High-density lipoproteins (HDL) play a critical role in lipid transport and vascular lipid homeostasis. HDL is heterogeneous, with particles varying in size, composition, and functionality. Proteomic studies have shown that HDL can host as many as 94 different proteins which can segregate into distinct subclasses. The two major apolipoproteins (apo)A-I and apoA-II account for 70% and 15-20%, respectively of HDL total protein. We hypothesized that the presence or absence of apoA-II can affect the binding of other proteins thus influencing HDL function. Immunoaffinity chromatography was used to isolate particles containing both apoA-I and apoA-II (LpA-I/LpA-II) and those with apoA-I with no apoA-II (LpA-I). These were isolated from both
i
) total HDL isolated by ultracentrifugation (UC) and
ii
) directly from plasma. The proteomic fingerprint of each population was determined by mass spectrometry. Within both populations, 54 total proteins were identified in UC isolated HDL compared to 98 proteins in particles isolated directly from plasma. In the UC isolated HDL, LpA-I/A-II particles contained 39 proteins compared to LpA-I particles which contained about 30. Interestingly, the opposite was true in particles isolated from plasma where upwards of 77 proteins on LpA-I/A-II particles were identified compared to LpA-I particles where 98 proteins were identified. Limited tryptic digestion experiments showed that LpA-I/A-II were more prone to proteolysis compared to LpA-I. Additionally, we found that LpA-I/A-II exhibited an increased capacity to efflux cholesterol from cultured macrophages (normalized by phospholipid content) suggesting a protein mediated difference in HDL functionality. Taken together, these results suggest the proteomic fingerprint of HDL subpopulations is heavily influenced by the method of isolation and that the proteomic pattern on HDL influences the physiological function of the particle.
Characterization of apolipoprotein A-IV complexes and A-IV isoforms in human lymph and plasma lipoproteins.
