Human High Density Lipoprotein Subclasses Research Articles

Human High Density Lipoproteins Are Platforms for the Assembly of Multi-component Innate Immune Complexes

Human innate immunity to non-pathogenic species of African trypanosomes is provided by human high density lipoprotein (HDL) particles. Here we show that native human HDLs containing haptoglobin-related protein (Hpr), apolipoprotein L-I (apoL-I) and apolipoprotein A-I (apoA-I) are the principle antimicrobial molecules providing protection from trypanosome infection. Other HDL subclasses containing either apoA-I and apoL-I or apoA-I and Hpr have reduced trypanolytic activity, whereas HDL subclasses lacking apoL-I and Hpr are non-toxic to trypanosomes. Highly purified, lipid-free Hpr and apoL-I were both toxic to Trypanosoma brucei brucei but with specific activities at least 500-fold less than those of native HDLs, suggesting that association of these apolipoproteins within the HDL particle was necessary for optimal cytotoxicity. These studies show that HDLs can serve as platforms for the assembly of multiple synergistic proteins and that these assemblies may play a critical role in the evolution of primate-specific innate immunity to trypanosome infection.

Discrimination between subclasses of human high-density lipoproteins by the HDL binding sites of bovine liver

The binding of human 125I-labeled HDL 3 (high-density lipoproteins, ρ 1.125–1.210 g/ cm 3) to a crude membrane fraction prepared from bovine liver closely fit the paradigm expected of a ligand binding to a single class of identical and independent sites, as demonstrated by computer-assisted binding analysis. The dissociation constant ( K d ), at both 37 and 4°C, was 2.9 μg protein/ml (approx. 2.9 · 10 −8 M); the capacity of the binding sites was 490 ng HDL 3 (approx. 4.9 pmol) per mg membrane protein at 37°C and 115 at 4°C. Human low-density lipoproteins (LDL) and very-low-density lipoproteins (VLDL) also bound to these sites ( K d = 41 μg protein/ml, approx. 6.7 · 10 −8 M for LDL, and K d = 5.7 μg protein/ml, approx. 7.0 · 10 −9 M for VLDL), but this observation must be considered in light of the fact that the normal circulating concentrations of these lipoproteins are much lower than those of HDL. The binding of 125I-labeled HDL 3 to these sites was inhibited only slightly by 1 M NaCl, suggesting the presence of primarily hydrophobic interactions at the recognition site. The binding was not dependent on divalent cations and was not displaceable by heparin; the binding sites were sensitive to both trypsin and pronase. Of exceptional note was the finding that various subclasses of human HDL (including subclasses of immunoaffinity-isolated HDL) displaced 125I-Iabeled HDL 3 from the hepatic HDL binding sites with different apparent affinities, indicating that these sites are capable of recognizing highly specific structural features of ligands. In particular, apolipoprotein A-I-containing lipoproteins with prebeta electrophoretic mobility bound to these sites with a strikingly lower affinity ( K d = 130 μg protein/ml) than did the other subclasses of HDL.

Comparative study of the lipid dynamics in the surface layer of porcine and human high density lipoprotein subclasses by spin labeling

In order to obtain information on the determinants of the lipid dynamics in the high density lipoproteins (HDL), we have compared the structural properties of human HDL subclasses with porcine HDL artificially subdivided into density subfractions corresponding to those of human HDL. Four different positional isomers of spin labeled fatty acids and spin labeled androstanol experienced more restricted motion in porcine HDL than in the human HDL 2 and HDL 3 subclasses. The differences in the spin label motion could not be accounted for on the basis of the differences in the chemical composition of the lipoproteins examined. They are, however, most probably due to the specific properties of the interactions between lipids and proteins that differ among the lipoproteins.

Factors affecting the exchange of apoproteins between human high density lipoprotein subclasses in vitro

We have presented evidence that apoproteins may exchange, in vitro, between all HDL subclasses tested, including HDL 2b, HDL 2a, HDL 3, and HDL 4. This exchange process is influenced by various factors including the concentrations of the subclasses and the presence of added apoprotein. This exchange process should be considered when designing experiments using HDL subclasses in vitro, and the importance of exchange in the in vivo situation should be a subject of further investigation.

Interchange of apoprotein components between the human plasma high density lipoprotein subclasses HDL2 and HDL3 in vitro.

The major apoproteins of human high density lipoproteins (HDL) labeled with 125I have been shown to exchange between the two major HDL subclasses HDL2 and HDL3 in vitro. This bidirectional exchange process is inhibited by cross-linking with bifunctional reagents and is apparently dependent upon the formation of collision complexes. This exchange has been demonstrated both when the subclasses of HDL are free in solution and also when one of them is covalently bound to Sepharose. Using system involving Sepharose-bound HDL, it could be shown that not only free apoprotein molecules but subunits consisting of lipid-apoprotein combinations were exchanged between HDL2 and HDL3. The rate of exchange in these processes is significant in the lifetime of the protein particles in vivo equalling approximately 2.5% per h for apoprotein exchange. These experiments suggest that there is a dynamic relationship between HDL2 and HDL3 even though each of them exists alone in vitro as stable separate entities; when they are placed together in solution significant interaction occurs between the particles. Apoprotein exchange occurs between HDL2:HDL2 and HDL3:HDL3 as well as between HDL2 and HDL3 molecules. These data also suggest that the interconversion of HDL2 and HDL3 may be affected by the availability of lipids.