atherosclerotic segments of apoE/LDL receptor-knockout mice. In distal segments of the thoracic aorta of apoE/LDL receptor-deficient mice, atherosclerotic lesions were minimal or absent, and the endothelium-dependent relaxation to acetylcholine and calcium ionophore was normal. Thus, in apoE/LDL receptor-knockout mice (a genetic model of hyperlipidemia), there is vascular remodeling with preservation of the aortic lumen despite marked intimal thickening, with impairment of endothelium-dependent relaxation to receptor- and nonreceptor-mediated agonists. Atherosclerosis may be accelerated in the apoE/LDL receptor-double-knockout mouse compared with the apoE-knockout strain alone. We speculate that other factors, such as the absence of LDL receptors, may contribute to the differences in the extent of atherosclerosis in these two models of hyperlipidemia.

Very low density lipoproteins stimulate surfactant lipid synthesis in vitro

Surfactant synthesis is critically dependent on the availability of fatty acids. One fatty acid source may be circulating triglycerides that are transported in VLDL, and hydrolyzed to free fatty acids by lipoprotein lipase (LPL). To evaluate this hypothesis, we incubated immortalized or primary rat alveolar pre-type II epithelial cells with VLDL. The cells were observed to surface bind, internalize, and degrade VLDL, a process that was induced by exogenous LPL. LPL induction of lipoprotein uptake significantly increased the rates of choline incorporation into phosphatidylcholine (PC) and disaturated PC, and these effects were associated with a three-fold increase in the activity of the rate-regulatory enzyme for PC synthesis, cytidylyltransferase. Compared with native LPL, a fusion protein of glutathione S-transferase with the catalytically inactive carboxy-terminal domain of LPL did not activate CT despite inducing VLDL uptake. A variant of the fusion protein of glutathione S-transferase with the catalytically inactive carboxy-terminal domain of LPL that partially blocked LPL-induced catabolism of VLDL via LDL receptors also partially blocked the induction of surfactant synthesis by VLDL. Taken together, these observations suggest that both the lipolytic actions of LPL and LPL-induced VLDL catabolism via lipoprotein receptors might play an integral role in providing the fatty acid substrates used in surfactant phospholipid synthesis.

Lipoprotein lipase binds to low density lipoprotein receptors and induces receptor-mediated catabolism of very low density lipoproteins in vitro

Lipoprotein lipase (LPL), the major enzyme responsible for the hydrolysis of plasma triglycerides, promotes binding and catabolism of triglyceride-rich lipoproteins by various cultured cells. Recent studies demonstrate that LPL binds to three members of the low density lipoprotein (LDL) receptor family, including the LDL receptor-related protein (LRP), GP330/LRP-2, and very low density lipoprotein (VLDL) receptors and induces receptor-mediated lipoprotein catabolism. We show here that LDL receptors also bind LPL and mediate LPL-dependent catabolism of large VLDL with Sf 100-400. Up-regulation of LDL receptors by lovastatin treatment of normal human foreskin fibroblasts (FSF cells) resulted in an increase in LPL-induced VLDL binding and catabolism to a level that was 10-15-fold greater than in LDL receptor-negative fibroblasts, despite similar LRP activity in both cell lines. This indicates that the contribution of LRP to LPL-dependent degradation of VLDL is small when LDL receptors are maximally up-regulated. Furthermore studies in LRP-deficient murine embryonic fibroblasts showed that the level of LPL-dependent degradation of VLDL was similar to that in normal murine embryonic fibroblasts. LPL also promoted the internalization of protein-free triglyceride emulsions; lovastatin-treatment resulted in 2-fold higher uptake in FSF cells, indicating that LPL itself could bind to LDL receptors. However, the lower induction of emulsion catabolism as compared with native VLDL suggests that LPL-induced catabolism via LDL receptors is only partially dependent on receptor binding by LPL and instead is primarily due to activation of apolipoproteins such as apoE. A fusion protein between glutathione S-transferase and the catalytically inactive carboxyl-terminal domain of LPL (GST-LPLC) also induced binding and catabolism of VLDL. However GST-LPLC was not as active as native LPL, indicating that lipolysis is required for a maximal LPL effect. Mutations of critical tryptophan residues in GST-LPLC that abolished binding to VLDL converted the protein to an inhibitor of lipoprotein binding to LDL receptors. In solid-phase assays using immobilized receptors, LDL receptors bound to LPL in a dose-dependent manner. Both LPL and GST-LPLC promoted binding of VLDL to LDL receptor-coated wells. These results indicate that LPL binds to LDL receptors and suggest that the carboxyl-terminal domain of LPL contributes to this interaction.

