OxLDL stimulates lipoprotein-associated phospholipase A2 expression in THP-1 monocytes via PI3K and p38 MAPK pathways

Lipoprotein-associated phospholipase A(2) (lp-PLA(2)) has been detected in human and rabbit atherosclerotic lesions, where it co-localizes with its substrate, oxidized LDL (oxLDL). Here, we investigated whether oxLDL may exert a regulatory effect on lp-PLA(2) expression. Using human monocytic THP-1 cells as a model system, we found that oxLDL up-regulated the expression of lp-PLA(2) while another substrate of the enzyme, platelet activating factor, had no such effect. The up-regulatory effect of oxLDL could be conferred by its oxidized phospholipids (oxPCs, the exact substrates of lp-PLA(2)), but not their hydrolyzed products, lysophosphatidylcholines (lysoPCs). OxLDL induced the activation of p38 mitogen-activating protein kinase (MAPK) through phosphatidylinositol 3-kinase (PI3K). Inhibition of either PI3K or p38 MAPK completely blocked oxLDL-induced lp-PLA(2) expression. In addition, inhibition of lp-PLA(2) activity in the conditioned medium significantly decreased lipid accumulation in macrophages as detected by oil red staining. The present study shows that oxLDL, and more specifically its unhydrolyzed oxidized phospholipids, can up-regulate lp-PLA(2) expression in monocytes through the PI3K and p38 MAPK pathway. In turn, lp-PLA(2) promotes lipoprotein uptake in macrophages. Our results uncover a new link between oxLDL and lp-PLA(2), and may provide insight into this interaction in the context of atherosclerosis.