Abstract 430: PCPE2 a Novel Determinant in HDL Biogenesis and Metabolism

Recent attempts to reduce cardiovascular events by pharmacologically raising plasma HDL concentration have not proven efficacious. This has lead to investigations on how to increase functional versus dysfunctional plasma HDL. Our work shows that a functional subfraction from ABCA1 mediated biogenesis of nascent HDL (nHDL) consists of 3 apoA-I, 105 phosphotidylcholine, 27 sphingomyelin and 108 free cholesterol molecules, compositionally reminiscent of lipid rafts. Our studies are now focused on procollagen C-proteinase enhancer 2 (PCPE2), first described by Francone, OL, et al. (2011), to play a significant role in the ABCA1 mediated efflux of cholesterol to apoA-I. Based on previous studies, we hypothesize that PCPE2 binds lipid-free apoA-I thereby aligning specific helices among 3 molecules of apoA-I for lipidation. To test this, we created loss-of-function apoA-I mutant proteins to identify specific helical repeats that participated in nHDL formation. The various mutations span the central helices as well as the N- and C- termini of apoA-I. Opposing cysteines were placed within 3-5 å using a composite 3-D model of lipid-free apoA-I conformation. The mutant apoA-I disulfide “locked” (oxidized) and “unlocked” (reduced) status was monitored using mass spectrometric sequencing. Using both locked and unlocked versions of each of the mutant proteins, secondary structure as well as the extent of lipidation was investigated. In addition, studies, utilizing LDL receptor, PCPE2 double knockout mice (LDLr -/- , PCPE2 -/- ) fed an atherogenic diet have been carried out. Despite having a higher HDL concentration LDLr -/- , PCPE2 -/- mice had significantly more aortic lipid deposition compared to LDLr -/- mice. The absence of PCPE2 substantially reduced the ability to efflux cholesterol suggesting that the HDL was dysfunctional. These data strongly suggest that PCPE2 participates in lipidation by helping apoA-I to either open or target a site on ABCA1 that promotes apoA-I opening.