The very low density lipoprotein receptor mediates the cellular catabolism of lipoprotein lipase and urokinase-plasminogen activator inhibitor type I complexes

The very low density lipoprotein (VLDL) receptor binds apolipoprotein E-rich lipoproteins as well as the 39-kDa receptor-associated protein (RAP). Ligand blotting experiments using RAP and immunoblotting experiments using an anti-VLDL receptor IgG detected the VLDL receptor in detergent extracts of human aortic endothelial cells, human umbilical vein endothelial cells, and human aortic smooth muscle cells. To gain insight into the role of the VLDL receptor in the vascular endothelium, its ligand binding properties were further characterized. In vitro binding experiments documented that lipoprotein lipase (LpL), a key enzyme in lipoprotein catabolism, binds with high affinity to purified VLDL receptor. In addition, urokinase complexed with plasminogen activator-inhibitor type I (uPA.PAI-1) also bound to the purified VLDL receptor with high affinity. To assess the capacity of the VLDL receptor to mediate the cellular internalization of ligands, an adenoviral vector was used to introduce the VLDL receptor gene into a murine embryonic fibroblast cell line deficient in the VLDL receptor and the LDL receptor-related protein, another endocytic receptor known to bind LpL and uPA.PAI-1 complexes. Infected fibroblasts that express the VLDL receptor mediate the cellular internalization of 125I-labeled LpL and uPA.PAI-1 complexes, leading to their degradation. Non-infected fibroblasts or fibroblasts infected with the lacZ gene did not internalize these ligands. These studies confirm that the VLDL receptor binds to and mediates the catabolism of LpL and uPA.PAI-1 complexes. Thus, the VLDL receptor may play a unique role on the vascular endothelium in lipoprotein catabolism by regulating levels of LpL and in the regulation of fibrinolysis by facilitating the removal of urokinase complexed with its inhibitor.

Glycoprotein 330/low density lipoprotein receptor-related protein-2 mediates endocytosis of low density lipoproteins via interaction with apolipoprotein B100

The ability of glycoprotein 330/low density lipoprotein receptor-related protein-2 (LRP-2) to function as a lipoprotein receptor was investigated using cultured mouse F9 teratocarcinoma cells. Treatment with retinoic acid and dibutyryl cyclic AMP, which induces F9 cells to differentiate into endoderm-like cells, produced a 50-fold increase in the expression of LRP-2. Levels of the other members of the low density lipoprotein (LDL) receptor (LDLR) family, including LDLR, the very low density lipoprotein receptor, and LRP-1, were reduced. When LDL catabolism was examined in these cells, it was found that the treated cells endocytosed and degraded at 10-fold higher levels than untreated cells. The increased LDL uptake coincided with increased LRP-2 activity of the treated cells, as measured by uptake of both 125I-labeled monoclonal LRP-2 antibody and the LRP-2 ligand prourokinase. The ability of LDL to bind to LRP-2 was demonstrated by solid-phase binding assays. This binding was inhibitable by LRP-2 antibodies, receptor-associated protein (the antagonist of ligand binding for all members of the LDLR family), or antibodies to apoB100, the major apolipoprotein component of LDL. In cell assays, LRP-2 antibodies blocked the elevated 125I-LDL internalization and degradation observed in the retinoic acid/dibutyryl cyclic AMP-treated F9 cells. A low level of LDL endocytosis existed that was likely mediated by LDLR since it could not be inhibited by LRP-2 antibodies, but was inhibited by excess LDL, receptor-associated protein, or apoB100 antibody. The results indicate that LRP-2 can function to mediate cellular endocytosis of LDL, leading to its degradation. LRP-2 represents the second member of the LDLR family identified as functioning in the catabolism of LDL.

gp330 on type II pneumocytes mediates endocytosis leading to degradation of pro-urokinase, plasminogen activator inhibitor-1 and urokinase-plasminogen activator inhibitor-1 complex

Glycoprotein 330 (gp330) is a member of a family of receptors related to the low density lipoprotein receptor (LDLR). Although several ligands have been shown to bind gp330 in solid-phase assays, the ability of gp330 to mediate ligand endocytosis has not been demonstrated. To develop a cellular model for gp330 function we screened a variety of cultured cell lines and identified several that expressed this protein, including immortalized rat type II pneumocytes and a human and two rodent tumor cell lines. Using type II pneumocytes, endocytosis of a previously described gp330 ligand, urokinase (uPA) complexed with plasminogen activator inhibitor-1 (uPA:PAI-1) and two new ligands, PAI-1 and pro-uPA, was demonstrated. RAP, the 39 kDa receptor-associated protein known to antagonize ligand binding to gp330 in solid-phase binding assays, completely inhibited both internalization and degradation of the radiolabeled ligands by type II pneumocytes. This suggested that the clearance of these ligands was dependent on either gp330 or the LDLR-related protein (LRP), which shares several ligand-binding characteristics with gp330. By using polyclonal antibodies to gp330, the cellular internalization and degradation of the ligands were inhibited by 30–50%; remaining ligand internalization and degradation activity could be partially inhibited by polyclonal antibodies against LRP. These findings indicate that gp330, like other LDLR family members, mediates endocytosis of its ligands. In addition, gp330 acts in concert with LRP in type II pneumocytes to mediate clearance of a variety of proteins involved in plasminogen activation, including uPA:PAI-1 complexes PAI-1 and pro-uPA.(ABSTRACT TRUNCATED AT 250 WORDS)

