Review VOLUME 9 | ISSUE 4 | OCTOBER - DECEMBER 2018 High density lipoprotein: the role of apolipoprotein A2 Eva Xepapadaki§, Christina Kalogeropoulou§, Evangelia Zvintzou§, Serafoula Filou and Kyriakos E. Kypreos* 1University of Patras, School of Medicine, Department of Pharmacology, Rio Achaias, TK. 26500, Greece; §Equal contribution to this work Abstract Atherosclerosis is a multistep process that progresses over a long period of time and displays a broad range of severity. In its final form, it manifests as a lesion of the intimal layer of the arterial wall. Multiple epidemiological and clinical studies in the past suggested that reduced HDL cholesterol (HDL-C) levels may correlate with increased risk for atherosclerosis. More recent data indicated that high density lipo- protein (HDL) particle functionality rather than HDL-C levels is a much more important parameter for human health and disease. Recent data from clinical paradigms and studies in mice support the interest- ing hypothesis that variations in HDL proteome may set the basis for its functionality. Apolipoprotein A2 (APOA2) is the second most abundant protein of HDL and plays a crucial role in HDL particle syn- thesis. Studies in mice suggested a proatherogenic role for APOA2 though studies in humans failed thus far to establish a clear role for APOA2 in atherosclerosis. Interestingly, though APOA2 increases HDL-C levels, the effects of this protein on HDL functionality are not adequately investigated. Understanding how APOA2 affects HDL and the lipoprotein transport system may provide another important piece in the puzzle of the mechanisms linking plasma lipoproteins with atherosclerosis. Key words: High density lipoprotein; Atherosclerosis; Apolipoprotein A2 SUBMISSION: 25-1-2019 | ACCEPTANCE: 07-3-2019 Xepapadaki E, Kalogeropoulou C, Zvintzou E, Filou S and Kypreos KE. High density lipoprotein: the Citation role of apolipoprotein A2. Hell J Atheroscler 2018; 9: 128-137 *Address correspondence at University of Patras Medical School, Department of Medicine, Pharmacology laboratory, Panepistimioupolis, Rio, TK. 26500, Greece, Tel: +302610969120, Fax: +302610996103, email: [email protected], web: http://www.kyriakoskypreos.com 128 © 2018 Hellenic Atherosclerosis Society E. Xepapadaki et al. High density lipoprotein: the role of apolipoprotein A2 igh density lipoprotein (HDL) has been bly of proteins and lipids synthesized in the cir- Hfor years an intriguing lipoprotein that at- culation and the main lipid cargo of mature HDL tracted the attention of biomedical community, particles is esterified cholesterol. However, other mainly because of its important role in atheropro- lipids (phospholipids, sphingolipids, ceramides tection [1]. In particular, the inverse correlation etc) are also part of HDL lipidome [18]. Current between HDL cholesterol (HDL-C) levels and the studies indicate that HDL is rather a mixture of risk for developing coronary heart disease (CHD) lipoprotein particles with densities in the range [2-7] suggested that high HDL-C levels in plas- of 1.063 to 1.21 g/ml and depending on their lip- ma may be protective against the development id composition these particles may assume a dis- of atherosclerosis while lower than physiological coidal or spherical geometry. Proteomic analyses levels of HDL-C may constitute a risk factor for revealed that HDL proteome also includes more this disease. than 85 different proteins, identified in particles Based on these clinical observations, investi- isolated by different methods[1;5;7]. Moreover gational drugs aiming at raising HDL-C levels these studies supported that HDL associated pro- have been tested recently as an additional mode teins may be found on structurally distinct parti- of protection against atherosclerosis and CHD cles, that are differentially distributed across the morbidity and mortality. One such approach was HDL density spectrum [5]. inhibition of cholesterol-ester transfer protein Recent data from experimental mice and clini- (CETP)[8]. The first 3 CETP inhibitors, torcetrapib cal trials have indicated that HDL functionality, [9], dalcetrapib [10] and evacetrapib [11] showed as determined by its lipidome and proteome, is no clinical benefit or even increased mortality more important in atheroprotection than HDL-C (torcetrapib), supporting the notion that CETP levels alone [5;7]. may not be the proper pharmacological target. The main protein component of HDL is apoli- More recently anacetrapib, a fourth experimental poprotein A-I (APOA1) which plays a key role in drug of this category, showed some benefit in pa- the biogenesis and functions of HDL [19]. Studies tients under intense statin treatment [12] though in cell cultures and experimental mice showed it was not clear