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, extra-hepatic tissues significantly contribute to PCSK9 production and, potentially, local regulation of LDL receptor expression.
Methods and results: In the present study we show that, among vascular cells, PCSK9 is expressed in smooth muscle cells (SMCs) but not in endothelial cells, macrophages and monocytes. PCSK9 was 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 knockdown of PCSK9 with small interfering RNA and by using recombinant PCSK9. Moreover, the proteolytic activity of PCSK9 was not required for LDLR downregulation 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 β-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.
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