Loss-of-function PCSK9 mutants evade the unfolded protein response sensor GRP78 and fail to induce endoplasmic reticulum stress when retained.

Abstract

The proprotein convertase subtilisin/kexin type-9 (PCSK9) plays a central role in cardiovascular disease (CVD) by degrading hepatic low-density lipoprotein receptor (LDLR). As such, loss-of-function (LOF) PCSK9 variants that fail to exit the endoplasmic reticulum (ER) increase hepatic LDLR levels and lower the risk of developing CVD. The retention of misfolded protein in the ER can cause ER stress and activate the unfolded protein response (UPR). In this study, we investigated whether a variety of LOF PCSK9 variants that are retained in the ER can cause ER stress and hepatic cytotoxicity. Although overexpression of these PCSK9 variants caused an accumulation in the ER of hepatocytes, UPR activation or apoptosis was not observed. Furthermore, ER retention of endogenous PCSK9 via splice switching also failed to induce the UPR. Consistent with these in vitro studies, overexpression of PCSK9 in the livers of mice had no impact on UPR activation. To elucidate the cellular mechanism to explain these surprising findings, we observed that the 94-kDa glucose-regulated protein (GRP94) sequesters PCSK9 away from the 78-kDa glucose-regulated protein (GRP78), the major activator of the UPR. As a result, GRP94 knockdown increased the stability of GRP78-PCSK9 complex and resulted in UPR activation following overexpression of ER-retained PCSK9 variants relative to WT secreted controls. Given that overexpression of these LOF PCSK9 variants does not cause UPR activation under normal homeostatic conditions, therapeutic strategies aimed at blocking the autocatalytic cleavage of PCSK9 in the ER represent a viable strategy for reducing circulating PCSK9.

• Authors: Paul Lebeau, Khrystyna Platko, Ali A Al-Hashimi, Jae Hyun Byun, Šárka Lhoták, Nicholas Holzapfel, Gabriel Gyulay, Suleiman A Igdoura, David R Cool, Bernardo Trigatti, Nabil G Seidah, Richard C Austin
• Journal: The Journal of biological chemistry (J Biol Chem) Vol. 293 Issue 19 Pg. 7329-7343 (05 11 2018) ISSN: 1083-351X [Electronic] United States
• PMID: 29593095 (Publication Type: Journal Article, Research Support, N.I.H., Extramural, Research Support, Non-U.S. Gov't)
• Copyright: © 2018 by The American Society for Biochemistry and Molecular Biology, Inc.

Chemical References

• Endoplasmic Reticulum Chaperone BiP
• HSPA5 protein, human
• Heat-Shock Proteins
• Hspa5 protein, mouse
• Membrane Glycoproteins
• endoplasmin
• PCSK9 protein, human
• Pcsk9 protein, mouse
• Proprotein Convertase 9
• Proteasome Endopeptidase Complex

Topics

• Animals
• Apoptosis
• Catalytic Domain
• Cell Line
• Endoplasmic Reticulum (metabolism)
• Endoplasmic Reticulum Chaperone BiP
• Endoplasmic Reticulum Stress (genetics)
• Gene Knockdown Techniques
• Heat-Shock Proteins (metabolism)
• Hepatocytes (metabolism)
• Humans
• Loss of Function Mutation
• Membrane Glycoproteins (genetics, metabolism)
• Mice
• Proprotein Convertase 9 (genetics, metabolism)
• Proteasome Endopeptidase Complex (metabolism)
• RNA Interference
• RNA Splicing
• Unfolded Protein Response (genetics)