PCSK9 Biology and Its Role in Atherothrombosis

International Journal of Molecular Sciences Review PCSK9 Biology and Its Role in Atherothrombosis Cristina Barale, Elena Melchionda, Alessandro Morotti and Isabella Russo * Department of Clinical and Biological Sciences, Turin University, I-10043 Orbassano, TO, Italy; [email protected] (C.B.); [email protected] (E.M.); [email protected] (A.M.) * Correspondence: [email protected]; Tel./Fax: +39-011-9026622 Abstract: It is now about 20 years since the first case of a gain-of-function mutation involving the as- yet-unknown actor in cholesterol homeostasis, proprotein convertase subtilisin/kexin type 9 (PCSK9), was described. It was soon clear that this protein would have been of huge scientific and clinical value as a therapeutic strategy for dyslipidemia and atherosclerosis-associated cardiovascular disease (CVD) management. Indeed, PCSK9 is a serine protease belonging to the proprotein convertase family, mainly produced by the liver, and essential for metabolism of LDL particles by inhibiting LDL receptor (LDLR) recirculation to the cell surface with the consequent upregulation of LDLR- dependent LDL-C levels. Beyond its effects on LDL metabolism, several studies revealed the existence of additional roles of PCSK9 in different stages of atherosclerosis, also for its ability to target other members of the LDLR family. PCSK9 from plasma and vascular cells can contribute to the development of atherosclerotic plaque and thrombosis by promoting platelet activation, leukocyte recruitment and clot formation, also through mechanisms not related to systemic lipid changes. These results further supported the value for the potential cardiovascular benefits of therapies based on PCSK9 inhibition. Actually, the passive immunization with anti-PCSK9 antibodies, evolocumab and alirocumab, is shown to be effective in dramatically reducing the LDL-C levels and attenuating CVD. Citation: Barale, C.; Melchionda, E.; While monoclonal antibodies sequester circulating PCSK9, inclisiran, a small interfering RNA, is a Morotti, A.; Russo, I. PCSK9 Biology new drug that inhibits PCSK9 synthesis with the important advantage, compared with PCSK9 mAbs, and Its Role in Atherothrombosis. Int. to preserve its pharmacodynamic effects when administrated every 6 months. Here, we will focus on J. Mol. Sci. 2021, 22, 5880. https:// the major understandings related to PCSK9, from its discovery to its role in lipoprotein metabolism, doi.org/10.3390/ijms22115880 involvement in atherothrombosis and a brief excursus on approved current therapies used to inhibit its action. Academic Editors: Valentina Pallottini, Marco Segatto Keywords: proprotein convertase subtilisin/kexin type 9; hypercholesterolemia; low-density lipoprotein; and Antimo Cutone low-density lipoprotein receptor; atherosclerosis; platelets; thrombosis Received: 14 May 2021 Accepted: 28 May 2021 Published: 30 May 2021 1. Introduction Publisher’s Note: MDPI stays neutral Hypercholesterolemia is unequivocally the most well-established risk factor associated with regard to jurisdictional claims in with the development of atherosclerotic cardiovascular (CV) disease (CVD), which, in published maps and institutional affil- turn, accounts for the plurality of worldwide morbidity and mortality [1]. Disorders iations. in cholesterolemia clearance can be primary (genetic) or secondary to other pathologies. Among the primitive forms, familial hypercholesterolemia is the most frequent among the genetic causes of early CVD because of lifelong exposure to dyslipidemia. With the advent of statins, a sustained and effective reduction in low-density lipopro- Copyright: © 2021 by the authors. tein cholesterol (LDL-C) levels has been achieved and large-scale clinical trials show that Licensee MDPI, Basel, Switzerland. the reduction in LDL-C is associated with a significant reduction in adverse CV events [2,3]. This article is an open access article Despite the success of LDL-C lowering by statins, less than half of recurrent CV events distributed under the terms and can be prevented and, even with adjunctive lipid-lowering therapies, such as ezetimibe, conditions of the Creative Commons certain patients do not achieve satisfactory control of dyslipidemia. Attribution (CC BY) license (https:// Proprotein convertase subtilisin kexin 9 (PCSK9) has lately received considerable atten- creativecommons.org/licenses/by/ tion as a target for the reduction of LDL-C levels in patients with hypercholesterolemia [4]. 