Abstract
Apolipoprotein (apo) E plays a central role in the transport of lipids among different organs and cell types, whereas α2-macroglobulin (α2M) is responsible for the binding and inactivation of plasma proteases, as well as the transport of various cytokines, growth factors, and hormones. In the present study, evidence is presented for direct binding of apoE with α2M in human plasma, based on the observation that two-dimensional non-denaturing gradient gel electrophoretic separation of plasma resulted in co-migration of apoE with α2M in a complex intermediate in size (18.5 nm in diameter) between low (LDL) and high density lipoproteins (HDL). ApoE associated with α2M could be immunoprecipitated from plasma with anti-human α2M antiserum. Purified apoE, labeled with 125I, bound to native and methylamine-activated α2M (α2M-MA) in vitro in a time- and concentration-dependent manner. ApoE bound to α2M-MA with greater affinity than α2M. The binding of apoE to both α2M and α2M-MA did not depend on the presence of lipid. Ingestion of an oral fat load resulted in a reduction in the amount of apoE associated with α2M. Sphingomyelin vesicles and very low density lipoproteins (VLDL), but not phosphatidylcholine vesicles or HDL3, inhibited the in vitro binding of 125I-labeled apoE3 to α2M and α2M-MA. Binding of 125I-labeled apoE3 was also partially inhibited by an excess of platelet-derived growth factor and β-amyloid protein, but not interferon-γ. Subjects with an apoE 4/4 phenotype had less apoE associated with α2M in plasma than subjects with an apoE 3/3 or 2/2 phenotype, corresponding to reduced in vitro binding of apoE4 with α2M or α2M-MA. Although the functional significance of apoE binding to α2M remains to be determined, the present results demonstrate that: 1) apoE is non-covalently bound to α2M in human plasma, 2) α2M-MA has a greater capacity to bind apoE than α2M, 3) various proteins or lipoproteins known to bind apoE or α2M can potentially affect the interaction of apoE with α2M, and 4) association of apoE with α2M or α2M-MA is dependent on apoE phenotype. —Krimbou, L., M. Tremblay, J. Davignon, and J. S. Cohn. Association of apolipoprotein E with α2-macroglobulin in human plasma. J. Lipid Res. 1998. 39: 2373–2386.
Highlights
Apolipoprotein E plays a central role in the transport of lipids among different organs and cell types, whereas ␣2-macroglobulin (␣2M) is responsible for the binding and inactivation of plasma proteases, as well as the transport of various cytokines, growth factors, and hormones
Apolipoprotein E (34.2 kD) plays a central role in the transport of lipids among different organs and cell types [1]. It is produced in a variety of cells and is a component of a number of circulating plasma lipoproteins, including very low density lipoproteins (VLDL), intermediate density lipoproteins (IDL), and high density lipoproteins (HDL)
Separation of human plasma by two-dimensional nondenaturing gradient gel electrophoresis has previously been used in our laboratory to investigate apoE-containing lipoproteins in the HDL size range [14, 27]
Summary
Apolipoprotein (apo) E plays a central role in the transport of lipids among different organs and cell types, whereas ␣2-macroglobulin (␣2M) is responsible for the binding and inactivation of plasma proteases, as well as the transport of various cytokines, growth factors, and hormones. Apolipoprotein (apo) E (34.2 kD) plays a central role in the transport of lipids among different organs and cell types [1] It is produced in a variety of cells and is a component of a number of circulating plasma lipoproteins, including very low density lipoproteins (VLDL), intermediate density lipoproteins (IDL), and high density lipoproteins (HDL). ApoE is almost entirely associated with lipoproteins containing apoB or apoA-I [12], though several studies have demonstrated the existence of minor lipoprotein subfractions containing apoE as their only apoprotein component [13, 14] These lipoproteins are found to be intermediate in size between LDL and HDL, and have a diameter between 9 and 18.5 nm, as determined by two-dimensional non-denaturing polyacrylamide gradient gel electrophoresis [14]. It was the aim of the present study to provide both in vivo and in vitro evidence for the association of apoE with ␣2M, and to demonstrate how this interaction could be affected by native and activated forms of ␣2M, by various proteins or lipoproteins known to bind apoE or ␣2M, or by apoE isoform
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