Evolutionary origin and sequence signatures of the heterodimeric ABCG5/ABCG8 transporter.

ATP-binding cassette (ABC) systems, characterized by ABC-type nucleotide-binding domains (NBDs), play crucial roles in various aspects of human physiology. Human ABCG5 and ABCG8 form a heterodimeric transporter that functions in the efflux of sterols. We used sequence similarity search, multiple sequence alignment, phylogenetic analysis, and structure comparison to study the evolutionary origin and sequence signatures of ABCG5 and ABCG8. Orthologs of ABCG5 and ABCG8, supported by phylogenetic analysis and signature residues, were identified in bilaterian animals, Filasterea, Fungi, and Amoebozoa. Such a phylogenetic distribution suggests that ABCG5 and ABCG8 could have originated in the last common ancestor of Amorphea (the unikonts), the eukaryotic group including Amoebozoa and Opisthokonta. ABCG5 and ABCG8 were missing in genomes of various lineages such as snakes, jawless vertebrates, non-vertebrate chordates, echinoderms, and basal metazoan groups. Amino-acid changes in key positions in ABCG8 Walker A motif and/or ABCG5 C-loop were observed in most tetrapod organisms, likely resulted in the loss of ATPase activity at one nucleotide-binding site. ABCG5 and ABCG8 in Ecdysozoa (such as insects) exhibit elevated evolutionary rates and accumulate various changes in their NBD functional motifs. Alignment inspection revealed several residue positions that show different amino-acid usages in ABCG5/ABCG8 compared to other ABCG subfamily proteins. These residues were mapped to the structural cores of transmembrane domains (TMDs), the NBD-TMD interface, and the interface between TMDs. They serve as sequence signatures to differentiate ABCG5/ABCG8 from other ABCG subfamily proteins, and some of them may contribute to substrate specificity of the ABCG5/ABCG8 transporter.

Genetic control of the mouse HDL proteome defines HDL traits, function, and heterogeneity.

HDLs are nanoparticles with more than 80 associated proteins, phospholipids, cholesterol, and cholesteryl esters. The potential inverse relation of HDL to coronary artery disease (CAD) and the effects of HDL on myriad other inflammatory conditions warrant a better understanding of the genetic basis of the HDL proteome. We conducted a comprehensive genetic analysis of the regulation of the proteome of HDL isolated from a panel of 100 diverse inbred strains of mice (the hybrid mouse diversity panel) and examined protein composition and efflux capacity to identify novel factors that affect the HDL proteome. Genetic analysis revealed widely varied HDL protein levels across the strains. Some of this variation was explained by local cis-acting regulation, termed cis-protein quantitative trait loci (QTLs). Variations in apoA-II and apoC-3 affected the abundance of multiple HDL proteins, indicating a coordinated regulation. We identified modules of covarying proteins and defined a protein-protein interaction network that describes the protein composition of the naturally occurring subspecies of HDL in mice. Sterol efflux capacity varied up to 3-fold across the strains, and HDL proteins displayed distinct correlation patterns with macrophage and ABCA1-specific cholesterol efflux capacity and cholesterol exchange, suggesting that subspecies of HDL participate in discrete functions. The baseline and stimulated sterol efflux capacity phenotypes were associated with distinct QTLs with smaller effect size, suggesting a multigenetic regulation. Our results highlight the complexity of HDL particles by revealing the high degree of heterogeneity and intercorrelation, some of which is associated with functional variation, and support the concept that HDL-cholesterol alone is not an accurate measure of HDL's properties, such as protection against CAD.

In vitro oxidized HDL and HDL from type 2 diabetes patients have reduced ability to efflux oxysterols from THP-1 macrophages.

Oxidized LDL (OxLDL) that are enriched in products of lipid peroxidation including oxysterols have been shown to induce cellular oxidative stress and cytotoxicity therefore accelerating atheroma plaque formation. Upon oxLDL exposure of THP-1 macrophages, intracellular oxidation of LDL derived-cholesterol as well as endogenous cholesterol was increased. The oxysterols intracellularly produced were efficiently exported to HDL whereas apolipoprotein A1 was inefficient. These findings prompted us to investigate the consequences of modification of HDL by oxidation and glycation as observed in type 2 diabetes with respect to oxysterol and cholesterol efflux. We show that efflux of oxysterols was significantly impaired after in vitro oxidation and glycoxidation of HDL whereas glycation alone had no impact. Cholesterol efflux was only slightly decreased by oxHDL or glycoxidized HDL and not changed with glycated HDL. The defect of HDL towards oxysterol efflux was also observed with HDL isolated from diabetic subjects as compared to healthy controls. These findings support a deleterious cellular retention of oxysterols due to dysfunctional HDL in type 2 diabetes.