Proprotein convertase 7 (PCSK7) reduces apoA-V levels

The locus of the human proprotein convertase subtilisin-kexin type-7 (PC7) gene (PCSK7) is on chromosome 11q23.3 close to the gene cluster APOA5/APOA4/APOC3/APOA1, a region implicated in the regulation of lipoprotein metabolism. A GWAS reported the association of PCSK7 SNPs with plasma triglyceride (TG), and exome sequencing of African Americans revealed the association of a low-frequency coding variant of PC7 (R504H; SNP rs142953140) with a ~ 30% TG reduction. Another PCSK7 SNP rs508487 is in linkage disequilibrium with a promoter variant of the liver-derived apolipoprotein A-V (apoA-V), an indirect activator of the lipoprotein lipase (LpL), and is associated with elevated TG levels. We thus hypothesized that PC7 regulates the levels/activity of apoA-V. Studies in the human hepatic cell line HuH7 revealed that wild-type (WT) PC7 and its endoplasmic reticulum (ER)-retained forms bind to and enhance the degradation of human apoA-V in acidic lysosomes in a nonenzymatic fashion. PC7-induced degradation of apoA-V is inhibited by bafilomycin A1 and the alkalinizing agents: chloroquine and NH₄ Cl. Thus, the PC7-induced apoA-V degradation implicates an ER-lysosomal communication inhibited by bafilomycin A1. In vitro, the natural R504H mutant enhances PC7 Ser⁵⁰⁵ phosphorylation at the structurally exposed Ser-X-Glu⁵⁰⁷ motif recognized by the secretory kinase Fam20C. Co-expression of the phosphomimetic PC7-S505E with apoA-V resulted in lower degradation compared to WT, suggesting that Ser⁵⁰⁵ phosphorylation of PC7 lowers TG levels via reduced apoA-V degradation. In agreement, in Pcsk7^-/- mice fed high-fat diet, plasma apoA-V levels and adipocyte LpL activity are increased, providing an in vivo mechanistic link for a role of liver PC7 in enhanced TG storage in adipocytes.