Metformin-Induced Proprotein Convertase Subtilisin/Kexin Type 9 Inhibition Further Decreases Low-Density Lipoprotein Cholesterol Following Statin Treatment in Patients With Coronary Artery Disease and Without Diabetes

ABSTRACT

In vitro investigations have established metformin's capacity to downregulate proprotein convertase subtilisin/kexin type 9 (PCSK9) expression, suggesting a potential beneficial effect on atherogenic lipoprotein particles when combined with metformin therapy. Our objective was to assess whether metformin could mitigate statin-induced adverse effects on PCSK9, thereby improving lipid profiles in patients with coronary artery disease (CAD) but without diabetes. Employing an open-label, placebo-controlled, randomized trial, we randomized patients with CAD but without diabetes into CLA (cholesterol-lowering agents alone: atorvastatin ± ezetimibe, n = 38) and Met + CLA groups (metformin plus CLA, n = 33) in a 1:1 ratio. The primary end point was the therapeutic impact of 1-month metformin combination treatment on low-density lipoprotein cholesterol (LDL-C) and PCSK9 levels. Baseline LDL-C and PCSK9 levels were 76.18 mg·dL -1 and 80.54 ng·mL -1 , respectively. After 1 month, metformin significantly reduced LDL-C (-20.81%, P < 0.001), enabling 72% of patients to attain guideline-recommended LDL-C goals. Noteworthy reductions in PCSK9 levels (-15.03%, P < 0.001) were observed. Moreover, Met + CLA markedly reduced LDL particle number more than CLA alone (-10.65% vs. 1.45%, P = 0.009), primarily due to diminished small-dense LDL particle count. Mechanistically, our study demonstrated metformin's inhibition of statin-induced PCSK9 expression in human hepatocellular cells. In summary, a 1-month metformin combination regimen reduced LDL-C levels in patients with CAD but without diabetes by inhibiting PCSK9 expression.

TRIAL REGISTRATION

Chinese Clinical Trial Registry identifier: ChiCTR1900026925 (26/10/2019).

Therapeutic Monoclonal Antibodies for Metabolic Disorders: Major Advancements and Future Perspectives

PURPOSE OF REVIEW

Globally, the prevalence of metabolic disorders is rising. Elevated low-density lipoprotein (LDL) cholesterol is a hallmark of familial hypercholesterolemia, one of the most prevalent hereditary metabolic disorders and another one is Diabetes mellitus (DM) that is more common globally, characterised by hyperglycemia with low insulin-directed glucose by target cells. It is still known that low-density lipoprotein cholesterol (LDL-C) increases the risk of cardiovascular disease (CVD). LDL-C levels are thought to be the main therapeutic objectives.

RECENT FINDINGS

The primary therapy for individuals with elevated cholesterol levels is the use of statins and other lipid lowering drugs like ezetimibe for hypercholesterolemia. Even after taking statin medication to the maximum extent possible, some individuals still have a sizable residual cardiovascular risk. To overcome this proprotein convertase subtilisin/kexin type 9 (PCSK9) inhibitors-monoclonal antibodies (mAbs) are a novel class of systemic macromolecules that have enhanced LDL-C-lowering efficacy. Along with this other inhibitor are used like Angiopoeitin like 3 inhibitors. Research on both humans and animals has shown that anti-CD3 antibodies can correct autoimmune disorders like diabetes mellitus. Individuals diagnosed with familial hypercholesterolemia (FH) may need additional treatment options beyond statins, especially when facing challenges such as statin tolerance or the inability of even the highest statin doses to reach the desired target cholesterol level. Here is the summary of PCSK9, ANGPTL-3 and CD3 inhibitors and their detailed information. In this review we discuss the details of PCSK9, ANGPTL-3 and CD3 inhibitors and the current therapeutic interventions of using the monoclonal antibodies in case of the metabolic disorder. We further present the present studies and the future prospective of the same.

