Impact of isoform and Cys-thiol modifications of apolipoprotein E on the LRP1-mediated metabolism of triglyceride-rich lipoproteins

Background: The low-density lipoprotein (LDL) receptor-related protein (LRP)1 participates in the metabolism of apolipoprotein (apo) E-containing lipoproteins; however, the details of its function have not been fully elucidated. Methods We investigated the effects of the isoform and modifications of the cysteine (Cys)-thiol of apoE on LRP1-mediated metabolism using a cell-based assay for the interaction between apoE-containing fluorescence-labeled emulsion particles (apoE-F-EP) and human fibroblasts expressing the LRP1 and lacking the LDL receptor. Results Among the three isoforms, apoE3-F-EP were most effectively bound to LRP1 and were catabolized. ApoE2-F-EP exhibited the lowest affinity to LRP1 but were significantly catabolized, whereas apoE4-F-EP were sufficiently bound to LRP1 but showed the lowest catabolic capability. Redox modifications of Cys112-thiol and Cys158-thiol had an antagonistic effect on the LRP1-mediated interaction of apoE-F-EP. The Tris (2-carboxyethyl) phosphine-reduction enhanced the binding and suppressed the catabolism of apoE3-F-EP, but had no effect on apoE2-F-EP. Interestingly, the formation of disulfide-linked complexes with apoAII suppressed binding, but enhanced the catabolism of apoE2-F-EP. Conclusions Redox modifications of apoE-Cys-thiol may modulate the LRP1-mediated metabolism of apoE2 or apoE3 containing lipoproteins, whereas apoE4, which has no Cys, essentially lacks this function. The failure or deficiency of this regulatory function may be a critical trigger for the development of dyslipidemia and related atherosclerosis.