Citation

  • Authors: Susan-Resiga D. et al.
  • Year: 2021
  • Journal: J Biol Chem
  • Applications: in vitro / DNA, siRNA / INTERFERin, jetPEI, jetPRIME
  • Cell type: Hep G2
    Description: Human hepatocarcinoma cells

Method

Small interfering RNA (siRNA) targeted against ASGR1 or PCSK9 were purchased from siGenome, Horizon Discoveries and transfections were carried out using INTERFERin (PolyPlus) as per manufacturer’s instructions. HepG2 naïve cells were transfected with siPCSK9 at final concentrations of 10 nM and analyzed 48 hours post-transfection, HepG2 naïve cells & HepG2-PCSK9-KO with siASGR1 at final concentrations of 20 nM and analyzed by quantitative RT-PCR or WB. Protein overexpression was achieved using JetPEI (PolyPlus) transfection reagent in HepG2 and HepG2-PCSK9-KO cells (2 μg total DNA/well in 12 well plate) and JetPRIME (PolyPlus) in HEK293 cells (1 μg total DNA/well in 12 well plate), following the manufacturer’s recommendations.

Abstract

The hepatic carbohydrate-recognizing asialoglycoprotein receptor (ASGR1) mediates the endocytosis/lysosomal degradation of desialylated glycoproteins following binding to terminal galactose/N-acetylgalactosamine. Human heterozygote-carriers of ASGR1-deletions exhibit ∼34% lower risk of coronary artery disease and ∼10-14% reduction of non-HDL-cholesterol. Since the proprotein convertase PCSK9 is a major degrader of the low-density lipoprotein receptor (LDLR), we investigated the degradation and functionality of LDLR and/or PCSK9 by endogenous/overexpressed ASGR1 using Western-blot and immunofluorescence in HepG2 naïve and HepG2-PCSK9-knockout cells. ASGR1, like PCSK9, targets LDLR, and both independently interact with/enhance the degradation of the receptor. This lack of cooperativity between PCSK9 and ASGR1 was confirmed in livers of wild-type (WT) and Pcsk9-/- mice. ASGR1-knockdown in HepG2 naïve cells significantly increased total (∼1.2-fold) and cell-surface (∼4-fold) LDLR protein. In HepG2-PCSK9-knockout cells, ASGR1 silencing led to ∼2-fold higher levels of LDLR protein and DiI (1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate)-LDL uptake associated with ∼9-fold increased cell-surface LDLR. Overexpression of WT-ASGR1/2 primarily reduced levels of immature non-O-glycosylated LDLR (∼110 kDa), whereas the triple Ala-mutant of Gln240/Trp244/Glu253 (characterized by loss of carbohydrate binding) reduced expression of the mature form of LDLR (∼150 kDa), suggesting that ASGR1 binds the LDLR in both a sugar-dependent and -independent fashion. The protease furin cleaves ASGR1 at the RKMK103↓ motif into a secreted form, likely resulting in a loss-of-function on LDLR. Altogether, we demonstrate that LDLR is the first example of a liver-receptor ligand of ASGR1. We conclude that silencing of ASGR1 and PCSK9 may lead to higher LDL-uptake by hepatocytes, thereby providing a novel approach to further reduce LDL-cholesterol levels.

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