ApoA-I is a member of blood apolipoproteins, which constitutes a major component of HDL (1). In addition to participating in cholesterol reversal, ApoA-I also has powerful anti-inflammatory functions (2). It is considered an anti-inflammatory protein, with levels reduced by at least 25% during acute inflammation (3). ApoA-I attenuates the inflammatory response through inhibit the production of TNF-a and IL-1 in rheumatoid arthritis, Crohn’s disease and other immune diseases (4). A negative correlation between ApoA-I levels and the severity of pancreatitis has also been found in pancreatitis (5–7). Furthermore, our previous study revealed a negative correlation between ApoA-I levels and disease severity in hypertriglyceridemic pancreatitis (8). In cases of sepsis, there is a negative correlation between ApoA-I and the severity of the condition (9). Additionally, the administration of ApoA-I mimetic peptide has been shown to improve survival rates in septic rats (10). The anti-inflammatory function of ApoA-I also plays an important role in the inhibition of atherosclerosis and anti-tumor growth (11). A negative correlation between reduced ApoA-I and disease severity was also found in COVID-19 infections (12, 13). However, the exact anti-inflammatory mechanism of ApoA-I is not well understood. In this article, we review the production, assembly and possible anti-inflammatory mechanism of ApoA-I.

The human ApoA-I gene is located in the 11q23 region of human chromosome 11,and is thought to be of the same genetic origin as Apolipoprotein A-II, Apolipoprotein A-IV, Apolipoprotein C-I, Apolipoprotein C-III, and Apolipoprotein E (14). The liver and intestine are the main sites of ApoA-I production in human tissues, but small amounts of ApoA-mRNA expression have been demonstrated in other organs, such as the pancreas and heart (15). The regulation of human ApoA-I gene expression is very complex and is controlled at multiple levels. Hepatocyte Nuclear Factor 4, Liver Receptor Homologue 1 and ApoA-I Regulatory Protein 1 are thought to be the major regulators of ApoA-1 initiation and repression (16). Translated ApoA-I is pruned intracellularly and then secreted as a lipid-poor protein or lipid-free protein ( Figure 1 ). ApoA-I gene mutations have been demonstrated to be associated with the development of numerous diseases, including those observed in diabetic patients. In particular, polymorphisms at the -75 bp locus of the ApoA-I gene have been linked to an increased risk of myocardial infarction in these patients (17). Individuals with the CC genotype of SNP rs5069 are more susceptible to oxidative imbalance and are at a higher risk of developing pancreatitis (18).

The primary structure of ApoA-I consists of 4 Tryptophan (Trp), 21 Lysine (Lys), 5Histidine (His), 16 Arginase (Arg), 16 Aspartic acid (Asp), 10 Threonine (Thr), 15 Serine (Ser), 27 Glutamic acid (Glu), 10 Proline(Pro), 10 Glycine (Gly), 19 Alanine (Ala), 13 Valine (Val), 3 Methionine (Met), 37 Leucine (Leu), 7 Tyrosine (Tyr), and 6 Phenylalanine (Phe), a total of 243 amino acid residues to form a single-chain peptide, which molecular weight is 28 KDa (19). This single-chain polypeptide may contain a plurality of 11 and 22 repeating amino acid residues, and these residues are typically separated by Pro residues. Among them, the 22 repeating amino acid residues can form the α-helix of ApoA-I, and the secondary structure of each ApoA-I contains about 8-9 tandem alpha helices (20). These α-helix play an important role in the biological function of ApoA-I (21–23). ApoA-I binds to the first extracellular domains(ECD1) of ATP-binding Cassette transport A1 (ABCA1), resulting in the transfer of cholesterol and cytosolic phospholipids from the cell membrane to ApoA-I, which then forms an initially lipidated ApoA-I (24). Initially lipidated ApoA-I can continue to form discoidal HDL in the presence of Dimyristoyl Phosphatidylcholine and egg phosphatidylcholine or palmitoyloleyl phosphatidylcholine (25). Studies of discoidal HDL mainly come from vitro models, as it is short-lived and difficult to isolate in plasma (26). Initially, it was believed that the binding of the α-helix to phospholipids in discoidal HDL was in the form of a spiked hedge fence. The α-helix consists of 22 repeating amino acids centered on a repeating proline residue crossing the edges of a bilayer parallel to the acyl chain (27). However, with research progressed, the ‘two-band’ model of discoidal HDL was considered the best possible model of α-helix binding to phospholipids. In this model, two cyclic ApoA-I molecules wrap a phospholipid bilayer in an antiparallel orientation. Discoidal HDL convers to spherical HDL by lecithin-cholesterol acyltransferase (LCAT) (28) ( Figure 1 ). The globular HDLsubpopulation can be classified according to size and density into five major subpopulations: HDL3c, HDL3b, HDL3a, HDL2a, and HDL2b. Each subpopulation is distinguished by its unique molecular composition and biological function (29). ApoA-I is floating in the phospholipid molecules and serves to regulate HDL diameter in globular HDL (30).

