Apolipoprotein E-containing lipoproteins and their extracellular interactions with LRP1 affect LPS-induced inflammation.

The linkage between low-density lipoprotein receptor-related protein (LRP)1-mediated metabolism of apolipoprotein (apo) E-containing lipoproteins (apoE-LP) and the lipopolysaccharide (LPS)-induced inflammatory response contributes to the pathogenesis of sepsis; however, the underlying mechanisms are unclear. Therefore, in this study, the effects of apoE-LP and their constituents on the mRNA expression of interleukin (IL)-6 and LRP1 were evaluated using a culture system of human fibroblasts supplemented with LPS and apoE-containing emulsion particles (apoE-EP). The affinity of apoE-LP for LPS was examined using the interaction between fluorescence-labeled LPS and serum lipoprotein fractions. LPS-induced inflammation significantly upregulated the mRNA expression of IL-6 and LRP1. This upregulation was markedly suppressed by pre-incubation of LPS with apoE-EP or its constituents (apoE or EP). The suppressive effect of apoE-EP on IL-6 upregulation was attenuated in the presence of lactoferrin, an inhibitor of LRP1. The prepared apoE-EP and serum triglyceride-rich lipoproteins showed significant affinity for LPS. However, these affinities appeared to be lower than expected based on the extent to which IL-6 upregulation was suppressed by pre-incubation of LPS with apoE-EP. Overall, these results indicate that LPS-induced inflammation may be regulated by 1) the LPS-neutralizing effect of apoE-LP, 2) anti-inflammatory effect of apoE, and 3) LRP1-mediated metabolic pathways.

Apolipoprotein E isoforms and their Cys-thiol modifications impact LRP1-mediated metabolism of triglyceride-rich lipoproteins.

The low-density lipoprotein (LDL) receptor-related protein (LRP)1 participates in the metabolism of apolipoprotein (apo) E-containing lipoproteins (apoE-LP). We investigated the effects of modifications of cysteine (Cys)-thiol of apoE on LRP1-mediated metabolism. Among the three isoforms, apoE2-LP exhibited the lowest affinity for LRP1 but was significantly catabolized, whereas apoE4-LP was sufficiently bound to LRP1 but showed the lowest catabolic capability. The reduction enhanced the binding and suppressed the catabolism of apoE3-LP, but had no effect on apoE2-LP. The formation of disulfide-linked complexes with apoAII suppressed binding, but enhanced the catabolism of apoE2-LP. Redox modifications of apoE-Cys-thiol may modulate the LRP1-mediated metabolism of apoE2- or apoE3-LP, but not apoE4-LP. The failure of this function may be involved in the pathophysiology of dyslipidemia.