The biology and therapeutic targeting of the proprotein convertases

Key Points

  • The secretory proprotein convertases comprise a family of nine subtilisin-like serine proteases called proprotein convertase 1 (PC1), PC2, furin, PC4, PC5, paired basic amino acid cleaving enzyme 4 (PACE4), PC7, subtilisin kexin isozyme 1 (SKI-1; also known as S1P) and proprotein convertase subtilisin kexin 9 (PCSK9), and their genes have been named PCSK1 to PCSK9.

  • Knowledge gained from in vitro and ex vivo studies, as well as the characterization of the phenotypes of knockout mice and those associated with human mutations showed that these enzymes can have both redundant and unique physiological roles.

  • Furin and PACE4 have a role in cancer and associated metastasis, in arthritis and in viral infections, which makes them attractive therapeutic targets. Small-molecule inhibitors, biologics or antisense silencing of these proprotein convertases are now being considered as therapeutic options.

  • PC1 is implicated in obesity and type 2 diabetes, PC4 in reproduction and PC7 in the regulation of anxiety; these proprotein convertases are therefore attractive targets in these settings.

  • Although SKI-1 has fundamental functions, such as the regulation of steroid and lipid synthesis, it also enhances viral infectivity, suggesting that the short-term use of pharmacological agents to block its activity could be beneficial.

  • Hepatic PCSK9 is a major circulating protein that regulates the half-life of the low-density lipoprotein receptor (LDLR) as well as the very-low-density lipoprotein receptor (VLDLR). It is upregulated by statins and hence its inhibition (in combination with or without statins) is considered to be one of the most promising new treatment approaches to effectively lower levels of LDL-cholesterol.

  • Multiple strategies are now in clinical trials (Phase I–III) to evaluate the efficacy and safety of blocking the function of PCSK9 and/or decrease its levels in the circulation. These include the use of monoclonal antibodies and adnectins, as well as antisense oligonucleotides and small-molecule inhibitors.

Abstract

The mammalian proprotein convertases constitute a family of nine secretory serine proteases that are related to bacterial subtilisin and yeast kexin. Seven of these (proprotein convertase 1 (PC1), PC2, furin, PC4, PC5, paired basic amino acid cleaving enzyme 4 (PACE4) and PC7) activate cellular and pathogenic precursor proteins by cleavage at single or paired basic residues, whereas subtilisin kexin isozyme 1 (SKI-1) and proprotein convertase subtilisin kexin 9 (PCSK9) regulate cholesterol and/or lipid homeostasis via cleavage at non-basic residues or through induced degradation of receptors. Proprotein convertases are now considered to be attractive targets for the development of powerful novel therapeutics. In this Review, we summarize the physiological functions and pathological implications of the proprotein convertases, and discuss proposed strategies to control some of their activities, including their therapeutic application and validation in selected disease states.

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Figure 1: Schematic representation of the primary structures of the human proprotein convertases.
Figure 2: Subcellular localization of proprotein convertases.
Figure 3: Intracellular versus extracellular pathway and dominant negative effect of proPCSK9.

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Acknowledgements

This research was supported by: Canadian Institutes of Health Research grants MOP-44363, MOP-93792 and MOP-102741; CTP-82946; and a Canada Chair grant 216684 (to N.G.S.). We would like to dedicate this article to M. Chretien who has been our mentor and a major driver in the discovery of the prohormone theory. He was the first to report that cleavage at pairs of basic residues of the inactive hormone precursor β-LPH (lipotropin) generates the active secretory products γ-LPH and β-MSH (melanocyte-stimulating hormone).

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FURTHER INFORMATION

Nabil G. Seidah's homepage

Alnylam Pharmaceuticals website — 4 January 2012 press release

Amgen website — 25 March 2012 press release

MEROPS Peptidase Database

MEROPS Peptidase Database (search results for 'S08.071: furin')

MEROPS Peptidase Database (search results for 'S08.077: PCSK7 peptidase')

Regeneron website — 26 March 2012 press release

Santaris Pharma website — 4 May 2011 press release

Serometrix website

UCL website (PCSK9 database information)

World Intellectual Property Organization (WIPO) website (WO/2011/051961)

Glossary

Proprotein convertases

Mammalian secretory serine proteases related to bacterial subtilases. They process various precursor proteins, mostly resulting in the release of more active products, but sometimes they inactivate some of their substrates.

