Cellular and molecular biology of lipoprotein metabolism: characterization of lipoprotein receptor-ligand interactions.
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Amino Acid Sequence↗
Biomedical subjects
D Y Hui
Publications and source records attributed to D Y Hui.
Human apolipoprotein E expression in Escherichia coli: structural and functional identity of the bacterially produced protein with plasma apolipoprotein E.
Human apolipoprotein E (apoE) was produced in Escherichia coli by transforming cells with an expression vector containing a reconstructed apoE cDNA, a lambda PL promoter regulated by the thermolabile cI repressor, and a ribosomal binding site derived from the lambda cII or the E. coli beta-lactamase gene. Transformed cells induced at 42 degrees C for short periods of time (less than 20 min) produced apoE, which accumulated in the cells at levels of approximately equal to 1% of the total soluble cellular protein. Longer induction periods resulted in cell lysis and the proteolytic destruction of apoE. The bacterially produced apoE was purified by heparin-Sepharose affinity chromatography, Sephacryl S-300 gel filtration, and preparative Immobiline isoelectric focusing. The final yield was approximately equal to 20% of the initial apoE present in the cells. Except for an additional methionine at the amino terminus, the bacterially produced apoE was indistinguishable from authentic human plasma apoE as determined by NaDodSO4 and isoelectric focusing gel electrophoresis, amino acid composition of the total protein as well as its cyanogen bromide fragments, and partial amino acid sequence analysis (residues 1-17 and 109-164). Both the bacterially produced and authentic plasma apoE bound similarly to apolipoprotein B,E(low density lipoprotein) receptors of human fibroblasts and to hepatic apoE receptors. Intravenous injection resulted in similar rates of clearance for both the bacterially produced and authentic apoE from rabbit and rat plasma (approximately equal to 50% removed in 20 min). The ability to synthesize a bacterially produced human apolipoprotein with biological properties indistinguishable from those of the native protein will allow the production of large quantities of apoE for use in further investigations of the biological and physiological properties of this apolipoprotein.
Amino Acid Sequence↗
Binding of chylomicron remnants and beta-very low density lipoproteins to hepatic and extrahepatic lipoprotein receptors. A process independent of apolipoprotein B48.
The ability of apolipoprotein (apo-) B48 to interact with lipoprotein receptors was investigated using three different types of lipoproteins. First, canine chylomicron remnants, which contained apo-B48 as their primary apoprotein constituent, were generated by the hydrolysis of chylomicrons with milk lipoprotein lipase. These apo-B48-containing chylomicron remnants are deficient in apo-E and reacted very poorly with apo-E receptors on adult dog liver membranes and the low density lipoprotein (apo-B,E) receptors on human fibroblasts. Addition of normal human apo-E3 restored the receptor binding activity of these lipoproteins. Second, beta-very low density lipoproteins (beta-VLDL) from cholesterol-fed dogs were subfractionated into distinct classes containing apo-E along with either apo-B48 or apo-B100. Both classes bound to the apo-B,E and apo-E receptors. Their binding was almost completely mediated by apo-E, as evidenced by the ability of the anti-apo-E to inhibit the receptor interaction. Third, beta-VLDL from type III hyperlipoproteinemic patients were subfractionated by immunoaffinity chromatography into lipoproteins containing apo-E plus either apo-B48 or apo-B100. Both subfractions bound poorly to apo-B,E and apo-E receptors due to the presence of defective apo-E2. However, the residual binding of the apo-B48-containing and apo-B100-containing human beta-VLDL was inhibited by the anti-apo-E. After lipase hydrolysis, apo-B100 became a more prominant determinant responsible for mediating receptor binding to the apo-B,E receptor. By contrast, lipase hydrolysis did not increase the binding activity of the apo-B48-containing beta-VLDL. These results indicate that apo-B48 does not play a direct role in mediating the interaction of lipoproteins with receptors on fibroblasts or liver membranes.
