The Multifaceted Biology of PCSK9.

This article reviews the discovery of PCSK9, its structure-function characteristics, and its presently known and proposed novel biological functions. The major critical function of PCSK9 deduced from human and mouse studies, as well as cellular and structural analyses, is its role in increasing the levels of circulating low-density lipoprotein (LDL)-cholesterol (LDLc), via its ability to enhance the sorting and escort of the cell surface LDL receptor (LDLR) to lysosomes. This implicates the binding of the catalytic domain of PCSK9 to the EGF-A domain of the LDLR. This also requires the presence of the C-terminal Cys/His-rich domain, its binding to the secreted cytosolic cyclase associated protein 1, and possibly another membrane-bound "protein X". Curiously, in PCSK9-deficient mice, an alternative to the downregulation of the surface levels of the LDLR by PCSK9 is taking place in the liver of female mice in a 17ß-estradiol-dependent manner by still an unknown mechanism. Recent studies have extended our understanding of the biological functions of PCSK9, namely its implication in septic shock, vascular inflammation, viral infections (Dengue; SARS-CoV-2) or immune checkpoint modulation in cancer via the regulation of the cell surface levels of the T-cell receptor and MHC-I, which govern the antitumoral activity of CD8+ T cells. Because PCSK9 inhibition may be advantageous in these processes, the availability of injectable safe PCSK9 inhibitors that reduces by 50% to 60% LDLc above the effect of statins is highly valuable. Indeed, injectable PCSK9 monoclonal antibody or small interfering RNA could be added to current immunotherapies in cancer/metastasis.

The Role of PCSK9 in Infectious Diseases.

BACKGROUND: In recent years, many aspects of the physiological role of PCSK9 have been elucidated, in particular regarding its role in lipid metabolism, cardiovascular risk but also its role in innate immunity. Increasing evidence is available on the involvement of PCSK9 in the pathogenesis of viral infections, mainly HCV, as well as in the regulation of host response to bacterial infections, mainly sepsis and septic shock. Moreover, the action of PCSK9 has been investigated as a crucial step in the pathogenesis of malaria infection and disease severity. OBJECTIVE: Aim of this paper is to review available published literature on the role of PCSK9 in a wide array of infectious diseases. CONCLUSION: Besides the ongoing investigation on PCSK9 inhibition among HIV-infected patients for the treatment of HIV- and ART-related hyperlipidemia, preclinical studies indicate how PCSK9 is involved in reducing the replication of HCV. Moreover, a protective role of PCSK9 inhibition has also been proposed against dengue and SARS-CoV-2 viral infections. Interestingly, high plasmatic PCSK9 levels have been described in patients with sepsis. Finally, a loss of function in the PCSK9-encoding gene has been reported to possibly reduce mortality in malaria infection.

Proprotein convertases regulate trafficking and maturation of key proteins within the secretory pathway.

Proprotein Convertases (PCs) are serine endoproteases that regulate the homeostasis of protein substrates in the cell. The PCs family counts 9 members-PC1/3, PC2, PC4, PACE4, PC5/6, PC7, Furin, SKI-1/S1P, and PCSK9. The first seven PCs are known as Basic Proprotein Convertases due to their propensity to cleave after polybasic clusters. SKI-1/S1P requires the additional presence of hydrophobic residues for processing, whereas PCSK9 is catalytically dead after autoactivation and exerts its functions using mechanisms alternative to direct cleavage. All PCs traffic through the canonical secretory pathway, reaching different compartments where the various substrates reside. Despite PCs members do not share the same subcellular localization, most of the cellular organelles count one or more Proprotein Convertases, including ER, Golgi stack, endosomes, secretory granules, and plasma membranes. The widespread expression of these enzymes at the systemic level speaks for their importance in the homeostasis of a large number of biological functions. Among others, PCs cleave precursors of hormones and growth factors and activate receptors and transcription factors. Notably, dysregulation of the enzymatic activity of Proprotein Convertases is associated to major human pathologies, such as cardiovascular diseases, cancer, diabetes, infections, inflammation, autoimmunity diseases, and Parkinson. In the current COVID-19 pandemic, Furin has further attracted the attention as a key player for conferring high pathogenicity to SARS-CoV-2. Here, we review the Proprotein Convertases family and their most important substrates along the secretory pathway. Knowledge about the complex functions of PCs is important to identify potential drug strategies targeting this class of enzymes.

