High Density Lipoprotein‐Lipopolysaccharide Binding Lowers HDL's Basal Antioxidant Capacity
Authors: Fausnacht Dane W , Boutagy Nabil E , McMillan Ryan P , Hulver Matthew W 
Published: November 2023

Clinically, high density lipoprotein (HDL) is known to be a potent negative risk factor of cardiovascular disease due to its ability to stimulate anti‐atherosclerotic, anti‐inflammation and antioxidant pathways. It has been established that HDL has a role in the innate immune system through its ability to sequester lipopolysaccharide (LPS) or lipoteichoic acid (LTA) in the blood. This sequestering prevents the endotoxins from binding to their respective toll‐like receptors and subsequently stimulating the transcription of pro‐inflammatory cytokines. Elevated HDL levels are seen to attenuate sepsis by rapidly removing LPS molecules from the blood. LPS molecules appear to bind to the apoproteins contained on the periphery of the HDL particles. The binding of LPS to apoproteins, specifically ApoA1, causes a conformational change to the apoproteins. These conformational changes may inhibit the apoprotein's abilities to interact with cell surface receptors. It is not known how the binding of LPS affects HDL's other beneficial roles. Along with its anti‐inflammatory role, HDL's ability to prevent the oxidation of LDL particles is seen as one of the major mechanisms in preventing atherosclerosis. ApoA1 is necessary for reverse cholesterol transport through interaction with the cell membrane transporter ABCA1 and is needed for HDL's inhibition of NADPH oxidase. ApoA1 and/or the other apoproteins contained on the surface of the HDL molecule may be mechanistically involved in HDL's anti‐oxidative effects, effects which may be inhibited by the presence of LPS. It is not yet known how LPS from chronic inflammation or metabolic endotoxemia will affect HDL function. The ability of LPS to inhibit HDLs interactions with human skeletal muscle cells and prevent reactive oxygen species (ROS) production was assessed. Skeletal muscle myoblasts were isolated from the vastus lateralis of human skeletal muscle tissue collected from young (22.9 years +/− 0.9) lean (Mean BMI 23.2+/− 0.6) donors. Cells were grown to ~ 80% confluence and differentiated for seven days. For 24 hours, differentiated muscle cells were incubated in a starvation media (serum free, low glucose DMEM (5.5mM) or a high glucose DMEM (25 mM) plus LPS (20 EU/ml)(from E. coli). Each of these conditions were tested with and without the presence of human derived HDL (50ug/ml). The rate of ROS production was measured using CM‐H2DCFDA (general oxidative stress indicator) kit. Oxidative markers, including 4‐hydroxynonenal (4‐HNE), were measured through immunocytochemistry florescence. HDL treatment, under starvation conditions in the absence of LPS, was effective at reducing both ROS production (−14.5%) and oxidative stress (4‐HNE, −12.6%). HDL, when added to the high glucose plus LPS condition, had no effect on ROS production when compared to the high glucose plus LPS condition without HDL. The addition HDL to the high glucose plus LPS treatment increased 4‐HNE content by 13.0% compared to treatment of high glucose and LPS without HDL. The anti‐oxidative capacity of HDL was not observed in the presence of LPS and elicited greater oxidative stress, possibly due to increased LPS‐HDL binding. This data suggest that the anti‐inflammatory effect of HDL may come at the cost of its other biological functions. It also suggests that chronic low‐grade endotoxemia may counter the clinical significance of HDL as a negative risk factor for cardiovascular and metabolic disease disease.

Support or Funding Information

American Diabetes Association (ADA‐07–12) and National Institutes of Health (1RO1DK078765)