Lysine glycation of apolipoprotein A-I impairs its anti-inflammatory function in type 2 diabetes mellitus

Apolipoprotein A-I (apoA-I), the major protein compontent of high-density lipoprotein (HDL), exerts many anti-atherogenic functions. This study aimed to reveal whether nonenzymatic glycation of specific sites of apoA-I impaired its anti-inflammatory effects in type 2 diabetes mellitus (T2DM). LC-MS/MS was used to analyze the specific sites and the extent of apoA-I glycation either modified by glucose in vitro or isolated from T2DM patients. Cytokine release in THP-1 monocyte-derived macrophages was tested by ELISA. Activation of NF-kappa B pathway was detected by western blot. The binding affinity of apoA-I to THP-1 cells was measured using ¹²⁵I-labeled apoA-I. We identified seven specific lysine (Lys, K) residues of apoA-I (K12, K23, K40, K96, K106, K107 and K238) that were susceptible to be glycated either in vitro or in vivo. Glycation of apoA-I impaired its abilities to inhibit the release of TNF-α and IL-1β against lipopolysaccharide (LPS) in THP-1 cells. Besides, the glycation levels of these seven K sites in apoA-I were inversely correlated with its anti-inflammatory abilities. Furthermore, glycated apoA-I had a lower affinity to THP-1 cells than native apoA-I had. We generated mutant apoA-I (K107E, M-apoA-I) with a substitution of glutamic acid (Glu, E) for lysine at the 107th site, and found that compared to wild type apoA-I (WT-apoA-I), M-apoA-I decreased its anti-inflammatory effects in THP-1 cells. We also modeled the location of these seven K residues on apoA-I which allowed us to infer the conformational alteration of glycated apoA-I and HDL. In summary, glycation of these seven K residues altered the conformation of apoA-I and consequently impaired the protective effects of apoA-I, which may partly account for the increased risk of cardiovascular disease (CVD) in diabetic subjects.