Navigating the Link Between Non-alcoholic Fatty Liver Disease/Non-alcoholic Steatohepatitis and Cardiometabolic Syndrome

The global prevalence of non-alcoholic fatty liver disease (NAFLD) is nearly 25% and is increasing rapidly. The spectrum of liver damage in NAFLD ranges from simple steatosis to non-alcoholic steatohepatitis, characterised by the presence of lobular inflammation and hepatocyte ballooning degeneration, with or without fibrosis, which can further develop into cirrhosis and hepatocellular carcinoma. Not only is NAFLD a progressive liver disease, but numerous pieces of evidence also point to extrahepatic consequences. Accumulating evidence suggests that patients with NAFLD are also at increased risk of cardiovascular disease (CVD); in fact, CVDs are the most common cause of mortality in patients with NAFLD. Obesity, type 2 diabetes and higher levels of LDL are common risk factors in both NAFLD and CVD; however, how NAFLD affects the development and progression of CVD remains elusive. In this review, we comprehensively summarise current data on the key extrahepatic manifestations of NAFLD, emphasising the possible link between NAFLD and CVD, including the role of proprotein convertase substilisin/kenin type 9, extracellular vesicles, microbiota, and genetic factors.

Proprotein convertase subtisilin/kexin 9 levels decline with hepatitis C virus therapy in people with HIV/hepatitis C virus and correlate with inflammation

BACKGROUND

Proprotein convertase subtisilin/kexin 9 (PCSK9) raises low-density lipoprotein cholesterol (LDL-C) levels and is associated with inflammation, which is elevated in HIV and hepatitis C virus (HCV) infection. We compared PCSK9 levels in people with co-occurring HIV and HCV (HIV/HCV) vs. HIV alone, and evaluated the impact of HCV direct-acting antiviral (DAA) therapy on PCSK9.

DESIGN

A prospective, observational cohort study.

METHODS

Thirty-five adults with HIV/HCV and 37 with HIV alone were evaluated, all with HIV virologic suppression and without documented cardiovascular disease. Circulating PCSK9 and inflammatory biomarkers were measured at baseline and following HCV treatment or at week 52 (for HIV alone) and compared using Wilcoxon tests and Spearman correlations.

RESULTS

At baseline, PCSK9 trended higher in HIV/HCV vs. HIV alone (307 vs. 284 ng/ml, P  = 0.06). Twenty-nine participants with HIV/HCV completed DAA therapy with sustained virologic response. PCSK9 declined from baseline to posttreatment 1 (median 7.3 weeks after end of therapy [EOT]) and posttreatment 2 (median 43.5 weeks after EOT), reaching levels similar to HIV alone; median within-person reduction was -60.5 ng/ml ( P  = 0.003) and -55.6 ng/ml ( P  = 0.02), respectively. Decline in PCSK9 correlated with decline in soluble (s)E-selectin and sCD163 ( r  = 0.64, P  = 0.002; r  = 0.58, P  = 0.008, respectively), but not with changes in LDL-C or other biomarkers. No significant change in PCSK9 occurred in the HIV alone group over 52 weeks.

CONCLUSION

PCSK9 declined with DAA therapy in participants with HIV/HCV, correlating with declines in several inflammatory biomarkers but not LDL-C. Elevated PCSK9 with HCV may be linked to particular HCV-associated inflammatory pathways more so than cholesterol homeostasis.

The Application of Peptide Nucleic Acids (PNA) in the Inhibition of Proprotein Convertase Subtilisin/Kexin 9 (PCSK9) Gene Expression in a Cell-Free Transcription/Translation System

Proprotein convertase subtilisin/kexin 9 (PCSK9) is a protein that plays a key role in the metabolism of low-density lipoprotein (LDL) cholesterol. The gain-of-function mutations of the PCSK9 gene lead to a reduced number of surface LDL receptors by binding to them, eventually leading to endosomal degradation. This, in turn, is the culprit of hypercholesterolemia, resulting in accelerated atherogenesis. The modern treatment for hypercholesterolemia encompasses the use of biological drugs against PCSK9, like monoclonal antibodies and gene expression modulators such as inclisiran-a short, interfering RNA (siRNA). Peptide nucleic acid (PNA) is a synthetic analog of nucleic acid that possesses a synthetic peptide skeleton instead of a phosphate-sugar one. This different structure determines the unique properties of PNA (e.g., neutral charge, enzymatic resistance, and an enormously high affinity with complementary DNA and RNA). Therefore, it might be possible to use PNA against PCSK9 in the treatment of hypercholesterolemia. We sought to explore the impact of three selected PNA oligomers on PCSK9 gene expression. Using a cell-free transcription/translation system, we showed that one of the tested PNA strands was able to reduce the PCSK9 gene expression down to 74%, 64%, and 68%, as measured by RT-real-time PCR, Western blot, and HPLC, respectively. This preliminary study shows the high applicability of a cell-free enzymatic environment as an efficient tool in the initial evaluation of biologically active PNA molecules in the field of hypercholesterolemia research. This cell-free approach allows for the omission of the hurdles associated with transmembrane PNA transportation at the early stage of PNA selection.

