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Arterioscler Thromb Vasc Biol. 2019 Oct;39(10):1996-2013. doi: 10.1161/ATVBAHA.119.313247. Epub 2019 Sep 5.

Ser-Phosphorylation of PCSK9 (Proprotein Convertase Subtilisin-Kexin 9) by Fam20C (Family With Sequence Similarity 20, Member C) Kinase Enhances Its Ability to Degrade the LDLR (Low-Density Lipoprotein Receptor).

Author information

1
From the Laboratory of Biochemical Neuroendocrinology, Montreal Clinical Research Institute (IRCM; affiliated to the Université de Montréal), QC, Canada (A.B.D.O., D.S.-R., E.G., R.E., J.M., J.H., A.E., N.G.S.).
2
Translational Proteomics Research Unit, Clinical Research Institute of Montreal (IRCM, affiliated to the Université de Montréal), QC, Canada (M.-S.G., D.F., B.C.).
3
Department of Molecular Biology, University of Texas Southwestern Medical Center, Dallas (M.B., V.S.T.).
4
Institut de Pharmacologie, Université de Sherbrooke, QC, Canada (K.L., R.D.).
5
Department of Biochemistry, Université de Sherbrooke, and Centre de Recherche du CHUS, QC, Canada (L.G., F.C., A.Ç.).

Abstract

OBJECTIVE:

PCSK9 (proprotein convertase subtilisin-kexin 9) enhances the degradation of the LDLR (low-density lipoprotein receptor) in endosomes/lysosomes. This study aimed to determine the sites of PCSK9 phosphorylation at Ser-residues and the consequences of such posttranslational modification on the secretion and activity of PCSK9 on the LDLR. Approach and Results: Fam20C (family with sequence similarity 20, member C) phosphorylates serines in secretory proteins containing the motif S-X-E/phospho-Ser, including the cholesterol-regulating PCSK9. In situ hybridization of Fam20C mRNA during development and in adult mice revealed a wide tissue distribution, including liver, but not small intestine. Here, we show that Fam20C phosphorylates PCSK9 at Serines 47, 666, 668, and 688. In hepatocytes, phosphorylation enhances PCSK9 secretion and maximizes its induced degradation of the LDLR via the extracellular and intracellular pathways. Replacing any of the 4 Ser by the phosphomimetic Glu or Asp enhanced PCSK9 activity only when the other sites are phosphorylated, whereas Ala substitutions reduced it, as evidenced by Western blotting, Elisa, and LDLR-immunolabeling. This newly uncovered PCSK9/LDLR regulation mechanism refines our understanding of the implication of global PCSK9 phosphorylation in the modulation of LDL-cholesterol and rationalizes the consequence of natural mutations, for example, S668R and E670G. Finally, the relationship of Ser-phosphorylation to the implication of PCSK9 in regulating LDL-cholesterol in the neurological Fragile X-syndrome disorder was investigated.

CONCLUSIONS:

Ser-phosphorylation of PCSK9 maximizes both its secretion and activity on the LDLR. Mass spectrometric approaches to measure such modifications were developed and applied to quantify the levels of bioactive PCSK9 in human plasma under normal and pathological conditions.

KEYWORDS:

cholesterol; lipoprotein; mutations; phosphorylation; serine

PMID:
31553664
PMCID:
PMC6768095
[Available on 2020-10-01]
DOI:
10.1161/ATVBAHA.119.313247

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