Proprotein convertase subtilisin kexin type 9 (PCSK9) secreted by cultured smooth muscle cells reduces macrophages LDLR levels

Objective: Proprotein convertase subtilisin kexin type 9 (PCSK9) is an important regulator of hepatic low -density lipoprotein (LDL)-cholesterol levels. Although PCSK9 is mainly of hepatic origin, extrahepatic tissues significantly contribute to PCSK9 production and, potentially, local regulation of LDL receptor expression. Methods and results: In the present study w e show that, among vascular cells, PCSK9 is expressed in smooth muscle cells (SMCs) but not in endothelial cells, macrophages and monocytes. PCSK9 w as also detectable in human atherosclerotic plaques. Conditioned media from SMCs significantly reduced LDLR expression in human macrophage and in the macrophage cell line J774. Co-culture experiments also demonstrated the influence of SMCs on LDLR expression in J774. PCSK9 released from SMCs directly regulated LDLR expression in macrophages as demonstrated by retroviral overexpression or knockdow n of PCSK9 w ith small interfering RNA and by using recombinant PCSK9. Moreover, the proteolytic activity of PCSK9 w as not required for LDLR dow nregulation since cultured media containing either the catalytic inactive PCSK9 or PCSK9 WT had a similar effect on LDLR in J774. Finally, conditioned media from SMCs affected b-VLDL cholesterol uptake and PCSK9 expression reduced both LDLR and LDL uptake in J774. Conclusions: Taken together our data indicate that PCSK9 secreted by human SMCs is functionally active and capable of reducing LDLR expression in macrophages. A possible direct role for this protein in foam cell formation and atherogenesis is suggested.