Abstract 13327: Modulation of Proprotein Convertase Subtilisin/Kexin Type 9 Function via Interaction with Apolipoprotein B/Lipoprotein Particles

Abstract

Proprotein convertase subtilisin/kexin type 9 (PCSK9), is secreted from the liver and circulates in plasma where it modulates low density lipoprotein cholesterol (LDL-C) by mediating the degradation of the hepatic LDL receptor (LDLR). Human subjects with “loss of function” mutations in PCSK9 have low levels of plasma LDL-C and lower circulating levels of PCSK9 than normal subjects. However, there is no strong correlation between circulating PCSK9 and LDL-C levels in the general population. This may suggest that plasma PCSK9 levels do not accurately reflect the function of PCSK9 which could relate to tissue compartmental effects or to the existence of functional/non-functional pools of PCSK9. To investigate this, serum from healthy volunteers was subjected to fast performance liquid chromatography (FPLC) and the lipoprotein and PCSK9 distribution was determined. PCSK9 eluted with a distribution pattern that varied from subject to subject. The protein was detectable within the LDL and HDL peaks, assuming a much higher molecular weight than expected based on the monomeric molecular weight of 77 kDa. In vitro pull down studies showed that PCSK9 can interact with apolipoprotein B (apoB)/LDL both intra- and extra-cellularly. The interaction was stronger at pH 5.6 than at pH 7.4. PCSK9 over-expression in HepG2 cells increased ApoB secretion approximately 2-fold. Anti-PCSK9 antibodies directed against either the pro-domain or C-terminal-domain but not the catalytic domain, were able to disrupt the PCSK9/apoB/LDL interaction. LDL and VLDL, but not HDL reduced the binding of PCSK9 to the LDLR in vitro and inhibited the ability of PCSK9 to reduce cell LDLR expression. Together, these findings indicate that PCSK9 interacts with ApoB/LDL affecting the function of both proteins.