Background: Elevated plasma Lp-PLA2 (lipoprotein-associated phospholipase A2) activity is closely associated with an increased risk of cardiovascular events. However, whether and how Lp-PLA2 is directly involved in the pathogenesis of atherosclerosis is still unclear. To examine the hypothesis that Lp-PLA2 could be a potential preventative target of atherosclerosis, we generated Lp-PLA2 knockout rabbits and investigated the pathophysiological functions of Lp-PLA2.
Methods: Lp-PLA2 knockout rabbits were generated using CRISPR/Cas9 system to assess the role of Lp-PLA2 in plasma lipids regulation and identify its underlying molecular mechanisms. Homozygous knockout rabbits along with wild-type rabbits were fed a cholesterol-rich diet for up to 14 weeks and their atherosclerotic lesions were compared. Moreover, the effects of Lp-PLA2 deficiency on the key cellular behaviors in atherosclerosis were assessed in vitro.
Results: When rabbits were fed a standard diet, Lp-PLA2 deficiency led to a significant reduction in plasma lipids. The decreased protein levels of SREBP2 (sterol regulatory element-binding protein 2) and HMGCR (3-hydroxy-3-methylglutaryl coenzyme A reductase) in livers of homozygous knockout rabbits indicated that the cholesterol biosynthetic pathway was impaired with Lp-PLA2 deficiency. In vitro experiments further demonstrated that intracellular Lp-PLA2 efficiently enhanced SREBP2-related cholesterol biosynthesis signaling independently of INSIGs (insulin-induced genes). When fed a cholesterol-rich diet, homozygous knockout rabbits exhibited consistently lower level of hypercholesterolemia, and their aortic atherosclerosis lesions were significantly reduced by 60.2% compared with those of wild-type rabbits. The lesions of homozygous knockout rabbits were characterized by reduced macrophages and the expression of inflammatory cytokines. Macrophages of homozygous knockout rabbits were insensitive to M1 polarization and showed reduced DiI-labeled lipoprotein uptake capacity compared with wild-type macrophages. Lp-PLA2 deficiency also inhibited the adhesion between monocytes and endothelial cells.
Conclusions: These results demonstrate that Lp-PLA2 plays a causal role in regulating blood lipid homeostasis and Lp-PLA2 deficiency protects against dietary cholesterol-induced atherosclerosis in rabbits. Lp-PLA2 could be a potential target for the prevention of atherosclerosis.
Keywords: atherosclerosis; cardiovascular diseases; inflammation; lipoproteins; rabbits.