Apolipoprotein E recycling: implications for dyslipidemia and atherosclerosis

Enhanced apoB48 metabolism in lipoprotein lipase X447 homozygotes

RATIONALE

Lipoprotein lipase (LPL) X447 homozygotes are characterized by enhanced conversion of TRL apoB100. Here, we set out to investigate whether this LPL variant is also associated with enhanced apoB48 clearance. Therefore, we evaluated apoB48 kinetics in X447 homozygotes in the fed state by infusion of isotope L-[1-(13)C]-valine and subsequent compartmental modeling.

METHODS AND RESULTS

ApoB48 metabolism was assessed in five X447 homozygotes (X/X genotype) and five S447 homozygotes (S/S genotype). Subjects were continuously fed and received infusion of stable isotope L-[1-(13)C]-valine. Results were analyzed by SAAM II modeling. Fasting (2.4-fold, p=0.02) as well as non-fasting (1.6-fold, p=0.09) apoB48 concentration was increased in the X447 homozygotes compared to S447 homozygotes. In addition, the X447 homozygotes exhibited a 1.7-fold higher apoB48 poolsize (p=0.04). Interestingly, apoB48 fractional catabolic rate (FCR) was 1.9-fold higher (p=0.007) and apoB48 synthesis was more than two-fold higher (p=0.006) in the X447 homozygotes compared to S447 homozygotes.

CONCLUSION

In the present study, we show that X447 homozygotes exhibit enhanced apoB48 clearance. Previously, these homozygotes were shown to present with enhanced apoB100 TRL conversion. Combined, this LPLS447X gain of function variant affects apoB48 as well as apoB100 TRL metabolism.

Mechanisms of lipid-antigen generation and presentation to T cells

The presentation of lipid antigens by CD1 molecules follows precise rules imposed by the biochemical nature of lipids. The structures of CD1-lipid complexes are elucidating how T-cell receptors interact with hydrophobic antigens. The mechanism of lipid uptake and the pathways followed by lipids embedded in the cell membrane contribute to the efficient presentation of exogenous and self-lipids. Lipid presentation is further regulated by the trafficking route of CD1 proteins and their precise membrane localization within endosomal vesicles. Moreover, the generation of immunogenic lipids might require adequate processing, which occurs in the presence of lipid-binding proteins, including CD1e. Here, we review recent experimental evidence that has revealed new protagonists involved in generating immunogenic lipids and has indicated unexpected biological mechanisms contributing to immune recognition.