Animals↗
Defective hepatic lipoprotein receptor binding of beta-very low density lipoproteins from type III hyperlipoproteinemic patients. Importance of apolipoprotein E.
Apolipoprotein (apo-) E2 and beta-migrating very low density lipoproteins (beta-VLDL) (which were isolated from type III hyperlipoproteinemic subjects) both demonstrated defective binding to apo-E and apo-B,E receptors on dog liver membranes and to apo-B,E low density lipoproteins (LDL) receptors on fibroblasts. The defective binding activity of the apo-E2 and beta-VLDL varied from very poor to nearly normal. The ability of the beta-VLDL to interact with hepatic apo-E receptors was enhanced by the addition of normal apo-E3 to the beta-VLDL. Furthermore, cysteamine treatment of the apo-E2 in beta-VLDL enhanced binding of the beta-VLDL to both apo-E and apo-B,E receptors. The importance of apo-E in mediating the receptor binding of beta-VLDL to these receptors was confirmed by using monoclonal antibodies. The residual binding activity of beta-VLDL to apo-E and apo-B,E receptors was inhibited by greater than 90% with anti-apo-E, while the addition of anti-apo-B had little effect. The apo-B in the beta-VLDL was capable of binding to apo-B,E receptors after the hydrolysis of the beta-VLDL triglycerides with milk lipoprotein lipase. Lipase treatment yielded, two subfractions of beta-VLDL. One fraction (d = 1.02 to 1.03 g/ml) was enriched with apo-B100; the other fraction (d less than 1.006 g/ml) was enriched with apo-B48 and apo-E2. Significantly increased amounts of the apo-B100-enriched fraction bound to apo-B,E receptors. Inhibition of this binding caused by the addition of anti-apo-B indicated that the binding activity of this subfraction was mediated by apo-B100. The apo-B48-enriched fraction did not show a significant increase in receptor binding, suggesting that apo-B48 does not bind to these receptors. In a control experiment, it was shown that triglyceride-rich VLDL, which contain normal apo-E3 and apo-B100, bind significantly to both liver apo-E receptors and fibroblast apo-B,E receptors. This binding activity was inhibited by greater than 90% with anti-apo-E. Lipase hydrolysis of the VLDL did not further enhance their receptor-binding activity. These results demonstrate that apo-E, and not apo-B, is the major determinant mediating the receptor-binding activity of cholesterol-rich beta-VLDL and triglyceride-rich VLDL.
Animals↗
Reduced immunoregulatory potency of low density lipoproteins with selectively modified arginine and lysine residues of apolipoprotein B.
Human plasma low density lipoproteins (LDL) suppress lymphocyte activation in vitro by inhibiting the early, membrane-associated events such as phytohemagglutinin-enhanced Ca2+ accumulation and phosphatidylinositol turnover. Chemical modification of the arginine residues of the protein constituent of LDL by 1,2-cyclohexanedione/borate or of the lysine residues by reductive methylation substantially decreases the immunosuppressive potency of LDL. The decrease in inhibitory capability of LDL correlates with a reduction in the ability of the derivatized LDL to compete with 125I-labeled LDL for lymphocyte membrane receptors. This correlation indicates that immunoregulation by LDL is the direct result of the binding of LDL to specific receptors at the cell surface. The receptor recognition site of LDL may consist of a high content of basic amino acid residues such that chemical modification of the LDL apolipoprotein reduces the LDL-lymphocyte interaction by specifically altering the change and/or steric properties of the receptor recognition site. Alternatively, chemical modification of arginine or lysine may cause a conformational change of the lipoprotein which is transmitted to the receptor recognition site. Derivatization of lysine or arginine residues does not elicit gross structural alteration of LDL, as evidenced by chemical analysis and by fluorescence quenching analysis. The intrinsic fluorescence intensity of LDL is, however, decreased by chemical modification, indicative of a minor but perhaps biologically significant structural alteration.
Amino Acids↗
Binding of plasma low density lipoproteins to erythrocytes.
