Studies on the role of specific cell surface receptors in the removal of lipoprotein (a) in man.

Abstract

A B S T R A C T The binding of '25I-lipoprotein (a)[Lp(a)] to cell surface receptors was studied on cultured human fibroblasts. The results were compared with corresponding data obtained with 1251-low density lipoproteins (LDL). Equilibrium binding studies showed that Lp(a) is bound with high affinity by the cell surface receptors. The maximum binding capacity for Lp(a) was 37% lower than for LDL. For Lp(a) and LDL, the Scatchard plots displayed linearity, indicating a single category of binding sites. Half-maximal saturation occurred at a concentration of 9.52±1.04 nM for Lp(a) and 7.76±1.29 nM for LDL. Competition binding experiments revealed that Lp(a) and LDL are nearly equally potent in competing each other for the binding sites. Binding of Lp(a) and LDL were followed by suppression of 3-hydroxy-3-methylglutaryl coenzyme A reductase activity. Cyclohexanedione treatment of Lp(a) and LDL completely abolished receptor binding. Neither Lp(a) nor LDL were specifically bound by fibroblasts obtained from a patient with homozygous familial hypercholesterolemia (FH).The removal mechanisms for Lp(a) and LDL were further compared by in vivo studies. Radioiodinated Lp(a) and LDL [1431][1432][1433][1434][1435][1436][1437][1438][1439][1440][1441] LDL are nearly identical (6-8). The hexose, hexosamin, and sialic acid content, however, are significantly higher in Lp(a) (9). The main protein constituent of both lipoproteins is apolipoprotein B. Lp(a) has an additional apoprotein that has been termed "specific Lp(a) antigen" (9-11). On agarose gel or cellulose acetate Lp(a) migrates somewhat faster than LDL and has therefore also been described as -pre-fl-lipoprotein" (3,4,12).In spite of these remarkable similarities between Lp(a) and LDL, Lp(a) is not a metabolic product of very low density lipoproteins (VLDL), LDL, or METHODS Binding studiesIsolation of Lp(a) and LDL. The details of the method for isolation of Lp(a) and LDL were described recently (14). Plasma was obtained by plasmapheresis from healthy volunteers who had high levels of Lp(a) as checked by doubleimmunodiffusion using monospecific anti-Lp(a) antibodies (14). The plasma was subjected to sequential ultracentrifugation to obtain density fractions from 1.006 to 1.055 g/ml and from 1.055 to 1.110 g/ml. The densities were adjusted by addition of solid NaCl and checked with a density meter (Anton Paar K. G., Graz). All centrifugal procedures were performed in a Beckman L8-70 centrifuge using a Ti 50.2 rotor (Beckman Instruments, Inc., Fullerton, CA). The supernatants, containing the lipoproteins of the d 1.006-1.055 g/ml and 1.055-1.110 g/ml, were collected by tube slicing. These fractions were concentrated to a volume of -5 ml by dialysis against polyethyleneglycol and then applied to an agarose column (Bio Gel A-Sm, Bio-Rad Laboratories, Richmond, CA). Elution of the lipoproteins was performed with 0.15 M NaCl, pH adjusted to 8.5 by addition of NH40H. During all steps of the isolation procedure, Na2EDTA and NaN3 were present in a concentration of 1 mg/ml...

Interaction of Lp(a) and of apo(a) with liver cells.

“…18 ' 23 - 37 Our findings, however, do not explain the increase of Lp(a) in some individuals treated with HMG-CoA reductase inhibitors. 24 Taken together, we deduce from our results that little Lp(a) may be catabolized in vivo directly by the LDL receptor.…”

“…Reports from many laboratories provide evidence that Lp(a) binds to the LDL receptor in vitro. 18 - 22 However, binding is significantly reduced in comparison to LDL. Few in vivo studies that address this question have been carried out.…”

High-Level Expression of Various Apolipoprotein (a) Isoforms by "Transferrinfection": The Role of Kringle IV Sequences in the Extracellular Association with Low-Density Lipoprotein

“…Understanding the biosynthesis of Lp(a) is therefore important and may provide the rationale for the development of drug intervention protocols to control Lp(a) levels in human plasma. We and others have demonstrated that Lp(a) plasma levels are determined by the rate of synthesis and not by the catabolic rate (Krempler et al, 1983;Rader et al, 1993). Recent work by others using transgenic mice Linton et al, 1994) and primary cultures of baboon liver cells suggested that Lp(a) assembly may occur in the plasma compartment (White et al, 1993).…”

Differential Influence of LDL Cholesterol and Triglycerides on Lipoprotein(a) Concentrations in Diabetic Patients

“…Although Lp(a) can be catabolized by the LDL receptor, the affinity of Lp(a) to the LDL receptor is considerably lower than that of LDL (32)(33)(34). Thus, it seems that this common catabolic pathway is not the reason for the association between LDL cholesterol and Lp(a).…”