The cellular internalization and degradation of hepatic lipase is mediated by low density lipoprotein receptor-related protein and requires cell surface proteoglycans

Hepatic lipase (HL) and lipoprotein lipase (LpL) are structurally related lipolytic enzymes that have distinct functions in lipoprotein catabolism. In addition to its lipolytic activity, LpL binds to very low density lipoproteins and promotes their interaction with the low density lipoprotein receptor-related protein (LRP) (Chappell, D. A., Fry, G. L., Waknitz, M. A., Muhonen, L. E., Pladet M. W., Iverius, P. H., and Strickland, D. K. (1993) J. Biol. Chem. 268, 14168-14175). In vitro binding assays revealed that HL also binds to purified LRP with a KD of 52 nM. Its binding to LRP is inhibited by the 39-kDa receptor-associated protein (RAP), a known LRP antagonist, and by heparin. 125I-Labeled HL is rapidly internalized and degraded by HepG2 cell lines, and approximately 70% of the cellular internalization and degradation is blocked by either exogenously added RAP or anti-LRP IgG. Mouse fibroblasts that lack LRP display a greatly diminished capacity to internalize and degrade HL when compared to control fibroblasts. These data indicate that LRP-mediated cellular uptake of HL accounts for a substantial portion of the internalization of this molecule. Proteoglycans have been shown to participate in the clearance of LpL, and consequently a role for proteoglycans in HL clearance pathway was also investigated. Chinese hamster ovary cell lines that are deficient in proteoglycan biosynthesis were unable to internalize or degrade 125I-HL despite the fact that these cells express LRP. Thus, the initial binding of HL to cell surface proteoglycans is an obligatory step for the delivery of the enzyme to LRP for endocytosis. A small, but significant, amount of 125I-HL was internalized in LRP deficient cells indicating that an LRP-independent pathway for HL internalization does exist. This pathway could involve cell surface proteoglycans, the LDL receptor, or some other unidentified surface protein.

The 39-kDa receptor-associated protein modulates lipoprotein catabolism by binding to LDL receptors

The 39-kDa receptor-associated protein (RAP) is cosynthesized and co-purifies with the low density lipoprotein receptor-related protein (LRP)/alpha 2-macroglobulin receptor and is thought to modulate ligand binding to LRP. In addition to binding LRP, RAP binds two other members of the low density lipoprotein (LDL) receptor family, gp330 and very low density lipoprotein (VLDL) receptors. Here, we show that RAP binds to LDL receptors as well. In normal human foreskin fibroblasts, RAP inhibited LDL receptor-mediated binding and catabolism of LDL and VLDL with Sf 20-60 or 100-400. RAP inhibited 125I-labeled LDL and Sf 100-400 lipoprotein binding at 4 degrees C with KI values of 60 and 45 nM, respectively. The effective concentrations for 50% inhibition (EC50) of cellular degradation of 2.0 nM 125I-labeled LDL, 4.7 nM 125I-labeled Sf 20-60, and 3.6 nM 125I-labeled Sf 100-400 particles were 40, 70, and 51 nM, respectively. Treatment of cells with lovastatin to induce LDL receptors increased cellular binding, internalization, and degradation of RAP by 2.3-, 1.7-, and 2.6-fold, respectively. In solid-phase assays, RAP bound to partially purified LDL receptors in a dose-dependent manner. The dissociation constant (KD) of RAP binding to LDL receptors in the solid-phase assay was 250 nM, which is higher than that for LRP, gp330, or VLDL receptors in similar assays by a factor of 14 to 350. Also, RAP inhibited 125I-labeled LDL and Sf 100-400 VLDL binding to LDL receptors in solid-phase assays with KI values of 140 and 130 nM, respectively. Because LDL bind via apolipoprotein (apo) B100 whereas VLDL bind via apoE, our results show that RAP inhibits LDL receptor interactions with both apoB100 and apoE. These studies establish that RAP is capable of binding to LDL receptors and modulating cellular catabolism of LDL and VLDL by this pathway.

The 39-kDa receptor-associated protein regulates ligand binding by the very low density lipoprotein receptor

A 39-kDa receptor associated protein (RAP) binds and inhibits ligand binding by two members of the low density lipoprotein (LDL) receptor family, gp330 and low density lipoprotein receptor-related protein/alpha 2-macroglobulin receptor. To determine if additional members of the LDL receptor family may interact with RAP, Chinese hamster ovary cells were transfected with plasmids directing expression of the very low density lipoprotein (VLDL) receptor cDNA or the LDL receptor cDNA. Detergent-soluble extracts from these and normal Chinese hamster ovary cells were subjected to sodium dodecyl sulfate-polyacrylamide gel electrophoresis, after which the proteins were transferred to nitrocellulose membranes and incubated with RAP. When detergent extracts from normal cells were incubated with RAP, several polypeptides, including a 130-kDa protein, were observed to bind RAP. In cells transfected with the VLDL receptor cDNA, a substantial increase in RAP binding to the 130-kDa polypeptide was noted. This protein was identified as the VLDL receptor by immunoblotting. The VLDL receptor present in detergent extracts from transfected cells bound to RAP-Sepharose, and a KD of 0.7 nM for the interaction between RAP and the purified VLDL receptor was determined using enzyme-linked immunosorbent assay. The purified VLDL receptor bound 125I-labeled VLDL, but not 125I-labeled LDL, and the binding of 125I-labeled VLDL was completely inhibited by RAP. Further, RAP inhibited the uptake and degradation of 125I-VLDL by cells overexpressing the VLDL receptor. Thus the VLDL receptor represents the third member of the LDL receptor family whose ligand binding properties are antagonized by RAP. This suggests a common functional role for RAP in modulating ligand binding by members of the LDL receptor family.