Low density lipoproteins (LDL) containing apolipoprotein B bind to intact, freshly isolated erythrocytes. The LDL-erythrocyte interaction is of low affinity, with a Kd of 1.1 x 10(-6) M. Binding is noncooperative. There are about 200 binding sites per cell and, within the limits of experimental uncertainty, these sites comprise a homogeneous class. Binding of LDL is a temperature-independent process. The maximum amount of LDL blood increases following proteolytic digestion of the cells with trypsin or chymotrypsin. The specificity of the binding sites for LDL is not absolute: high density lipoproteins and lipid vesicles composed of phosphatidylcholine or phosphatidylcholine/cholesterol (equimolar) complete with LDL for occupancy of 60% of the binding sites. Modification of 5--6 of the 9 apolipoprotein B arginine residues with 1,2-cyclohexanedione/borate or of 10--15 of the 20 lysine residues by reductive methylation does not alter the ability of LDL to bind to erythrocytes. Native LDL and methylated-LDL alter erythrocyte morphology. However, LDL in which the arginine residues are derivatized with 1,2-cyclohexanedione/borate do not induce the discocyte leads to echinocyte transformation. Chemically modified and native LDL exchange cholesterol with erythrocytes at equal rates and to nearly equal extents. Taken together, the data suggest that the binding sites for LDL on the erythrocyte membrane are distinct from the LDL receptors at the surface of other cells--e.g., fibroblasts and lymphocytes--which do not bind HDL and which do not recognize LDL with derivatized arginine or lysine residues. It is proposed that the biological function of the erythrocyte binding sites is to mediate the exchange of cholesterol between the cell membrane and lipoproteins.
Apolipoproteins↗
Lipoprotein binding to canine hepatic membranes. Metabolically distinct apo-E and apo-B,E receptors.
Hepatic membranes from adult dog livers have receptors which bind to lipoproteins containing the E apoprotein (the apo-E HDLc) but lack specific receptors for the apo-B-containing low density lipoproteins (LDL). Scatchard analysis of direct binding data for 125I-apo-E-HDLc revealed nonlinearity of the binding which could be resolved into two components, suggesting the presence of two separate binding sites. The binding site for apo-E HDLc that possessed the highest affinity (Kd = 0.23 x 10(-9) M) was calcium-dependent and was sensitive to proteolytic digestion with pronase. The lower affinity (Kd = 20 x 10(-9) M) binding site for apo-E HDLc did not require calcium and was resistant to pronase digestion. Chemical modification of the arginyl or lysyl residues of the apo-E HDLc prevented the HDLc from binding to the higher affinity receptor but had no effect on their binding to the lower affinity site. Adult canine liver membranes also bound canine 125I-HDL. However, the binding of HDL was of lower affinity (Kd = 8.2 x 10(-8) M), did not require calcium, was not blocked by modification of the lysyl or arginyl residues, and may not be of physiologic significance. Although the liver membranes from normal chow-fed adult dogs did not bind canine LDL, it was possible to demonstrate specific high affinity binding of LDL under certain metabolic conditions in dogs. When adult dogs were treated with the hypocholesterolemic agent cholestyramine, the liver membranes from these animals readily bound canine LDL. The Kd for canine LDL binding to these liver membranes was 15 x 10(-9) M. Furthermore, it was possible to demonstrate high affinity binding of LDL to the liver membranes from young rapidly growing puppies (Kd = 11 x 10(-9) M). The binding of the 125I-LDL to the liver membranes from the adult cholestyramine-treated dogs or from the puppies appeared to be mediated by apo-B,E receptors which resembled the LDL receptor of human skin fibroblasts. The 125I-LDL binding of these liver membranes was competitively inhibited by the addition of unlabeled LDL or apo-E HDLc. On the other hand, 125I-apo-E HDLc, capable of binding to the apo-B,E or to the apo-E receptors, were only partially displaced by the addition of unlabeled LDL but were totally displaced by apo-E HDLc. In summary, the adult dog liver possessed only the apo-E receptor. An apo-B,E receptor capable of binding LDL and HDLc could be induced by treatment of adult dogs with cholestyramine. Similarly, the liver membranes of young growing puppies possessed the apo-E and po-B,E receptors and were capable of binding both apo-E HDLc and LDL. The mechanism responsible for the control of the expression of the hepatic apo-B,E and/or apo-E receptors remains to be determined. These data indicate that a unique receptor capable of interacting specifically with apo-E-containing lipoproteins, and not with apo-B-containing lipoproteins (LDL), exists in the adult canine liver.