ApoA-I constitutes the primary constituent of HDL, representing approximately 70%,ApoA-II accounts for 15-20%, while the remaining proteins are amphiphilic (1). ApoA-I not only determines the size and shape of HDL but is also a functional protein, such as determining the composition of HDL soluble lipids, transporting cholesterol from peripheral cells, activating LCAT activity to convert circulating cholesterol to cholesteryl esters, and delivering cholesteryl esters to the liver or to steroidogenic tissues via cell surface receptors (2). Most of ApoA-I is found in the blood associated with lipoproteins, with only about 5-10% present in a non-lipoprotein-associated state. This non-lipidated ApoA-I can be obtained from HDL remodeling or triglyceride-rich lipoproteins, or secreted directly by the liver or intestines (31). ApoA-I in plasma is about 100-150 mg/dl,and its half-life is about 4 days (32, 33). Most lipidated ApoA-I is metabolized in the liver (34), Non-lipidated ApoA-I can pass through glomerular filtration and be degraded, excreted in urine, or partially reabsorbed (35) ( Figure 1 ).

Based on the robustness of ApoA-I and findings in the clinic, ApoA-I supplementation is considered a viable approach to treatment disease. Therefore, ApoA-I which can be used as a medicine has become the focus of attention. Such as ApoA-I mimetic peptides and ApoA-I based infusions.

Anantharamaiah et al. synthesized the first apoA-I mimetic peptide 18A in, 1985,which comprises 18 amino acids (81). Subsequently, the amino acid and α-helix structures of ApoA-I mimetic peptides have been modified to enhance their functionality (82, 83). To date, ApoA-I mimetic peptides in the pilot study phase are 4F,6F,FX-5A,ATI-5261 and ETC-642 (84). Among the many mimetic peptides, 4F has the most prominent anti-inflammatory effects. In vitro,4 F reduces IL-6 secretion in SARS-CoV-2 infected Vero-E6 and Calu3 cells (85), and it has an inhibitory effect on IL-4-induced selective activation of macrophages (43), 4F also inhibits Reactive Oxygen Species(ROS) production and ameliorates oxidative damage in endothelial cells (86). It has been shown in animal studies that ApoA-I mimetic peptides have the inflammation inhibition (87). In mice,4F inhibits the expression of interleukin-6(IL-6), interleukin-1β(IL-1β) and tumor necrosis factor-α (TNF-α) (88), it reduce macrophage infiltration in the liver (89). It also upregulates vascular endothelial growth factor protein expression to improve endothelial cell function in animal studies (90). An in vivo study in humans also showed that oral administration of apoA-I mimetic peptides 6F and 4F reduced plasma and intestinal tissue cytokines (TNF-a, IL-6) and chemokines (CX3CL1), and reduced systemic and intestinal inflammation in chronic treatment of HIV (91).

ApoA-I based infusions include HDL-VHDL infusion, purified ApoA-I infusion, ApoA-I_Milano infusion, CSL-111 and CSL-112 infusion,CER-001 infusion and modified ApoA-I (84). Of these,CSL-112 is considered the most viable infusion. CSL-112 consists of purified human ApoA-I and phosphatidylcholine, because it binds less phosphatidylcholine than CSL-111, it does not have the dose toxicity of CSL-111 (92). In previous studies, CSL-111 has been shown to have the effect of downregulating macrophages to reduce inflammation (93, 94). CSL112 also has shown anti-inflammatory effects ex vivo studies, including markers of monocyte chemotactic factor-1 and proinflammatory cytokines interleukin-1β (95).

