Hepatic S1P deficiency lowers plasma cholesterol levels in apoB-containing lipoproteins when LDLR function is compromised

Nutrition & Metabolism

Weijun Jin¹  Xian-Cheng Jiang¹  M. Mahmood Hussain²  Jahangir Iqbal²  Afroza Huq¹  Debapriya Basu¹ 

[1] Department of Cell Biology, State University of New York, Downstate Medical Center, Brooklyn 11203, NY, USA;Department of Pediatrics, State University of New York, Downstate Medical Center, Brooklyn 11203, NY, USA

关键词: Hepatocytes;    SREBP;    PCSK9;    LDLR;    Ad libitum;    Blp-c;    cre;    Knockdown;    S1P;   

Others  :  1229048
DOI  :  10.1186/s12986-015-0031-4

 received in 2015-04-22, accepted in 2015-09-28,  发布年份 2015

【 摘 要 】

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/fand 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/fmice, 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.

【 授权许可】

   
2015 Basu et al.

【 图 表 】

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