Abstract 261: Liver-specific Pcsk9 Knockout And Transgenic Mice
Background: PCSK9 is primarily expressed in liver and intestine and was recently identified as the third locus implicated in autosomal dominant hypercholesterolemia, after LDLR and APOB. PCSK9 transits through the secretory pathway, associates with LDLR and triggers its degradation in endodomal/lysosomal compartments.
Hypothesis: Mice lacking PCSK9 specifically in liver or intestine will define the tissue-specific contribution of PCSK9 to cholesterol homeostasis, and whether the latter is exclusively dependent on LDLR.
Methods: Conditional Pcsk9 knockout mice in which the proximal promoter and exon 1 were framed with loxP sites were obtained. Using Cre-expressing mice, we generated complete (Pcsk9−/ −), hepatocyte (Alb-cre)- or enterocyte (Vil-cre)-specific knockouts. We also generated double knockout mice lacking both LDLR and PCSK9 and transgenic mice overexpressing mouse V5-tagged PCSK9 in hepatocytes.
PCSK9-deficient mice were hypocholesterolemic (−42% in plasma cholesterol), while liver-specific deficiency resulted in a 27% drop, indicating that liver PCSK9 contributes to 2/3 of the hypocholesterolemia.
In contrast, transgenic mice exhibited a 57% increase in plasma cholesterol.
Immunohistochemistry of liver sections from the different genotypes revealed an inversed correlation between the hepatocyte surface LDLR levels and PCSK9 expression.
Plasma FPLC analysis revealed superimposable cholesterol and triglyceride profiles for single PCSK9- or double PCSK9- and LDLR-deficient strains, revealing that PCSK9 does not regulate apoB secretion, except through its effect on LDLR.
The hepatocyte-specific knockout revealed that, in liver, PCSK9 is exclusively expressed in hepatocytes and that the circulating PCSK9 is > 95% from hepatic origin.
Conclusion: These mice models provide a unique approach to dissect the tissue-specific functions of PCSK9. We demonstrated that 1/3 of the plasma cholesterol drop observed in PCSK9-deficient mice is under the control of extrahepatic PCSK9. It remains to be proven whether the small intestine is the second organ in importance to regulate cholesterol homeostasis, though it does not significantly release PCSK9 in the blood stream.