Animals↗
Functional unit of the low density lipoprotein receptor of fibroblasts: a 100,000-dalton structure with multiple binding sites.
The low density lipoprotein (apoprotein B,E) receptors of fibroblasts bind plasma lipoproteins that contain either the B (apo-B) or E (apo-E) apoproteins. These include the low density lipoproteins (LDL) containing apo-B and certain high density lipoproteins containing apo-E (e.g., the cholesterol-induced apo-E HDLc). The same receptor binds both LDL and apo-E HDLc, but the apo-E HDLc bind with much higher affinity. This higher affinity is due to the binding of apo-E HDLc to multiple receptor sites. One possible structural model for this receptor is that each apo-B,E receptor possesses a single binding site. Thus, the multiple binding of apo-E HDLc would require the recruitment of four independent receptors to bind to a single apo-E HDLc particle. A second model is that each receptor unit possesses multiple binding sites capable of binding one apo-E HDLc particle or four LDL particles. This study characterizes the apo-B,E receptors in situ in the membranes of fibroblasts by radiation inactivation. This technique allows one to determine the functional size of the receptor on the basis of the amount of radiation required to inactivate the structure. The larger the molecular size of the structure, the less ionizing radiation (generated by a linear accelerator) that will be required to abolish the receptor--ligand interaction. The functional size of the apo-B,E receptor is 106,000 Mr as determined with both LDL and apo-E HDLc binding after radiation inactivation. Furthermore, the data derived from radiation inactivation and Scatchard analysis indicate that each apo-B,E receptor has multiple binding sites and that each functional receptor unit, capable of binding one apo-E HDLc or four LDL particles, has a molecular weight of 106,000.
Apolipoproteins↗
Two independent lipoprotein receptors on hepatic membranes of dog, swine, and man. Apo-B,E and apo-E receptors.
We have reported previously that canine livers possess two distinct lipoprotein receptors, an apoprotein (apo)-B,E receptor capable of binding the apo-B-containing low density lipoproteins (LDL) and the apo-E-containing cholesterol-induced high density lipoproteins (HDLc), and an apo-E receptor capable of binding apo-E HDLc but not LDL. Both the apo-B,E and apo-E receptors were found on the liver membranes obtained from immature growing dogs, but only the apo-E receptors were detected on th hepatic membranes of adult dogs. In this study, the expression of the apo-B,E receptors, as determined by canine LDL binding to the hepatic membranes, was found to be highly dependent on the age of the dog and decreased linearly with increasing age. Approximately 30 ng of LDL protein per milligram of membrane protein were bound via the apo-B,E receptors to the hepatic membranes of 7- to 8-wk-old immature dogs as compared with no detectable LDL binding in the hepatic membranes of adult dogs (greater than 1--1.5 yr of age). Results obtained by in vivo turnover studies of canine 125I-LDL correlated with the in vitro findings. In addition to a decrease in the expression of the hepatic apo-B,E receptors with age, these receptors were regulated, i.e., cholesterol feeding suppressed these receptors in immature dogs and prolonged fasting induced their expression in adult dogs. Previously, it was shown that the apo-B,E receptors were induced in adult livers following treatment with the hypocholesterolemic drug cholestyramine. In striking contrast, the apo-E receptors, as determined by apo-E HDLc binding, remained relatively constant for all ages of dogs studied (10--12 ng/mg). Moreover, the expression of the apo-E receptors was not strictly regulated by the metabolic perturbations that regulated the apo-B,E receptors. Similar results concerning the presence of apo-B,E and apo-E receptors were obtained in swine and in man. The hepatic membranes of adult swine bound only apo-E HDLc (apo-E receptors), whereas the membranes from fetal swine livers bound both LDL and apo-E HDLc (apo B,E and apo-E receptors). Furthermore, the membranes from adult human liver revealed the presence of the apo-E receptors as evidenced by the binding of 12--14 ng of HDLc protein per milligram of membrane protein and less than 1 ng of LDL protein per milligram. The membranes from the human liver also bound human chylomicron remnants and a subfraction of human HDL containing apo-E. These data suggest the importance of the E apoprotein and the apo-E receptors in mediating lipoprotein clearance, including chylomicron remnants, by the liver of adult dogs, swine, and man.