whether the benefit is a result of that biogenesis of classical APOA1-containing increased HDL-C levels or reduced non-HDL-C HDL particles (APOA1-HDL) involves the lipid levels. In any case, the failure of high-dose nia- transporters ATP-binding cassette A1 (ABCA1) cin, another HDL raising drug, to reduce the risk and G1 (ABCG1) and the plasma enzyme Leci- for cardiovascular events (AIM-HIGH [13] and thin:Cholesterol Acyl Transferase (LCAT) [20- HPS2-THRIVE [14] clinical trials), is additional 23]. However, studies in mice showed that, other evidence that simply raising HDL-C in plasma is apolipoproteins such as apolipoprotein E (APOE) not an effective strategy for the prevention and [24], apolipoprotein A-II (APOA2)[25], apoli- treatment of CHD as once thought. These results, poprotein C-III (APOC3) [26] and possibly oth- along with Mendelian randomization studies fail- er small exchangeable apolipoproteins are also ing to demonstrate a causative relationship be- capable of promoting the de novo biogenesis of tween HDL-C and cardiovascular diseases [15], HDL in the absence of a functional APOA1. and more recent epidemiological data demon- In addition to the studies in mice, we recent- strating a U-shape correlation between all-cause ly observed the existence of APOE-containing mortality and HDL-C levels further supported HDL (APOE-HDL) and APOC3-containing HDL that excessive increase in HDL-C may be detri- (APOC3-HDL) particles in the plasma of mor- mental to human health [16;17]. bidly obese human subjects [18]. Analysis of Even though HDL is usually referred to as the their HDL particle composition showed that rap- “good cholesterol”, it is actually more than just id weight loss was associated with a significant a “cholesterol”. HDL is a macromolecular assem- switch from primarily APOE-HDL and APOC3- © 2018 Hellenic Atherosclerosis Society 129 High density lipoprotein: the role of apolipoprotein A2 E. Xepapadaki et al. HDL to primarily APOA1-HDL [18] alongside Apolipoprotein A2 in HDL and beyond with a significant improvement of the antioxi- APOA2, the second most abundant protein of dant properties of HDL [18]. In another clinical HDL [35], is a 77 aminoacid amphipathic glyco- paradigm we observed that young asymptom- protein [5;19] which plays a crucial role in HDL atic subjects (≤35 years of age) who suffered an particle synthesis, structure, functions and plas- acute non-fatal myocardial infarction had ele- ma concentration [36] and is synthesized mainly vated plasma APOE-HDL and APOC3-HDL that by the liver and to a much lesser extent by the correlated with reduced antioxidant potential intestine [37;38]. The interactions of APOA2 with [27]. These clinical observations supported the APOA1, APOE and other proteins of the HDL interesting hypothesis that variations in HDL metabolic pathway may affect subpopulation proteome may set the basis for its functionality. distribution and functionality of HDL. In vitro To test this hypothesis, we turned back to experi- studies suggested that ΑPOA2 forms dimers with mental mice where we selectively produced HDL APOE and it was proposed that this interaction of different apolipoprotein composition using may affect the ability of APOE to associate with adenovirus-mediated gene transfer of APOA1, or HDL particles [39]. APOE or APOC3. Indeed, our findings support Transgenic mice, overexpressing human the notion that APOA1-HDL is functionally dis- APOA2, had abnormal lipoprotein composi- tinct from APOE-HDL and APOC3-HDL [18;28]. tion, increased HDL-C levels and were prone to The apparent differences in HDL apolipopro- atherosclerosis (40;41). Specifically, increased tein content, lipidome and functionality between APOA2 levels affected the size and subpopula- APOE3-HDL, APOC3-HDL and APOA1-HDL, tion distribution of HDL particles [25;42] as well identified by our preclinical and clinical studies, as the ratio of APOA1/APOA2 in HDL. These reinforce our theory that not all HDL particles are mice had smaller, predominantly APOA2-con- equally active and that HDL proteome dictates its taining HDL (APOA2-HDL), while APOA1-HDL lipidome and subsequently its functionality [18]. was significantly reduced [25;43;44] and had a Along the same line, genetic control of the mouse decreased proportion of esterified cholesterol to HDL proteome defines HDL traits, function, and total cholesterol, due to reduced LCAT activity heterogeneity [29]. [43;44]. In addition, in transgenic mice express- Structural and functional changes of HDL have ing human ΑPOA2, very low density lipoproteins also been reported in the chronic inflammatory (VLDL) contained significant levels of APOA2 process of atherosclerosis [30;31]. In pathological and the activities of both lipoprotein lipase (LpL) states such as