Targeting proprotein convertase subtilisin/kexin type 9 (PCSK9): from bench to bedside

Proprotein convertase subtilisin/kexin type 9 (PCSK9) has evolved as a pivotal enzyme in lipid metabolism and a revolutionary therapeutic target for hypercholesterolemia and its related cardiovascular diseases (CVD). This comprehensive review delineates the intricate roles and wide-ranging implications of PCSK9, extending beyond CVD to emphasize its significance in diverse physiological and pathological states, including liver diseases, infectious diseases, autoimmune disorders, and notably, cancer. Our exploration offers insights into the interaction between PCSK9 and low-density lipoprotein receptors (LDLRs), elucidating its substantial impact on cholesterol homeostasis and cardiovascular health. It also details the evolution of PCSK9-targeted therapies, translating foundational bench discoveries into bedside applications for optimized patient care. The advent and clinical approval of innovative PCSK9 inhibitory therapies (PCSK9-iTs), including three monoclonal antibodies (Evolocumab, Alirocumab, and Tafolecimab) and one small interfering RNA (siRNA, Inclisiran), have marked a significant breakthrough in cardiovascular medicine. These therapies have demonstrated unparalleled efficacy in mitigating hypercholesterolemia, reducing cardiovascular risks, and have showcased profound value in clinical applications, offering novel therapeutic avenues and a promising future in personalized medicine for cardiovascular disorders. Furthermore, emerging research, inclusive of our findings, unveils PCSK9's potential role as a pivotal indicator for cancer prognosis and its prospective application as a transformative target for cancer treatment. This review also highlights PCSK9's aberrant expression in various cancer forms, its association with cancer prognosis, and its crucial roles in carcinogenesis and cancer immunity. In conclusion, this synthesized review integrates existing knowledge and novel insights on PCSK9, providing a holistic perspective on its transformative impact in reshaping therapeutic paradigms across various disorders. It emphasizes the clinical value and effect of PCSK9-iT, underscoring its potential in advancing the landscape of biomedical research and its capabilities in heralding new eras in personalized medicine.

Insight into the role of PCSK9 in glucose metabolism

Diabetes mellitus (DM) is strongly associated with an increased risk of atherosclerotic cardiovascular disease (ASCVD). Proprotein convertase subtilisin/kexin type 9 (PCSK9) was recently identified as an important regulator of circulating low-density lipoprotein-cholesterol (LDL-C) levels via degradation of the LDL receptor, proving to be a valid target to improve lipoprotein profiles and cardiovascular outcomes in patients with ASCVD. Beyond LDL receptor processing and cholesterol homeostasis, the PCSK9 protein has recently been verified to be associated with glucose metabolism. Importantly, clinical trials suggest that treatment with PCSK9 inhibitors for patients with DM is more effective. Hence, in this review, we summarize the current findings derived from experimental, preclinical, and clinical studies regarding the association between PCSK9 and glucose metabolism, including the relationship of PCSK9 genetic mutations to glucose metabolism and diabetes, the link between plasma PCSK9 concentrations and glucose metabolic parameters, the effects of glucose-lowering drugs on plasma PCSK9 levels and the impacts of PCSK9 inhibitors on cardiovascular outcomes of patients with DM. Clinically, exploring this field may improve our understanding regarding the roles of PCSK9 in glucose metabolism and may offer an in-depth interpretation of how PCSK9 inhibitors exert effects on the treatment of patients with DM.