Adult↗
Inhibition by membrane-bound low-density lipoproteins of the primary inductive events of mitogen-stimulated lymphocyte activation.
Low-density lipoproteins (LDL) inhibit phytohemagglutinin (PHA)-enhanced accumulation of calcium ion and cyclic guanosine 3':5'-monophosphate, and phosphatidylinositol turnover in human lymphocytes. Inhibition by LDL of PHA-induced 32P incorporation into monophosphoinositide and cyclic guanosine 3':5'-monophosphate accumulation correlates directly with inhibition by LDL of calcium ion accumulation. These results suggest that calcium ion accumulation is required for PHA to direct turnover of monophosphoinositide and cyclic guanosine 3':5'-monophosphate synthesis in lymphocytes and that inhibition of this mitogen-induced response by LDL may be a direct consequence of the ability of these lipoproteins to prevent calcium ion accumulation by the activated cells. The ability of LDL to suppress PHA-enhanced calcium ion accumulation is directly proportional to the amount of LDL localized at the cell surface, indicating that internalization of the lipoproteins is not required for immunoregulation. This conclusion is supported by two additional lines of evidence: (a) LDL depleted of the bioregulatory constituent cholesterol are as effective as are native LDL in suppressing lymphocyte response to PHA; and (b) LDL-Sepharose complexes which cannot be endocytosed by the cells are as immunosuppressive as soluble LDL. The data clearly establish that membrane-bound LDL may have a regulatory role.
Calcium↗
Immunoregulatory plasma lipoproteins. Role of apoprotein E and apoprotein B.
Plasma lipoproteins containing either the B (apo-B) or E (apo-E) apoproteins, e.g. apo-B low density lipoproteins (LDL) isolated from normolipemic humans and apo-E-containing cholesterol-induced high density lipoproteins (apo-E HDLc) isolated from cholesterol-fed dogs, suppress phytohemagglutinin (PHA)-induced lymphocyte activation and inhibit early events such as mitogen-enhanced 45Ca2+ accumulation and late events such as enhanced DNA synthesis. On a molar basis, apo-E HDLc are 3.5 times more effective than apo-B LDL in inhibiting 45Ca2+ accumulation by 50% and 3.8 times more effective than apo-B LDL in suppressing DNA synthesis by 50%. Both lipoproteins bind to the lymphocyte surface, and in competitive binding assays apo-E HDLc and apo-B LDL bind to the same receptors. These receptors, termed immunoregulatory receptors, comprise a homogeneous class of binding sites which do not act cooperatively. The equilibrium dissociation constant (Kd) for apo-B LDL is 2.0 X 10(-7) M at 4 degrees C and 37 degrees C; the Kd for apo-E HDLc is 9.3 X 10(-8) M at 4 degrees C and 7.3 X 10(-8) M at 37 degrees C. At saturation, 16,000 and 20,000 LDL particles are bound/cell at 4 degrees C and 37 degrees C, respectively. The corresponding values for apo-E HDLc are 6,700 at 4 degrees C and 5,500 at 37 degrees C. The increased effectiveness of apo-E HDLc in supp]ressing the PHA-induced Ca2+ accumulation and DNA synthesis is due to the multiple receptor binding of apo-E HDLc. At 37 degrees C, each apo-E HDLc particle occupies multiple receptors at a ratio of 3.6:1 relative to apo-B LDL. The enhanced affinity of apo-E HDLc most likely results from the multiple interactions of this lipoprotein with the receptors.
