apoA-II is an amphipathic protein synthesized and secreted by the liver and small intestine as an 82-amino acid proprotein that is proteolytically cleaved to the mature 77-amino acid protein

This article is available online at http://www.jlr.org are important for the effi ciency of the RCT pathway and the inverse relationship with the incidence of atherosclerosis. Plasma HDL comprises a heterogeneous assembly of different size particles, with variations in lipid and protein content ( 6, 7 ). Apolipoprotein (apo)A-I and apoA-II are the major protein constituents, comprising 70% and 20% of total HDL protein, respectively ( 8 ). ApoA-I has received the most attention to this point, and its structure-function relationship in preventing atherosclerosis is relatively well understood ( 7 ). In comparison, less attention has been paid to apoA-II. Human apoA-II is an amphipathic protein synthesized and secreted by the liver and small intestine as an 82-amino acid proprotein that is proteolytically cleaved to the mature 77-amino acid protein ( 9 ). The concentration of apoA-II in plasma is 31 ± 6 mg/dl, much lower than the apoA-I level (123 ± 28 mg/dl) ( 10 ). ApoA-II circulates as a 17.4 kDa homodimer formed by two mature apoA-II molecules linked by a disulphide bond at residue 6 ( 11 ). HDL heterogeneity originates at the point of biogenesis at which apoA-I and ABCA1 interact to create discoidal nascent particles; the relative available lipid/apoA-I ratio controls the size distribution ( 12 ). The heterogeneity of mature spherical HDL is also affected by remodeling by lipases and lipid transfer proteins in the plasma ( 6 ). It is important to understand HDL heterogeneity because different HDL subspecies exhibit different functionalities ( 13 ). The existence of LpA-I and LpA-I+A-II particles is another aspect of HDL heterogeneity that infl uences HDL functionality. For instance, LpA-I+A-II HDL is less effective than LpA-I HDL at promoting selective cholesteryl ester uptake via SR-B1 ( 14 ) and the transfer of cholesteryl ester by CETP ( 15 ). Also, compared with LpA-I particles, LpAI+A-II particles are less effective at promoting cholesterol esterifi cation via LCAT ( 16 ), although the abilities of the two types of HDL to promote cellular cholesterol effl ux are similar ( 17, 18 ). To better understand the origins of nascent HDL heterogeneity arising from the coexistence of LpA-I and LpAI+A-II particles, we examined the particles formed when Abstract It is important to understand HDL heterogeneity because various subspecies possess different functionalities. To understand the origins of HDL heterogeneity arising from the existence of particles containing only apoA-I (LpA-I) and particles containing both apoA-I and apoA-II (LpA-I+A-II), we compared the abilities of both proteins to promote ABCA1-mediated effl ux of cholesterol from HepG2 cells and form nascent HDL particles. When added separately, exogenous apoA-I and apoA-II were equally effective in promoting cholesterol effl ux, although the resultant LpA-I and LpA-II particles had different sizes. When apoA-I and apoA-II were mixed together at initial molar ratios ranging from 1:1 to 16:1 to generate nascent LpA-I+A-II HDL particles, the particle size distribution altered, and the two proteins were incorporated into the nascent HDL in proportion to their initial ratio. Both proteins formed nascent HDL particles with equal effi ciency, and the relative amounts of apoA-I and apoA-II incorporation were driven by mass action. The ratio of lipid-free apoA-I and apoA-II available at the surface of ABCA1-expressing cells is a major factor in determining the contents of these proteins in nascent HDL. Manipulation of this ratio provides a means of altering the relative distribution of LpA-I and LpA-I+A-II HDL particles. —Alexander, E. T., and M. C. Phillips. Infl uence of apolipoprotein A-I and apolipoprotein A-II availability on nascent HDL heterogeneity. J. Lipid Res. 2013. 54: 3464–3470.