Subtilisin kexin isozyme 1

(SKI-1). Also called site 1 protease. The eighth member of the proprotein convertase family; SKI-1 is responsible for the activation of various membrane-bound transcription factors (for example, sterol regulatory element-binding proteins, activating transcription factor 6 and cyclic AMP-responsive element binding proteins) and other proteins transiting through the Golgi apparatus.

Proprotein convertase subtilisin kexin 9

The ninth member of the proprotein convertase family, which is mostly implicated in regulating the levels of circulating low-density lipoprotein (LDL) cholesterol via the induction of the intracellular degradation of the LDL receptor in endosomes and lysosomes.

Low-density lipoprotein receptor

(LDLR). The main receptor of circulating LDL-cholesterol.

Prosegment

The aminoterminal inhibitory domain of the proprotein convertases. This domain acts as an intramolecular chaperone assisting the folding of the convertase in the endoplasmic reticulum, and also keeps the enzyme in an inhibited state until it is separated from the active convertase either in the trans-Golgi network, in immature secretory granules or at the cell surface.

Transforming growth factor-β

(TGFβ). A growth factor that regulates multiple physiological functions.

PCSK9–LDLR complex

The complex formed by the binding of the catalytic subunit of proprotein convertase subtilisin kexin 9 (PCSK9) to the epidermal growth factor A domain of low-density lipoprotein receptor (LDLR). The formation of this complex leads to the degradation of the LDLR in endosomes and lysosomes.

Cys-His-rich domain

Carboxy-terminal domain of proprotein convertase subtilisin kexin 9 (PCSK9) that is crucial for the sorting of the PCSK9–LDLR to endosomes and/or lysosomes.

Knockout mice

Gene knockout in mice. The availability of these genetically engineered mice allowed the characterization of some of the physiological functions of the proprotein convertases.

Genome-wide association study

(GWAS). A genome study that allows the identification of genes associated with disease states.

Sterol regulatory element binding proteins

(SREBPs). Membrane-bound transcription factors that are cleaved first by subtilisin kexin isozyme 1 in the cis- and medial-Golgi and then by site 2 protease to release their aminoterminal cytosolic fragment, which acts as a transcription factor activating the production of various proteins and enzymes implicated in cholesterol and fatty acid synthesis. The activated SREBP2 upregulates the transcription of mRNA encoding proprotein convertase subtilisin kexin 9.

Very-low-density lipoprotein receptor

(VLDLR). The main receptor of circulating VLDL-cholesterol.

Pro-opiomelanocortin

(POMC). The precursor of adrenocorticotropic hormone, α-melanocyte-stimulating hormone and β-endorphin. These products produced by proprotein convertase 1 and/or proprotein convertase 2 regulate cortisol and corticosterone production, food intake and skin colour, as well as pain sensitivity.

Autosomal dominant hypercholesterolaemia

(ADH). A single-gene dominant disorder in hypercholesterolemia, where a mutation in one allele is sufficient to cause the disease.

Coronary artery disease

The end result of the accumulation of atheromatous plaques within the walls of the coronary arteries that supply the myocardium (the muscle of the heart) with oxygen and nutrients. Coronary artery disease is the leading cause of death worldwide.

Granulocyte–macrophage colony-stimulating factor

(GM-CSF). A cytokine that functions as an activator of the immune system by acting as a white blood cell growth factor and by stimulating stem cells to produce granulocytes (neutrophils, eosinophils and basophils) and monocytes. Monocytes exit the circulation and migrate into tissues, whereupon they mature into macrophages and dendritic cells.

Familial hypercholesterolaemia

A genetic disorder characterized by high cholesterol levels, specifically very high levels of low-density lipoprotein (LDL; also known as 'bad cholesterol') in the blood, and early cardiovascular disease. Many patients have mutations in the gene that encodes the LDL receptor (LDLR) protein, which normally removes LDL from circulation, or in the gene encoding apolipoprotein B, which is the part of LDL that binds to the receptor; mutations in other genes such as proprotein convertase subtilisin kexin 9 are less common.

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Seidah, N., Prat, A. The biology and therapeutic targeting of the proprotein convertases. Nat Rev Drug Discov 11, 367–383 (2012). https://doi.org/10.1038/nrd3699

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