Apolipoproteins↗
Phosphatidylinositol turnover in mitogen-activated lymphocytes. Suppression by low-density lipoproteins.
Low-density (LD) lipoproteins inhibit phytohaemagglutinin-enhanced turnover of phosphatidylinositol in human peripheral lymphocytes. Turnover was assessed by (32)P incorporation into phospholipids and by loss of (32)P from [(32)P]phosphatidylinositol. Inhibition of lipid turnover by LD lipoproteins is not the result of a change in the amount of phytohaemagglutinin required for maximum cellular response. Neither phytohaemagglutinin nor LD lipoproteins influence (32)P incorporation into phosphatidylethanolamine and phosphatidylcholine during the first 60min after mitogenic challenge. The extent of inhibition of phosphatidylinositol turnover by LD lipoproteins depends on the concentration of LD lipoproteins present in the incubation medium: 50% of maximum inhibition occurs at a low-density-lipoprotein protein concentration of 33mug/ml and maximum inhibition occurs at low-density-lipoprotein protein concentrations above 100mug/ml. Phytohaemagglutinin stimulates (32)P incorporation into phosphatidylinositol, phosphatidylinositol phosphate and phosphatidylinositol bisphosphate. However, LD lipoproteins abolish (32)P incorporation into phosphatidylinositol without affecting incorporation into phosphatidylinositol phosphate and phosphatidylinositol bisphosphate. The ability of LD lipoproteins to inhibit phytohaemagglutinin-induced phosphatidylinositol turnover is mimicked by EGTA. Furthermore, inhibition of LD lipoproteins by phytohaemagglutinin-induced (32)P incorporation into phosphatidylinositol correlates directly with inhibition by LD lipoproteins of Ca(2+) accumulation. These results suggest that Ca(2+) accumulation and turnover of phosphatidylinositol are coupled responses in lymphocytes challenged by mitogens. The step in phosphatidylinositol metabolism that is sensitive to LD lipoproteins and, by inference, that is coupled to Ca(2+) accumulation is release of [(32)P]phosphoinositol from phosphatidylinositol.
Calcium↗
Inhibition by low density lipoproteins of mitogen-stimulated cyclic nucleotide production by lymphocytes.
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Biological Transport, Active↗
Inhibition of Ca2+ accumulation in mitogen-activated lymphocytes: role of membrane-bound plasma lipoproteins.
Low density lipoproteins (LDL) in which apoB is the only protein constituent inhibit lymphocyte activation induced by the mitogen phytohemagglutinin. LDL suppress primary induction events, most notably enhanced Ca2+ accumulation, which occur as a result of mitogen-triggered alterations of the lymphocyte plasma membrane. Receptors in the lymphocyte membrane--designated "immunoregulatory receptors"--recognize and bind apoB-LDL. Under conditions of receptor saturation, about 22,500 +/- 4500 LDL particles are bound to the lymphocyte surface at 4 and 37 degrees C. The dissociation constant, Kd, is 1.76 X 10(-7) M and is also independent of temperature. Suppression of phytohemagglutinin-induced 45Ca2+ accumulation by LDL is a receptor-dependent process: inhibition is contingent on attachment of LDL to the immunoregulatory receptors. This conclusion is supported by three lines of evidence. First, heparin restores the susceptibility of the lymphocytes to mitogenic challenge by displacing membrane-bound LDL. Second, the immnoregulatory potency of LDL correlates directly with the amount of LDL bound at the cell surface. Third, LDL-Sepharose complexes, which cannot be internalized by the cells, are as immunosuppressive as soluble LDL. This demonstrates that membrane-bound plasma lipoproteins influence the behavior of intact cells.