ApoA-I goes through a series of complex processes from generation to assembly (96). Thus, both the expression of the ApoA-I gene and the changes in its α-helix structure are susceptible to interference by external environmental factors, and these changes may be associated with the development of the disease (23, 97). ApoA-I is the primary component of HDL, and the biological functions of HDL, including cholesterol regulation (98), inflammatory response modulation (99), and tumor growth modulation are believed to be closely related to ApoA-I (100), which is an effective performer of HDL’s biological functions (101). As described above, ApoA-I plays a pivotal role in the inhibition of the inflammatory response, and this is achieved through the inhibition of immune cell production and activity, the reduction of the recognition of immune cells by vascular endothelial cells, and the direct destructive effect on microorganisms and their products. Specific molecular regulatory mechanisms have focused on the study of immune cells. According to the literature review, most of the regulatory effects of ApoA-I on immune cell function are related to its regulation of cholesterol, in which the effect on lipid rafts is an important target for ApoA-I to regulate immune cells, but a clearer and more systematic mechanism is still not well described, and more and more in-depth studies are expected to be conducted in the future. The decrease of ApoA-I has been detected in many diseases (102–105), especially in those related to inflammation (4–13). However, the underlying cause of the decrease is not clear. Whether the reduction is due to a decrease in ApoA-I production or an increase in ApoA-I depletions is necessary to explore.

As previously described, several studies have shown that ApoA-I mimetic peptides have some anti-inflammatory effects. However, even with studies showing that ApoA-I mimetic peptide does not play a role in suppressing inflammation. In the aortic constriction rabbit model, ApoA-I mimetic infusions did not significantly improve echocardiographic parameters nor molecular markers of cardiac inflammation, oxidative stress and fibrosis (106). This includes previous trial in patients with coronary artery disease or at high risk for cardiovascular disease that showed conflicting results on the effect of oral and parenteral administration of 4F on HDL inflammation indices (107). The contradictory test results may be attributed to the fact that the functions of the simulated components are distinct (108, 109) and that the final evaluation criteria differ for each test. ApoA-I mimetic peptide is a synthetic substance that is similar to ApoA-I. However, its molecular structure differs from ApoA-I, which means that it cannot exert the same effect when it enters the body. Furthermore, the pharmacokinetics and pharmacokinetics of ApoA-I mimetic peptides in vivo require further comprehensive and in-depth investigation. Research on the anti-inflammatory effects of ApoA-I mimetic peptides have been focused on in vitro and in vivo studies, with few clinical trials in humans. The main factors limiting the development of these may be related to the contradictory results of ApoA-I mimetic peptide tests and concerns about their safety. The development of ApoA-I mimetic peptides that are closer to the molecular structure of ApoA-I and safer may require more difficult work. However, from the theoretical standpoint, the structure of CSL-112 is more closely aligned with ApoA-I mimetic peptide, which should result in a more robust anti-inflammatory effect and enhanced safety profile. But the majority studies of CSL-111/112 on immune cells come from atheromatous plaque formation, lack of extensively studied as the ApoA-I mimetic peptides in inflammation. And concerns have been raised about the efficacy of CSL-112 due to the findings of recent clinical studies, which indicated that patients with acute myocardial infarction treated with CSL-112 did not experience a significant reduction in major adverse cardiovascular events (110). We are looking forward to deeper study and bigger breakthroughs in the anti-inflammatory treatment of CSL-112.

The production and assembly of ApoA-I is influenced by multiple factors, and ensuring the structural integrity of ApoA-I is a prerequisite for its anti-inflammatory effects. ApoA-I can exert its anti-inflammatory effects through regulate immune cells, vascular endothelial cells, and direct interaction with microorganisms and their products. Several experiments have demonstrated that ApoA-I mimetic peptides and ApoA-I based infusions inhibit inflammatory responses. It is anticipated that safer and more efficacious ApoA-I drugs will emerge in the near future as research progresses.

XT: Writing – original draft. TR: Writing – original draft. KW: Conceptualization, Writing – review & editing. LW: Writing – review & editing.