Cellular catabolism of normal very low density lipoproteins via the low density lipoprotein receptor-related protein/alpha 2-macroglobulin receptor is induced by the C-terminal domain of lipoprotein lipase

Lipoprotein lipase (LPL) binds to the low density lipoprotein receptor-related protein (LRP)/alpha 2-macroglobulin receptor and induces catabolism of normal human very low density lipoproteins (VLDL) via LRP in vitro. Recent studies showed that the C-terminal domain of LPL can bind LRP in solid phase assays and inhibit cellular catabolism of two LRP ligands, activated alpha 2-macroglobulin and the 39-kDa receptor-associated protein (Williams, S.E., Inoue, I., Tran, H., Fry, G. L., Pladet, M.W., Iverius, P.-H., Lalouel, J.-M., Chappell, D.A., and Strickland, D.K. (1994) J. Biol. Chem. 269, 8653-8658). The current study investigated the potential for this region of LPL to promote cellular catabolism of VLDL via LRP. A fragment comprising the C-terminal domain of LPL (designated LPLC) was expressed in bacteria and found to promote cellular binding, uptake, and degradation of normal human VLDL in a dose-dependent manner. These effects were present whether LPLC was added simultaneously with 125I-VLDL or was prebound to cell surfaces prior to the assay. Mutations involving Lys407, Trp393, Trp394, or deletion of the C-terminal 14 residues reduced the effects of LPLC. Three LRP-binding proteins, the receptor-associated protein, lactoferrin, and a polyclonal antibody against LRP, competed for 125I-VLDL degradation induced by LPLC. Heparin or heparinase treatment of cells prevented LPLC-induced 125I-VLDL catabolism. Thus, cell-surface proteoglycans play an important role in this pathway. Interestingly, either LPLC or LPL when added in excess could block LPL-induced 125I-VLDL degradation presumably by interacting directly with LRP. However, unlabeled VLDL could not prevent catabolism of 125I-labeled LPLC or LPL. These data show that cellular fates for VLDL versus LPLC or LPL are divergent. This is probably due to independent catabolism of the latter via cell-surface proteoglycans. In summary, these in vitro studies indicate that a fragment of LPL corresponding to the C-terminal domain mimics the native enzyme with respect to induction of VLDL catabolism via LRP. Because LPLC lacks the catalytic site of native LPL, these studies establish that lipase activity is not required for LRP-mediated lipoprotein catabolism.

The carboxyl-terminal domain of lipoprotein lipase binds to the low density lipoprotein receptor-related protein/alpha 2-macroglobulin receptor (LRP) and mediates binding of normal very low density lipoproteins to LRP

Lipoprotein lipase (LPL) binds with high affinity to the low density lipoprotein receptor-related protein/alpha 2-macroglobulin receptor (LRP) and promotes binding, uptake, and degradation of normal triglyceride-rich lipoproteins in a process mediated by LRP (Chappell, D. A., Fry, G. L., Naknitx, M.A., Muhonen, L. E., Pladet, M. W., Iverius, P-H., and Strickland, D. K. (1993) J. Biol. Chem. 268, 14168-14175). To localize the portion of LPL that is responsible for interacting with LRP, fragments of LPL were expressed in bacteria. A fragment of human LPL containing the COOH-terminal domain (residues 313-448, designated LPLC) which lacks the catalytic site was able to bind to LRP. Purified LRP bound specifically to microtiter wells coated with LPL or LPLC with KD values of 2.8 and 5 nM, respectively. The effects of several mutations of LPLC were tested. Mutation of Lys407 to Ala reduced the affinity of LPLC for LRP by approximately 10-fold. Like native LPL, LPLC prevented the binding of activated alpha 2-macroglobulin and the 39-kDa receptor-associated protein to LRP and inhibited the internalization and degradation of activated alpha 2-macroglobulin and receptor-associated protein in cultured fibroblasts. LPLC also bound to 125I-labeled human normal triglyceride-rich lipoproteins and promoted their binding to purified LRP and to cultured cells. Mutation of Trp393 and Trp394 to Ala completely abolished the ability of LPLC to bind to lipoproteins, but had little effect on its interaction with LRP. These data indicate that the COOH-terminal domain of LPL may function both in binding lipoproteins and mediating their interaction with LRP.

