Abstract 318: Hepatic Site-1 Protease Knockdown in Mice Decreases Proprotein Convertase Subtilisin/kexin Type 9 Expression

Introduction— Site-1 protease (S1P) is a proprotein convertase involved in activation of transcription factors from their membrane-bound precursors in a process known as Regulated Intramembrane Proteolysis. These factors include Sterol Regulatory Element Binding Proteins (SREBPs) and CRE Binding Protein-H. SREBPs are the transcriptional master regulators of de novo cholesterol and fatty acid biosynthetic genes, and also positively regulate Low density lipoprotein receptor (LDLR) and Proprotein convertase subtilisin/kexin type 9 (PCSK9) expression. Inhibition of PCSK9 is a promising drug targeting strategy for treatment of dyslipidemia since the major function of PCSK9 is to degrade LDLR. Hypothesis— S1P controls PCSK9 expression and function in the liver. Results— Inducing Cre-LoxP mediated DNA recombination in the liver of S1P flox mice by three different approaches, we consistently observed that S1P knockdown significantly decreases PCSK9 mRNA expression in the liver and reduces plasma PCSK9 concentrations compared to S1P flox control mice. This effect is largely due to a decrease in PCSK9 mRNA synthesis without a change in its mRNA stability. The reduction of PCSK9 mRNA correlates with downregulation of the SREBP pathways and has a positive effect on hepatic LDLR protein levels as evident in mice after food entrainment. Also, we found that S1P differentially regulates PCSK9 and LDLR mRNA levels in a dose- and time-dependent manner. Finally a constitutively active form of SREBP, but not CREBH, almost restores PCSK9 expression in isolated hepatocytes from S1P knockdown mice to that of S1P flox control. Hence, S1P regulates PCSK9 expression through SREBPs. Conclusion— Taken together, our study reveals the physiological role of liver S1P in regulating hepatic and plasma PCSK9 levels and influencing its function. This may provide a novel strategy to inhibit PCSK9.