Cholesterol-Lowering Activity of Vitisin A Is Mediated by Inhibiting Cholesterol Biosynthesis and Enhancing LDL Uptake in HepG2 Cells

Pyranoanthocyanins have been reported to possess better chemical stability and bioactivities than monomeric anthocyanins in some aspects. The hypocholesterolemic activity of pyranoanthocyanins is unclear. In view of this, this study was conducted to compare the cholesterol-lowering activities of Vitisin A with the anthocyanin counterpart Cyanidin-3-O-glucoside(C3G) in HepG2 cells and to investigate the interaction of Vitisin A with the expression of genes and proteins associated with cholesterol metabolism. HepG2 cells were incubated with 40 μM cholesterol and 4 μM 25-hydroxycholeterol with various concentrations of Vitisin A or C3G for 24 h. It was found that Vitisin A decreased the cholesterol levels at the concentrations of 100 μM and 200 μM with a dose-response relationship, while C3G exhibited no significant effect on cellular cholesterol. Furthermore, Vitisin A could down-regulate 3-hydroxy-3-methyl-glutaryl coenzyme A reductase (HMGCR) to inhibit cholesterol biosynthesis through a sterol regulatory element-binding protein 2 (SREBP2)-dependent mechanism, and up-regulate low-density lipoprotein receptor (LDLR) and blunt the secretion of proprotein convertase subtilisin/kexin type 9 (PCSK9) protein to promote intracellular LDL uptake without LDLR degradation. In conclusion, Vitisin A demonstrated hypocholesterolemic activity, by inhibiting cholesterol biosynthesis and enhancing LDL uptake in HepG2 cells.

SKI-1/S1P Facilitates SARS-CoV-2 Spike Induced Cell-to-Cell Fusion via Activation of SREBP-2 and Metalloproteases, Whereas PCSK9 Enhances the Degradation of ACE2

Proprotein convertases activate various envelope glycoproteins and participate in cellular entry of many viruses. We recently showed that the convertase furin is critical for the infectivity of SARS-CoV-2, which requires cleavage of its spike protein (S) at two sites: S1/S2 and S2'. This study investigates the implication of the two cholesterol-regulating convertases SKI-1 and PCSK9 in SARS-CoV-2 entry. The assays used were cell-to-cell fusion in HeLa cells and pseudoparticle entry into Calu-3 cells. SKI-1 increased cell-to-cell fusion by enhancing the activation of SREBP-2, whereas PCSK9 reduced cell-to-cell fusion by promoting the cellular degradation of ACE2. SKI-1 activity led to enhanced S2' formation, which was attributed to increased metalloprotease activity as a response to enhanced cholesterol levels via activated SREBP-2. However, high metalloprotease activity resulted in the shedding of S2' into a new C-terminal fragment (S2″), leading to reduced cell-to-cell fusion. Indeed, S-mutants that increase S2″ formation abolished S2' and cell-to-cell fusion, as well as pseudoparticle entry, indicating that the formation of S2″ prevents SARS-CoV-2 cell-to-cell fusion and entry. We next demonstrated that PCSK9 enhanced the cellular degradation of ACE2, thereby reducing cell-to-cell fusion. However, different from the LDLR, a canonical target of PCSK9, the C-terminal CHRD domain of PCSK9 is dispensable for the PCSK9-induced degradation of ACE2. Molecular modeling suggested the binding of ACE2 to the Pro/Catalytic domains of mature PCSK9. Thus, both cholesterol-regulating convertases SKI-1 and PCSK9 can modulate SARS-CoV-2 entry via two independent mechanisms.

PCSK9 facilitates melanoma pathogenesis via a network regulating tumor immunity

BACKGROUND

PCSK9 regulates cholesterol homeostasis and promotes tumorigenesis. However, the relevance of these two actions and the mechanisms underlying PCSK9's oncogenic roles in melanoma and other cancers remain unclear.