Low density lipoprotein receptors bind and mediate cellular catabolism of normal very low density lipoproteins in vitro

Very low density lipoproteins (VLDL) are heterogeneous, triglyceride-rich particles that are precursors of low density lipoproteins (LDL). Before conversion to LDL, the majority of VLDL are irreversibly cleared from plasma by uncertain mechanisms. To investigate one potential mechanism for VLDL clearance, we studied the ability of LDL receptors to mediate VLDL uptake in vitro. Small, intermediate, and large VLDL from normolipidemic humans were found to bind and undergo catabolism via LDL receptors on normal human fibroblasts. Binding to cell surfaces was up-regulated by lovastatin, an inducer of LDL receptors. Both LDL and a monoclonal antibody against the LDL receptor (IgG-C7) prevented binding of 125I-VLDL. Also, VLDL binding to mutant fibroblasts lacking LDL receptors was low. Thus, LDL receptors mediated VLDL interactions with cells. Binding affinity decreased near saturation, and the apparent number of high affinity sites decreased with increasing VLDL particle size. Because LDL receptors are small (M(r) 115,000) relative to VLDL (M(r) 9-24 x 10(6)) and are clustered in clathrin-coated pits, these findings suggest that steric hindrance becomes an important binding determinant near saturation and are consistent with a lattice model for LDL receptor-ligand interactions. The capacity for cellular catabolism of VLDL decreased with increasing particle size, consistent with a lattice model. The lattice model was also supported by differences between 125I-VLDL binding to cell surfaces and binding to partially purified LDL receptors in solid-phase assays in which steric constraints resulting from clustering in clathrin-coated pits are not present. In both cell-surface and solid-phase assays, VLDL bound via apoE, not apoB-100. Our studies establish that normal VLDL interact with LDL receptors and that steric hindrance due to crowding of particles on clustered LDL receptors is an important determinant of their binding and catabolism. These findings suggest that LDL receptors may participate in normal VLDL clearance in vivo.

Lipoprotein lipase induces catabolism of normal triglyceride-rich lipoproteins via the low density lipoprotein receptor-related protein/alpha 2-macroglobulin receptor in vitro. A process facilitated by cell-surface proteoglycans

Bovine milk lipoprotein lipase (LPL) induced binding, uptake, and degradation of 125I-labeled normal human triglyceride-rich lipoproteins by cultured mutant fibroblasts lacking LDL receptors. The induction was dose-dependent and occurred whether LPL and 125I-lipoproteins were added to incubation media simultaneously or LPL was allowed to bind to cell surfaces, and unbound LPL was removed by washing prior to the assay. Lipolytic modification of lipoproteins did not appear to be necessary for increased catabolism because the effect of LPL was not prevented by inhibitors of LPL's enzymatic activity, p-nitrophenyl N-dodecylcarbamate or phenylmethylsulfonyl fluoride. However, the effect was abolished by boiling LPL prior to the assay suggesting that major structural features of LPL were required. Also, LPL-induced binding to cells was blocked by an anti-LPL monoclonal antibody but not by antibodies that are known to block apolipoprotein E- or B-100-mediated binding to low density lipoprotein (LDL) receptors. This indicates that LPL itself mediated 125I-lipoprotein binding to cells. Cellular degradation of 125I-lipoproteins was partially or completely blocked by two previously described ligands for the LDL receptor-related protein/alpha 2-macroglobulin receptor (LRP): activated alpha 2-macroglobulin (alpha 2M*), and the 39-kDa receptor-associated protein. These data implicated LRP as mediating LPL-induced lipoprotein degradation and were confirmed by showing that LPL's effects were prevented by an immunoaffinity-isolated polyclonal antibody against LRP. Furthermore, LPL promoted binding of 125I-lipoproteins to highly purified LRP in a solid-phase assay. Heparin or heparinase treatment of cells markedly decreased LPL-induced binding, uptake, and degradation of lipoproteins, but had no effect on catabolism of alpha 2M*. Thus, cell-surface proteoglycans were obligatory participants in the effects of LPL but were not required for LRP-mediated catabolism of alpha 2M*. Taken together, these in vitro findings establish that through interaction with cell-surface proteoglycans, LPL induces catabolism of normal human triglyceride-rich lipoproteins via LRP.

Glycoprotein 330, a member of the low density lipoprotein receptor family, binds lipoprotein lipase in vitro

