Abstract 13997: Fructose Diet Elevates Circulating PCSK9 and LDL-cholesterol Levels and Reduces Liver LDL Receptor Protein Abundance in Hamsters through a Novel Mechanism
The liver secreted serum protein PCSK9 binds to hepatic LDL receptor (LDLR) and triggers its intracellular degradation. Thus, circulating PCSK9 concentrations influence plasma LDL-c levels and impact cardiovascular disease risk. High fructose diet (HFD) induces dyslipidemia and insulin resistance in animals and humans; however, whether PCSK9 has a causal role in the development of dyslipidmia by high fructose consumption remains elusive. In this study, we showed that serum levels of LDL-c were elevated in hamsters fed HFD as compared to animals fed a normal diet (ND). The increase in circulating LDL-c was accompanied by a 50% reduction of liver LDLR protein abundance in HFD group without changes in LDLR mRNA levels. Examinations of serum PCSK9 levels and liver PCSK9 expressions revealed that hepatic PCSK9 mRNA and protein were reduced by HFD, but serum PCSK9 levels in HFD group were 2.2-fold higher than ND group. PCSK9 in hamster serum was detected as two forms of ~60 kDa and ~53 kDa, the latter being the dominant one. It was previously reported that proprotein convertases furin and PC5A cleave human PCSK9 at Arg218-Gln219 to produce PCSK9-ΔN218 (53 kDa) that circulates in blood along with the intact form. To assess the activity of the equivalent hamster 53 kDa PCSK9-ΔN224 in degrading hepatic LDLR, we conducted in vitro experiments in Huh7 cells by exogenous overexpression of hamster PCSK9 alone or with furin/PC5A. Western blotting of culture medium from transfected Huh7 cells with anti-hamster PCSK9 antibody demonstrated that coexpression of hamster PCSK9 with either furin or PC5A increased the 53 kDa form to 82% and 70% of total PCSK9 species respectively, compared to 18% of the PCSK9 species in the absence of furin or PC5A coexpression. Importantly, the corresponding LDLR protein abundances were reduced by similar extents in Huh7 cells expressing hamster PCSK9 without or with furin/PC5A coexpression. Comparable results were obtained by treating naïve Huh7 cells with culture medium of HEK293 cells expressing hamster PCSK9 alone or with furin/PC5A. Altogether, these new findings suggest that HFD may reduce serum LDL-c uptake by LDLR in hamster liver through a novel mechanism that elevates circulating PCSK9 concentration without increasing its hepatic synthesis.
- © 2013 by American Heart Association, Inc.