Effect of proprotein convertase subtilisin kexin type 9 (PCSK9) on smooth muscle cell proliferation and migration

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. On these basis, we have previously shown that PCSK9 is expressed in cultured smooth muscle cells (SMCs) and it is detectable in human carotid atherosclerotic plaques. The aim of the present study was to investigate the role of PCSK9 on SMC proliferation, migration and differentiation. SMCs were isolated from PCSK9-/- mice (PCSK9null cells) and the expression of PCSK9 reconstituted by retroviral vector encoding for PCSK9-FLAG tag (PCSK9rec cells). The proliferation rate of PCSK9null cells and PCSK9rec was then determined by cell counting in response to 10%FCS to up of 6 days. The calculated doubling time of PCSK9null cells was significantly higher than that of PCSK9rec (41.2±1.9 h vs 32.2±3.1 h, respectively; p=0.01). Under these experimental conditions, PCSK9rec cells expressed lower levels of LDLR, determined by western blot analysis, while the mRNA levels were 10 fold higher. The reconstitution of PCSK9 expression in SMCs also determined a significant induction of the contractile phenotypic markers of SMCs, such as SM22 (15.5±4.4 fold p<0.01), -actin (3.1±1.1 fold; p=0.05) and KLF4 (2.9±0.5 fold p<0.01). The migratory capacity of both cell lines were also investigated by Boyden chamber chemotaxis assay in response to PDGF-BB (5÷20 ng/ml). Although the number of migrated cells were higher in PCSK9rec cells, either under basal or stimulated conditions, the entity of the chemotactic response (expressed as fold of induction vs basal condition) was not significantly different between the two cell lines. Taken together our data indicate that PCSK9 promotes the SMC proliferation and migration. A possible direct role for PCSK9 on SMC accumulation in atherogenesis is suggested.