Calcium↗
Mitogen-stimulated calcium ion accumulation by lymphocytes. Influence of plasma lipoproteins.
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Biological Transport, Active↗
Interaction of plasma lipoproteins with erythrocytes. I. Alteration of erythrocyte morphology.
Intact erythrocytes incubated in the presence of low density lipoproteins (LDL) undergo a time-dependent morphologic transformation from biconcave discs to spherocytes within 4 h. No shape change is observed when erythrocytes are incubated with high density lipoproteins (HDL). The LDL-induced change in erythrocyte morphology occurs without concomitant leakage of hemoglobin from the cell or depletion of intracellular ATP; no change in the distribution of the major lipids of the erythrocyte membranes was detected. The alteration of morphology does require attachment of LDL to the erythrocyte surface. The LDL-induced morphologic alteration is inhibited by HDL, but not by serum albumin. HDL prevent the attachment of LDL to the cell membrane; however, the HDL subfractions, HDL2 and HDL3, are only partially effective. These data suggest that normal erythrocyte morphology and cell function may depend on the concentration and composition of the circulating lipoproteins.
Adenosine Triphosphate↗
Interaction of plasma lipoproteins with erythrocytes. II. Modulation of membrane-associated enzymes.
When incubated with intact erythrocytes, low density lipoproteins (LDL) decrease the phosphate content of erythrocyte spectrin allowing the cells to undergo morphological transformation. The phosphate content of spectrin depends on the balance between the activity of membrane-associated cyclic AMP-independent protein kinases and phosphoprotein phosphates. LDL do not influence the activity of membrane-associated cyclic AMP-independent protein kinases; these lipoproteins activate by 2-fold and greater membrane-associated phosphatases as determined by hydrolysis of p-nitrophenyl phosphate and by phosphate hydrolysis of phosphorylated erythrocyte membrane proteins. We conclude that LDL interact at the exterior surface of the erythrocyte to stimulate dephosphorylation of spectrin. The significance of this conclusion is augmented by the fact that spectrin, the target for LDL-induced dephosphorylation, specifies cell morphology and modulates the distribution of cell-surface receptors. LDL also render erythrocyte acetylcholinesterase less susceptible to inhition by F-. Lipoproteins in the high density class (HDL) do not stimulate dephosphorylation of spectrin, and they are consequently unable to alter erythrocyte morphology. HDL do prevent the LDL-induced activation of membrane phosphatase. The inhibitory capacity of HDL is observed over the range of LDL:HDL (w/w) which exists in the plasma of normolipemic humans.
Acetylcholinesterase↗
Erythrocyte spectrin alteration induced by low-density lipoprotein.
Addition of human plasma low-density lipoproteins (LDL) to intact human erythrocytes induces the erythrocytes to undergo morphologic transition from biconcave disks to echinocytes and spherocytes. The transformation is time-dependent. Two hours are required before echinocytes are detected by scanning electron microscopy. After two hours, LDL also decrease the phosphate content of spectrin by 40% relative to the control, suggesting that these lipoproteins modulate cell shape by influencing phosphorylation-dephosphorylation of a membrane-associated cytoskeletal protein. LDL do not induce depletion of intracellular adenosine triphosphate (ATP), nor do they inhibit cyclic adenosine monophosphate-independent protein kinases which phosphorylate spectrin. LDL stimulate membrane-bound phosphatases by a factor of two, thereby reducing the amount of phosphate covalently bound to membrane proteins. The observed effects are specific for LDL. High-density lipoproteins (HDL) do not stimulate dephosphorylation of spectrin or alter erythrocyte morphology. However, HDL protect the erythrocytes against LDL-induced alterations. These data suggest that the circulating lipoproteins have a role in maintaining erythrocyte morphology by regulating the extent of phosphorylation of spectrin.
Erythrocyte Membrane↗