Glycoprotein 330 (gp330), a cell-surface protein that is localized in clathrin-coated pits, is structurally related to both the low density lipoprotein receptor (LDLR) and the LDLR-related protein/alpha 2-macroglobulin receptor (LRP). We recently demonstrated that gp330 and LRP may be functionally related as well; both bind the 39-kDa polypeptide referred to as receptor-associated protein (Kounnas, M. Z., Argraves, W. S., and Strickland, D. K. (1992) J. Biol. Chem. 267, 21162-21166). In this report, we tested several other LRP ligands for their ability to interact with human and rat gp330 in vitro. Gp330 did not exhibit detectable binding to the LRP ligands, alpha 2-macroglobulin protease complex or Pseudomonas aeruginosa exotoxin A. However, we found that gp330 (purified from human or rat) bound the lipolytic enzyme lipoprotein lipase (LPL) with high affinity (Kd = 6.1 and 2.7 nM, respectively). The binding was saturable, divalent cation dependent, and inhibited by heparin or receptor-associated protein. Because LRP has also been shown to bind LPL, the present findings further extend the functional similarities between gp330 and LRP. By analogy to the postulated role of the LRP-LPL interaction in facilitating hepatic clearance of LPL-associated lipoproteins from the blood (Beisiegel, U., Weber, W., and Bengtsson-Olivercrona, G. (1991) Proc. Natl. Acad. Sci. U.S.A. 88, 8342-8346; Chappell, D. A., Fry, G. L., Waknitz, M. A., Iverius, P. H., Williams, S. E., and Strickland, D. K. (1992) J. Biol. Chem. 267, 25764-25767), we speculate that the gp330-LPL interaction described herein may contribute to the uptake of LPL-associated lipoproteins in tissues expressing gp330. Consistent with this possibility, we found that LPL promoted in vitro binding of 125I-lipoproteins to gp330.

Regulation of LDL receptor expression by luminal sterol flux in CaCo-2 cells

The regulation of expression of the intestinal low density lipoprotein (LDL) receptor by luminal (apical) sterol flux was investigated in the human intestinal cell line CaCo-2. Cells were cultured on semipermeable micropore filters, which separated an upper and lower well. To the apical media were added solutions containing either taurocholate micelles alone or micelles containing sterols. Because of an efflux of cholesterol, which occurred from cells incubated with micelles alone, LDL receptor mRNA levels increased threefold. With an influx of micellar sterols, receptor mRNA levels decreased in a dose-dependent manner. Synthesis and degradation of the LDL receptor were addressed by pulse-chase experiments. In cells incubated with micelles containing 25-hydroxycholesterol, the rate of receptor synthesis was significantly decreased, whereas the rate of receptor turnover remained unchanged. As assessed by immunoblots and steady-state labeling of proteins followed by immunoprecipitation of the LDL receptor, cells incubated with micellar 25-hydroxycholesterol contained substantially less receptor protein. These cells also bound and degraded less LDL. In contrast, in cells incubated with micelles alone, the rate of receptor synthesis was increased and cells contained more LDL receptor protein, although this was not reflected in an increased in LDL binding. The results suggest that LDL receptor expression in CaCo-2 cells is regulated by luminal sterol flux and that this regulation occurs at the level of transcription.

The low density lipoprotein receptor-related protein/alpha 2-macroglobulin receptor binds and mediates catabolism of bovine milk lipoprotein lipase

Lipoprotein lipase (LPL), the major lipolytic enzyme involved in the conversion of triglyceride-rich lipoproteins to remnants, was found to compete with binding of activated alpha 2-macroglobulin (alpha 2M*) to the low density lipoprotein receptor-related protein (LRP)/alpha 2-macroglobulin receptor. Bovine milk LPL displaced both 125I-labeled alpha 2M* and 39-kDa alpha 2M receptor-associated protein (RAP) from the surface of cultured mutant fibroblasts lacking LDL receptors with apparent KI values at 4 degrees C of 6.8 and 30 nM, respectively. Furthermore, LPL inhibited the cellular degradation of 125I-alpha 2M* at 37 degrees C. Because both alpha 2M* and RAP interact with LRP, these data suggest that LPL binds specifically to this receptor. This was further supported by observing that an immunoaffinity-isolated polyclonal antibody against LRP blocked cellular degradation of 125I-LPL in a dose-dependent manner. In addition, 125I-LPL bound to highly purified LRP in a solid-phase assay with a KD of 18 nM, and this binding could be partially displaced with alpha 2M* (KI = 7 nM) and RAP (KI = 3 nM). Taken together, these data establish that LPL binds with high affinity to LRP and undergoes LRP-mediated cellular uptake. The implication of these findings for lipoprotein catabolism in vivo may be important if LRP binding is preserved when LPL is attached to lipoproteins. If so, LPL might facilitate LRP-mediated clearance of lipoproteins.

Evidence for isomerization during binding of apolipoprotein-B100 to low density lipoprotein receptors

To determine the kinetics of human low density lipoproteins (LDL) interacting with LDL receptors, 125I-LDL binding to cultured human fibroblasts at 4 degrees C was studied. Apparent association rate constants did not increase linearly as 125I-LDL concentrations were increased. Instead, they began to plateau which suggested that formation of initial receptor-ligand complexes is followed by slower rearrangement or isomerization to complexes with higher affinity. To test this, 125I-LDL were allowed to associate for 2, 15, or 120 min, then dissociation was followed. The dissociation was biphasic with the initial phase being 64-110-fold faster than the terminal phase. After binding for 2 min, a greater percentage of 125I-LDL dissociated rapidly (36%) than after association for 15 min (24%) or 120 min (11%). Neither the rate constants nor the relative amplitudes of the two phases were dependent on the degree of receptor occupancy. Thus, the duration of association, but not the degree of receptor occupancy affected 125I-LDL dissociation. To determine if binding by large LDL, which is predominantly via apolipoprotein (apo) E, also occurs by an isomerization mechanism, the d = 1.006-1.05 g/ml lipoproteins were fractionated by ultracentrifugation. In contrast to small LDL which bound via apoB-100 and whose dissociation was similar to that of unfractionated LDL, large LDL dissociation after 2, 15, or 120 min of binding did not show isomerization to a higher affinity. This suggests that large and small LDL bind by different mechanisms as a result of different modes of interaction of apoE and apoB-100 with LDL receptors.