4.0/). Int. J. Mol. Sci. 2021, 22, 5880. https://doi.org/10.3390/ijms22115880 https://www.mdpi.com/journal/ijms Int. J. Mol. Sci. 2021, 22, 5880 2 of 22 Therefore, PCSK9 inhibitors have become effective and exciting new therapies for lipid management, especially for patients who could not achieve adequate lipid-lowering with combination therapy of maximally tolerated statin and ezetimibe. Given that enhanced circulating levels of LDL-C are crucial not only for the development and the progression but also for the outcomes of atherosclerotic CV disease [5], therapies including PCSK9 inhibitors are expected to slow the progression of atherosclerosis and reduce CV events and death. Indeed, the pharmacological approach with PCSK9 inhibitors significantly and safely improves cardiovascular outcomes, even though a significant CV benefit in terms of mortality has not been shown yet and there being an absence of specific long-term trials [6–8]. The central role of PCSK9 remains the regulation of cholesterol homeostasis even if growing evidence confirms that PCSK9 can exert a plethora of pleiotropic effects [9]. In particular, PCSK9 effects on vascular biology through additional atherogenic mechanisms are known to promote inflammation, plaque formation and thrombosis, events all involved in the pathogenesis of acute coronary syndrome (ACS). 2. PCSK9 Discovery Insights into the physiological function of PCSK9 (initially named neural apoptosis- regulated convertase-1) were derived initially from the finding that functional mutations in the PCSK9 gene cause dominant familial hypercholesterolemia (ADH). A paper published in 2003 described a natural mutant of PCSK9 discovered in French families with no mutation in the LDLR or apolipoprotein B (APOB) genes showing severely high LDL-C levels [10]. Single-point gain-of-function (GOF) mutations (p.S127R and p.F216L) in the PCSK9 gene revealed the involvement of an as-yet-unknown actor in cholesterol homeostasis. Additional GOF variants of PCSK9 linked to high plasma LDL-C [11] confirmed its pathophysiological role and PCSK9 became in a short time a promising treatment target in the clinic. The interest in PCSK9 function was also strengthened by the study carried out on a multiethnic population (50% African Americans) in Dallas County, USA [12], where two nonsense mutations in PCSK9 (p.Y142X and p.C679X) occurred at a combined frequency of ~2% in African Americans, and were associated with 40% lower levels of LDL-C. It was subsequently demonstrated that these nonsense mutations were associated with reduced life-long LDL-C exposure and reductions in coronary artery disease risk [13]. In 2006, Zhao et al. [14] described for the first time an individual, a healthy woman, with no immunodetectable circulating PCSK9 concentrations and very low plasma levels of LDL-C (14 mg/dL) because of the compound heterozygote of the two inactivating mutations that disrupt PCSK9 synthesis (paternal allele) and secretion (maternal allele). In another study where PCSK9 nonsense mutations were detected in African subjects, one woman was homozygous for C679X with severe hypocholesterolemia (LDL-C levels of 7 mg/dL) without apparent medical problems [15]. Since then, PCSK9 function has received such a remarkable bulk of research that it is considered a striking example of a rapid translation from a genetic-based discovery to an approved therapy. 3. PCSK9 Gene and Structure PCSK9 is the ninth and last member of a family of serine proteases closer to bacterial subtilisin 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 PCSK9 [16,17]. Data from in vitro and ex vivo studies as well as phenotypes associated with human mutations or knockout animal models show that enzymes of this family can exert unique physiological roles [16], and are implicated in the processing and/or modulation of various proteins that determine their activation or inactivation [18]. The first eight convertases—PC1, PC2, furin, PC4, PC5, PACE4, PC7 and SKI-1—cleave secretory protein precursors to generate mature, functional and bioactive peptides, polypeptides Int. J. Mol. Sci. 2021, 22, 5880 3 of 22 and hormones involved in regulating growth and metabolism [19,20]. In contrast, PCSK9 cleaves itself and then no longer functions as a protease [21], rather it acts in a non- enzymatic fashion to promote the endosomal and lysosomal degradation of the most prominent receptor involved in LDL-C homeostasis (LDLR) [22,23]. Therefore, the catalytic activity of PCSK9 is not needed for its functional enhancement on LDLR cycling. The 22-kb human PCSK9 gene is located at chromosome 1p32 and comprises 11 introns and 12 exons that encode a 72-kDa zymogen of 692-amino acids synthesized in the endo- plasmic reticulum (ER) as a glycoprotein, but is still unable to bind to LDLR (Figure1). Figure 1. Processing of the zymogen pre-proPCSK9 leading to the cleaved PCSK9 mature form. SP: signal peptide. The proprotein

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