Association of lipid-lowering drugs with COVID-19 outcomes from a Mendelian randomization study.

Background: Lipid metabolism plays an important role in viral infections. We aimed to assess the causal effect of lipid-lowering drugs (HMGCR inhibitiors, PCSK9 inhibitiors, and NPC1L1 inhibitior) on COVID-19 outcomes using two-sample Mendelian randomization (MR) study. Methods: We used two kinds of genetic instruments to proxy the exposure of lipid-lowering drugs, including expression quantitative trait loci of drugs target genes, and genetic variants within or nearby drugs target genes associated with low-density lipoprotein (LDL cholesterol from genome-wide association study). Summary-data-based MR (SMR) and inverse-variance-weighted MR (IVW-MR) were used to calculate the effect estimates. Results: SMR analysis found that a higher expression of HMGCR was associated with a higher risk of COVID-19 hospitalization (odds ratio [OR] = 1.38, 95% confidence interval [CI] = 1.06-1.81). Similarly, IVW-MR analysis observed a positive association between HMGCR-mediated LDL cholesterol and COVID-19 hospitalization (OR = 1.32, 95% CI = 1.00-1.74). No consistent evidence from both analyses was found for other associations. Conclusions: This two-sample MR study suggested a potential causal relationship between HMGCR inhibition and the reduced risk of COVID-19 hospitalization. Funding: Start-up Fund for high-level talents of Fujian Medical University.

The virus SARS-CoV-2 has caused millions of infections and deaths during the COVID-19 pandemic, but as of December 2021, no new drugs targeted to SARS-CoV-2 specifically exist. Thus, it is important to identify existing drugs that can reduce the infection and mortality of this virus, since repurposing old drugs is faster and cheaper than developing new ones. Fats, such as cholesterol, can play an important role in viral infections, meaning that drugs intended to lower the levels of fats in the blood could have a protective effect against SARS-CoV-2. To test this hypothesis, Huang, Xiao, et al. carried out a Mendelian randomization study to investigate if there is a link between drugs that lower fats and outcomes of SARS-CoV-2 infection, including susceptibility, hospitalization, and severe disease. This approach consists on grouping people according to their version of a particular gene, which minimizes the effect of variables that can cause spurious associations, something known as confounding bias. Thus, Mendelian randomization studies allow scientists to disentangle cause and effect. Using this method, Huang, Xiao, et al. found an association between statins (a type of drug that decreases the levels of bad cholesterol) and a reduced risk of being hospitalized after being infected with SARS-CoV-2. These findings suggest that statins could benefit patients infected with SARS-CoV-2, and indicate that they should be prioritized in future clinical trials for treating COVID-19.

How Do Enveloped Viruses Exploit the Secretory Proprotein Convertases to Regulate Infectivity and Spread?

Inhibition of the binding of enveloped viruses surface glycoproteins to host cell receptor(s) is a major target of vaccines and constitutes an efficient strategy to block viral entry and infection of various host cells and tissues. Cellular entry usually requires the fusion of the viral envelope with host plasma membranes. Such entry mechanism is often preceded by "priming" and/or "activation" steps requiring limited proteolysis of the viral surface glycoprotein to expose a fusogenic domain for efficient membrane juxtapositions. The 9-membered family of Proprotein Convertases related to Subtilisin/Kexin (PCSK) serine proteases (PC1, PC2, Furin, PC4, PC5, PACE4, PC7, SKI-1/S1P, and PCSK9) participate in post-translational cleavages and/or regulation of multiple secretory proteins. The type-I membrane-bound Furin and SKI-1/S1P are the major convertases responsible for the processing of surface glycoproteins of enveloped viruses. Stefan Kunz has considerably contributed to define the role of SKI-1/S1P in the activation of arenaviruses causing hemorrhagic fever. Furin was recently implicated in the activation of the spike S-protein of SARS-CoV-2 and Furin-inhibitors are being tested as antivirals in COVID-19. Other members of the PCSK-family are also implicated in some viral infections, such as PCSK9 in Dengue. Herein, we summarize the various functions of the PCSKs and present arguments whereby their inhibition could represent a powerful arsenal to limit viral infections causing the present and future pandemics.