METHODS

PCSK9's association with melanoma was analysed using the TCGA dataset. Empty vector (EV), PCSK9, gain-of-function (D374Y), and loss-of-function (Q152H) PCSK9 mutant were stably-expressed in murine melanoma B16 cells and studied for impact on B16 cell-derived oncogenesis in vitro and in vivo using syngeneic C57BL/6 and Pcsk9^-/- mice. Intratumoral accumulation of cholesterol was determined. RNA-seq was performed on individual tumor types. Differentially-expressed genes (DEGs) were derived from the comparisons of B16 PCSK9, B16 D374Y, or B16 Q152H tumors to B16 EV allografts and analysed for pathway alterations.

RESULTS

PCSK9 expression and its network negatively correlated with the survival probability of patients with melanoma. PCSK9 promoted B16 cell proliferation, migration, and growth in soft agar in vitro, formation of tumors in C57BL/6 mice in vivo, and accumulation of intratumoral cholesterol in a manner reflecting its regulation of the low-density lipoprotein receptor (LDLR): Q152H, EV, PCSK9, and D374Y. Tumor-associated T cells, CD8 + T cells, and NK cells were significantly increased in D374Y tumors along with upregulations of multiple immune checkpoints, IFNγ, and 143 genes associated with T cell dysfunction. Overlap of 36 genes between the D374Y DEGs and the PCSK9 DEGs predicted poor prognosis of melanoma and resistance to immune checkpoint blockade (ICB) therapy. CYTH4, DENND1C, AOAH, TBC1D10C, EPSTI1, GIMAP7, and FASL (FAS ligand) were novel predictors of ICB therapy and displayed high level of correlations with multiple immune checkpoints in melanoma and across 30 human cancers. We observed FAS ligand being among the most robust biomarkers of ICB treatment and constructed two novel and effective multigene panels predicting response to ICB therapy. The profiles of allografts produced by B16 EV, PCSK9, D374Y, and Q152H remained comparable in C57BL/6 and Pcsk9^-/- mice.

CONCLUSIONS

Tumor-derived PCSK9 plays a critical role in melanoma pathogenesis. PCSK9's oncogenic actions are associated with intratumoral cholesterol accumulation. PCSK9 systemically affects the immune system, contributing to melanoma immune evasion. Novel biomarkers derived from the PCSK9-network effectively predicted ICB therapy responses.

Molecular interactions of PCSK9 with an inhibitory nanobody, CAP1 and HLA-C: Functional regulation of LDLR levels

OBJECTIVE

The liver-derived circulating PCSK9 enhances the degradation of the LDL receptor (LDLR) in endosomes/lysosomes. PCSK9 inhibition or silencing is presently used in clinics worldwide to reduce LDL-cholesterol, resulting in lower incidence of cardiovascular disease and possibly cancer/metastasis. The mechanism by which the PCSK9-LDLR complex is sorted to degradation compartments is not fully understood. We previously suggested that out of the three M1, M2 and M3 subdomains of the C-terminal Cys/His-rich-domain (CHRD) of PCSK9, only M2 is critical for the activity of extracellular of PCSK9 on cell surface LDLR. This likely implicates the binding of M2 to an unknown membrane-associated "protein X" that would escort the complex to endosomes/lysosomes for degradation. We reported that a nanobody P1.40 binds the M1 and M3 domains of the CHRD and inhibits the function of PCSK9. It was also reported that the cytosolic adenylyl cyclase-associated protein 1 (CAP1) could bind M1 and M3 subdomains and enhance the activity of PCSK9. In this study, we determined the 3-dimensional structure of the CHRD-P1.40 complex to understand the intricate interplay between P1.40, CAP1 and PCSK9 and how they regulate LDLR degradation.

METHODS

X-ray diffraction of the CHRD-P1.40 complex was analyzed with a 2.2 Å resolution. The affinity and interaction of PCSK9 or CHRD with P1.40 or CAP1 was analyzed by atomic modeling, site-directed mutagenesis, bio-layer interferometry, expression in hepatic cell lines and immunocytochemistry to monitor LDLR degradation. The CHRD-P1.40 interaction was further analyzed by deep mutational scanning and binding assays to validate the role of predicted critical residues. Conformational changes and atomic models were obtained by small angle X-ray scattering (SAXS).

