BACKGROUND: Site-1 protease (S1P) is the key enzyme required for activation of 
the sterol regulatory element binding proteins (SREBPs) that govern lipid 
synthesis. While S1P has been speculated to influence plasma apoB-containing 
lipoprotein (Blp) metabolism, there has been little investigative work. LDL 
receptor (LDLR) is the major receptor for clearing plasma LDL cholesterol 
(LDL-c). Proprotein convertase subtilisin kexin type 9 (PCSK9) modulates LDL-c 
through post-translational degradation of the LDLR.
METHODS: A hepatic-specific knockdown (KD) of S1P was achieved using floxed S1P 
mouse models (S1P(f/f) and LDLR(-/-)S1P(f/f)) and hepatic expression of Cre 
recombinase. Lipids were measured in total plasma and size fractionated plasma 
using colorimetric assays. Realtime polymerase chain reaction, western blotting 
and ELISA were used to determine hepatic expression of key genes/protein. 
Plasmid mediated overexpression and siRNA mediated knockdown of genes were 
performed in mouse primary hepatocytes to determine the mechanistic basis of 
PCSK9 gene regulation.
RESULTS: A hepatic-specific KD of S1P resulted in a 45 % and 38 % reduction in 
plasma total cholesterol and triglyceride levels, respectively. Hepatic S1P KD 
had a minimal effect on plasma Blp cholesterol (Blp-c) in S1P(f/f) mice, despite 
significantly reducing VLDL secretion. Notably, hepatic S1P KD decreased the LDL 
receptor (LDLR) mRNA expression by 50 %. However, the reduction in LDLR protein 
levels was less than that of mRNA expression, especially under fed conditions. 
Further assessment of hepatic S1P deficiency revealed that it increased LDLR 
protein stability in vivo. Mechanistically, hepatic S1P KD was shown to decrease 
the liver and plasma levels of the protein proprotein convertase 
subtilisin/kexin type 9 (PCSK9), which degrades LDLR protein. This effect was 
more prominent in the fed condition and sufficient to account for the 
discordance in LDLR mRNA and protein levels. Furthermore, hepatic S1P was shown 
to regulate PCSK9 expression through activation of the SREBPs. In the LDLR(-/-) 
background, hepatic S1P KD significantly reduced Blp-c levels.
CONCLUSION: Hepatic S1P is a physiological modulator of plasma Blp metabolism 
through its regulation of LDLR and PCSK9. Hepatic S1P is a valid target for 
lowering plasma Blp-c levels in the situation where LDLR function is 
compromised.