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.

Apolipoprotein A-V gene therapy for disease prevention / treatment: a critical analysis

Apolipoprotein (apo) A-V is a novel member of the class of exchangeable apo's involved in triacylglycerol (TG) homeostasis. Whereas a portion of hepatic-derived apoA-V is secreted into plasma and functions to facilitate lipoprotein lipase-mediated TG hydrolysis, another portion is recovered intracellularly, in association with cytosolic lipid droplets. Loss of apoA-V function is positively correlated with elevated plasma TG and increased risk of cardiovascular disease. Single nucleotide polymorphisms (SNP) in the APOA5 locus can affect transcription efficiency or introduce deleterious amino acid substitutions. Likewise, rare mutations in APOA5 that compromise functionality are associated with increased plasma TG and premature myocardial infarction. Genetically engineered mouse models and human population studies suggest that, in certain instances, supplementation with wild type (WT) apoA-V may have therapeutic benefit. It is hypothesized that individuals that manifest elevated plasma TG owing to deleterious APOA5 SNPs or rare mutations would respond to WT apoA-V supplementation with improved plasma TG clearance. On the other hand, subjects with hypertriglyceridemia of independent origin (unrelated to apoA-V function) may not respond to apoA-V augmentation in this manner. Improvement in the ability to identify individuals predicted to benefit, advances in gene transfer technology and the strong connection between HTG and heart disease, point to apoA-V supplementation as a viable disease prevention / therapeutic strategy. Candidates would include individuals that manifest chronic TG elevation, have low plasma apoA-V due to an APOA5 mutation/polymorphism and not have deleterious mutations/polymorphisms in other genes known to influence plasma TG levels.