Title : PCSK9’s role in immunomodulation
Abstract:
Atherosclerosis, the leading cause of cardiovascular diseases, is driven by high blood cholesterol levels and chronic inflammation. The disruption of the hepatic interaction between Proprotein Convertase Subtilisin/Kexin 9 (PCSK9) and Low-Density Lipoprotein Receptor (LDLR) downregulates blood cholesterol levels and reduces cardiovascular events. Recent data suggest that other members of the LDLR superfamily may be targets of PCSK9. In this presentation, I will show that LDLR-related protein 5 (LRP5) is a PCSK9 target, and both proteins participate in foam cell formation and hence, in the mechanism of lipid accumulation and atherosclerotic plaque formation. I will first show that LRP5 is needed for macrophage lipid uptake since LRP5-silenced macrophages have less intracellular cholesterol accumulation. Immunoprecipitation experiments will show that LRP5 forms a complex with PCSK9 in lipid-loaded macrophages opening the possibility that PCSK9 induces lysosomal LRP5 degradation in a similar manner than it does with LDLR. We have also studied the role of PCSK9 and LRP5 in the inflammatory response by TLR4/NFkB signaling pathway and show that PCSK9 gene interference decreases inflammation supporting a role for PCSK9 as an inflammatory mediator in atherosclerosis. We then validated our results in an in vivo mice model. We analyzed the differential expression of cholesterol-related genes and proteins including LRP5, PCSK9, VLDLR, LRP6, LRP2 and LRP1 in wildtype (Wt) and LRP5 knock-out (Lrp5-/-) mice fed a normocholesterolemic (NC) or a hypercholesterolemic (HC) diet. Lipid uptake was studied in liver resident cells (HepG2) and in liver fat-storing cells (hepatic stellate cells) with and without LRP5 and PCSK9. Results show that cholesterol accumulates in the livers of Wt and Lrp5-/- mice. This accumulation can be explained by the increased expression of LRP receptors in HC Wt mice or scavenger receptors in HC Lrp5-/- mice. More importantly, PCSK9 and LRP5 bind intracellularly in fat-storing liver cells but not in structural liver cells and both proteins are needed for lipid uptake.