Ligand size as a determinant for catabolism by the low density lipoprotein (LDL) receptor pathway. A lattice model for LDL binding

Low density lipoproteins (LDL) are large (Mr = 2.5 x 10(6)) in comparison to LDL receptors (Mr = 115,000). Since most LDL receptors are clustered in coated pits, we tested the hypothesis that crowding of receptor-bound LDL particles would cause steric effects. The apparent affinity of LDL for receptors on cultured fibroblasts decreased near saturation causing concave-upward Scatchard plots. Both the higher and lower affinity components of binding were up-regulated by the cholesterol synthesis inhibitor, lovastatin, indicating that the entire binding curve was sterol-responsive. In contrast, neither component of LDL binding was present on lovastatin-treated or untreated null fibroblasts which are incapable of expressing LDL receptors. Therefore, the concave-upward Scatchard plots were entirely due to binding to LDL receptors. These results are consistent with a lattice model in which receptor-bound LDL are large enough to decrease binding to adjacent receptors. A lattice model implies that large LDL should produce steric effects at a lower receptor occupancy than should small LDL. This was tested using seven LDL fractions that differed in diameter from 20 to 27 nm. Fewer large than small LDL were bound to the cell surface at 4 degrees C and 37 degrees C, and fewer were internalized and degraded at 37 degrees C. Since large LDL bound via both apolipoprotein (apo) E and apoB100, receptor cross-linking could have caused fewer large LDL to be bound at saturation. However, when the potential for cross-linking was prevented by an apo-E-specific monoclonal antibody (1D7), the difference in binding by large versus small LDL was not eliminated; instead, it was exaggerated. Taken together, these results support a lattice model for LDL binding and indicate that steric hindrance associated with crowding of LDL particles on receptor lattices is a major determinant for catabolism by the LDL receptor pathway in vitro.

High receptor binding affinity of lipoproteins in atypical dysbetalipoproteinemia (type III hyperlipoproteinemia)

Familial dysbetalipoproteinemia (or type III hyperlipoproteinemia) is characterized by the presence of abnormal, cholesteryl ester-rich beta-very low density lipoproteins (beta-VLDL) in the plasma. Subjects with typical dysbetalipoproteinemia are homozygous for an amino acid substitution in apolipoprotein (apo-) E at residue 158 and have defective apo-E-mediated binding of both pre-beta-VLDL and beta-VLDL to apo-B,E(LDL) (or LDL) receptors (1988. Chappell, D.A., J. Clin. Invest. 82:628-639). To understand the effect of substitutions in apo-E at sites other than residue 158, nine dysbetalipoproteinemic (dys-beta) subjects who were either homozygous or heterozygous for substitutions in apo-E at atypical sites were studied. These substitutions occurred at residue 142 (n = 6), 145 (n = 2), or 146 (n = 1) and are known to cause less defective binding than does the 158 substitution. The chemical composition and electrophoretic mobility of pre-beta-VLDL and beta-VLDL from atypical and typical dys-beta subjects were indistinguishable. However, lipoproteins from atypical and typical dys-beta subjects differed in their affinity for the apo-B,E(LDL) receptor on cultured human fibroblasts. The pre-beta-VLDL and beta-VLDL from atypical dys-beta subjects had 640- or 17-fold higher affinity, respectively, than did corresponding lipoproteins from typical dys-beta subjects. The higher binding affinity of lipoproteins from atypical dys-beta subjects was associated with a higher ratio of apo-E to total apo-C. Since higher binding affinity should cause more rapid receptor-mediated clearance of beta-VLDL in atypical than in typical dys-beta subjects in vivo, the mechanism of beta-VLDL accumulation may differ in these two groups.

Pre-beta-very low density lipoproteins as precursors of beta-very low density lipoproteins. A model for the pathogenesis of familial dysbetalipoproteinemia (type III hyperlipoproteinemia)

