Haptoglobin Binding to Apolipoprotein A-I Prevents Damage from Hydroxyl Radicals on Its Stimulatory Activity of the Enzyme Lecithin-Cholesterol Acyl-Transferase

Abstract

Apolipoprotein A-I (ApoA-I), a major component of HDL, binds haptoglobin, a plasma protein transporting to liver or macrophages free Hb for preventing hydroxyl radical production. This work aimed to assess whether haptoglobin protects ApoA-I against this radical. Human ApoA-I structure, as analyzed by electrophoresis and MS, was found severely altered by hydroxyl radicals in vitro. Lower alteration of ApoA-I was found when HDL was oxidized in the presence of haptoglobin. ApoA-I oxidation was limited also when the complex of haptoglobin with both high-density lipoprotein and Hb, immobilized on resin beads, was exposed to hydroxyl radicals. ApoA-I function to stimulate cholesterol esterification was assayed in vitro by using ApoA-I-containing liposomes. Decreased stimulation was observed when liposomes oxidized without haptoglobin were used. Conversely, after oxidative stress in the presence of haptoglobin (0.5 microM monomer), the liposome activity did not change. Plasma of carrageenan-treated mice was analyzed by ELISA for the levels of haptoglobin and ApoA-I, and used to isolate HDL for MS analysis. Hydroxyproline-containing fragments of ApoA-I were found associated with low levels of haptoglobin (18 microM monomer), whereas they were not detected when the haptoglobin level increased (34-70 microM monomer). Therefore haptoglobin, when circulating at enhanced levels with free Hb during the acute phase of inflammation, might protect ApoA-I structure and function against hydroxyl radicals.

Apolipoprotein A-I (ApoA-I) Mimetic Peptide P2a by Restoring Cholesterol Esterification Unmasks ApoA-I Anti-Inflammatory Endogenous Activity In Vivo

“…This binding, by decreasing the amount of free ApoA-I available for LCAT stimulation, impairs cholesterol esterification Salvatore et al, 2007Salvatore et al, , 2009Cigliano et al, 2009). The peptide P2a, which presents the ApoA-I amino acidic sequence overlapping the domain required for LCAT stimulation, displaces Hpt from ApoA-I and rescues the enzyme activity in vitro .…”

“…Haptoglobin binds ApoA-I, the main stimulator of LCAT, thus inhibiting the enzyme activity and the efficiency of the reverse cholesterol transport Salvatore et al, 2007;Cigliano et al, 2009). The plasma level of Hpt was measured in carrageenan-treated mice (Fig.…”

“…In several studies, we have shown that the acute-phase protein haptoglobin (Hpt) binds the HDL protein components ApoA-I and apolipoprotein E, impairing their key functions in RCT (Porta et al, 1999;Spagnuolo et al, 2005;Salvatore et al, 2007Salvatore et al, , 2009Cigliano et al, 2009). Haptoglobin is a polymorphic glycoprotein that exhibits phenotype prevalence in cardiovascular diseases (Delanghe et al, 1997).…”

“…This protein stimulates the efflux of cellular cholesterol and activates the enzyme LCAT, which in turn converts the free cholesterol into cholesteryl esters (CEs), addressing them to HDLs for transport into the circulation (Rader and Daugherty, 2008). The binding of Hpt to ApoA-I is associated with inhibition of LCAT activity and reduction of ApoA-Imediated delivery of cholesterol to hepatocytes leading to 1) poor cholesterol removal from peripheral cells and 2) a low level of HDL cholesterol in the circulation Salvatore et al, 2007;Cigliano et al, 2009). Moreover, several studies have confirmed that high levels of Hpt are associated with increased risk of developing cardiovascular events or myocardial infarction (Braeckman et al, 1999;De Bacquer et al, 2001;Matuszek et al, 2003).…”

Application of Acute Phase Proteins for Monitoring Inflammatory States in Cattle

“…Iron present in heme catalyzes the generation of reactive oxygen species (ROS; Sadrzadeh et al, 1984). Moreover, Hp binds to apo A-1 to protect it from free radical-mediated damage and also to prevent HDL from forming adducts with other lipoprotein molecules (Salvatore et al, 2007). Another function of Hp is to inhibit both cyclooxygenase (COX) and lipooxygenase (LOX) activities, which provide a means to modulate responses to inflammation or infection that may be harmful to tissues (Saeed et al, 2007).…”