The physical, chemical, and receptor binding properties of very low density lipoprotein (VLDL) fractions from familial dysbetalipoproteinemic (dys-beta) subjects, homozygous for apolipoprotein (apo-) E2 (E2/2 phenotype), and subjects with the E3/3 phenotype were studied to gain insights into the pathogenesis of dysbetalipoproteinemia, a disorder characterized by the presence of beta-VLDL in the plasma. Pre-beta-VLDL from dys-beta subjects were larger (27 vs. 17 x 10(6) D) and more triglyceride rich (68 vs. 43% dry weight) than beta-VLDL. Pre-beta-VLDL predominated in the Sf greater than 100 flotation fraction, whereas beta-VLDL predominated in the Sf 20-60 fraction. Because lipolysis converts large VLDL (Sf greater than 100) in vivo to smaller, more cholesteryl ester-rich VLDL (Sf 20-60), it is likely that pre-beta-VLDL are precursors of beta-VLDL. Although beta-VLDL were not found in type V hyperlipidemic E3/3 subjects, they were induced by intravenous heparinization, suggesting that lipolysis of pre-beta-VLDL in vivo can result in beta-VLDL formation. Similarly, heparinization of a dys-beta subject produced more beta-VLDL, at the expense of pre-beta-VLDL. The pre-beta-VLDL from normolipidemic and type V hyperlipidemic E3/3 subjects, respectively, had 90 and 280 times the affinity for the apo-B,E(LDL) receptor than did the pre-beta-VLDL from dys-beta subjects. Heparin-induced beta-VLDL from type V hyperlipidemic subjects had a sixfold higher binding affinity than did heparin-induced beta-VLDL from dys-beta subjects. These data suggest that pre-beta-VLDL from E2/2 subjects interact poorly with lipoprotein receptors in vivo, decreasing their receptor-mediated clearance and increasing their conversion to beta-VLDL during lipolytic processing.

A trial of amitriptyline and fluphenazine in the treatment of painful diabetic neuropathy

We conducted a double-blind, placebo-controlled, crossover study of the effectiveness of amitriptyline and fluphenazine in alleviating the pain of diabetic peripheral neuropathy in six diabetic patients. Pain was evaluated by the patients with a graphic rating scale. A placebo response was found, but no additional effect of amitriptyline and fluphenazine was seen. Although the statistical power of this study was low, these data, when combined with a reevaluation of previous trials of amitriptyline and fluphenazine in the treatment of painful diabetic neuropathy, indicate that there is no justification for the use of these agents in the treatment of painful neuropathy outside of large, controlled clinical trials. Depression as a possible cause of this condition should not go unnoted or untreated.

Role of apolipoprotein E in the lipolytic conversion of beta-very low density lipoproteins to low density lipoproteins in type III hyperlipoproteinemia

The beta-very low density lipoproteins (beta-VLDL) that accumulate in type III hyperlipoproteinemic subjects can be divided into two fractions (fraction I and fraction II), which differ in size, lipid composition, and the type of apolipoprotein B (apo-B) present in the particles. The apo-B48-containing particles (fraction I) are of intestinal origin, while apo-B100-containing particles (fraction II) are derived from the liver. Both fractions contain a defective form of apo-E referred to as apo-E2. Intravenous infusion of heparin into two subjects with type III hyperlipoproteinemia resulted in the complete removal of fraction II particles from density less than 1.006 g/ml, while fraction I particles remained at this density. In vitro studies confirmed that fraction I particles did not change density when subjected to hydrolysis with lipoprotein lipase, while fraction II particles shifted to the intermediate density lipoprotein range (approximately equal to 1.02 g/ml). When the beta-VLDL were hydrolyzed by lipoprotein lipase in the presence of density greater than 1.21 g/ml lipoprotein-deficient plasma, the addition of normal apo-E (apo-E3), but not apo-E2, resulted in a shift of fraction II particles to the low density lipoprotein (LDL) range (approximately equal to 1.05 g/ml). Fraction I particles did not undergo a shift to this higher density, supporting previous observations that apo-B48-containing particles are not converted to LDL. The demonstration that apo-B100-containing particles in type III hyperlipoproteinemic subjects could be converted to particles with the density of LDL suggests that apo-E plays a role in the normal conversion of VLDL to LDL. The mutant form of apo-E (apo-E2) found in the beta-VLDL from type III hyperlipoproteinemic subjects appears to impede this conversion, whereas the addition of normal apo-E (apo-E3) allows the processing to occur.

Regulation of low density lipoprotein receptors by adrenocorticotropin in the adrenal gland of mice and rats in vivo

Membranes prepared from the adrenal gland of mice and rats possess high affinity binding sites that recognize 125I-labeled human low density lipoprotein (LDL). These binding sites resemble the functional LDL receptors that mediate the uptake of LDL by cultured mouse and bovine adrenal cells. The number of LDL binding sites per mg of membrane protein increased 2- to 5-fold over 24 h when mice or rats were treated with adrenocorticotropin (ACTH). In rats, this increase was accompanied by a similar ACTH-induced increase in the adrenal uptake of intravenously administered 125I-LDL, suggesting that the LDL binding sites mediate the uptake of LDL by the adrenal in the intact animal. The number of LDL binding sites on adrenal membranes rose by 5-fold when animals were rendered lipoprotein-deficient, either by treatment of mice with 4-aminopyrazolopyrimidine or by treatment of rats with 17 alpha-ethinyl estradiol. This increase was prevented when endogenous ACTH secretion was blocked by administration of dexamethasone, suggesting that ACTH was required. The current experiments suggest that LDL receptors provide one source of cholesterol for the mouse and rat adrenal in vivo and that the number of LDL receptors of this organ is regulated by ACTH.