RESULTS

We demonstrate that PCSK9 exists in a closed or open conformation and that P1.40 favors the latter by binding key residues in the M1 and M3 subdomains of the CHRD. Our data show that CAP1 is well secreted by hepatic cells and binds extracellular PCSK9 at distinct residues in the M1 and M3 modules and in the acidic prodomain. CAP1 stabilizes the closed conformation of PCSK9 and prevents P1.40 binding. However, CAP1 siRNA only partially inhibited PCSK9 activity on the LDLR. By modeling the previously reported interaction between M2 and an R-X-E motif in HLA-C, we identified Glu₅₆₇ and Arg₅₄₉ as critical M2 residues binding HLA-C. Amazingly, these two residues are also required for the PCSK9-induced LDLR degradation.

CONCLUSIONS

The present study reveals that CAP1 enhances the function of PCSK9, likely by twisting the protein into a closed configuration that exposes the M2 subdomain needed for targeting the PCSK9-LDLR complex to degradation compartments. We hypothesize that "protein X", which is expected to guide the LDLR-PCSK9-CAP1 complex to these compartments after endocytosis into clathrin-coated vesicles, is HLA-C or a similar MHC-I family member. This conclusion is supported by the PCSK9 natural loss-of-function Q554E and gain-of-function H553R M2 variants, whose consequences are anticipated by our modeling.

Genetic Spectrum of Familial Hypercholesterolaemia in the Malaysian Community: Identification of Pathogenic Gene Variants Using Targeted Next-Generation Sequencing

Familial hypercholesterolaemia (FH) is caused by mutations in lipid metabolism genes, predominantly in low-density lipoprotein receptor (LDLR), apolipoprotein B (APOB), proprotein convertase subtilisin/kexin-type 9 (PCSK9) and LDL receptor adaptor protein 1 (LDLRAP1). The prevalence of genetically confirmed FH and the detection rate of pathogenic variants (PV) amongst clinically diagnosed patients is not well established. Targeted next-generation sequencing of LDLR, APOB, PCSK9 and LDLRAP1 was performed on 372 clinically diagnosed Malaysian FH subjects. Out of 361 variants identified, 40 of them were PV (18 = LDLR, 15 = APOB, 5 = PCSK9 and 2 = LDLRAP1). The majority of the PV were LDLR and APOB, where the frequency of both PV were almost similar. About 39% of clinically diagnosed FH have PV in PCSK9 alone and two novel variants of PCSK9 were identified in this study, which have not been described in Malaysia and globally. The prevalence of genetically confirmed potential FH in the community was 1:427, with a detection rate of PV at 0.2% (12/5130). About one-fourth of clinically diagnosed FH in the Malaysian community can be genetically confirmed. The detection rate of genetic confirmation is similar between potential and possible FH groups, suggesting a need for genetic confirmation in index cases from both groups. Clinical and genetic confirmation of FH index cases in the community may enhance the early detection of affected family members through family cascade screening.

Expanding Biology of PCSK9: Roles in Atherosclerosis and Beyond

Purpose of Review

Since the discovery of PCSK9 in 2003, this proprotein convertase was shown to target specific receptors for degradation in endosomes/lysosomes, including LDLR and other family members and hence to enhance the levels of circulating LDL-cholesterol (LDLc). Accordingly, inhibitors of PCSK9, including monoclonal antibodies blocking its circulating activity and siRNA silencers of its hepatic expression, are now used in clinics worldwide to treat hypercholesterolemia patients effectively and safely in combination with statins and/or ezetimibe. These powerful treatments reduce the incidence of atherosclerosis by at least 20%. Since 2008, novel targets of PCSK9 began to be defined, thereby expanding its roles beyond LDLc regulation into the realm of inflammation, pathogen infections and cellular proliferation in various cancers and associated metastases.

Recent Findings

Some pathogens such as dengue virus exploit the ability of PCSK9 to target the LDLR for degradation to enhance their ability to infect cells. Aside from increasing the degradation of the LDLR and its family members VLDLR, ApoER2 and LRP1, circulating PCSK9 also reduces the levels of other receptors such as CD36 (implicated in fatty acid uptake), oxidized LDLR receptor (that clears oxidized LDLc) as well as major histocompatibility class-I (MHC-I) receptors (implicated in the immune response to antigens). Thus, these novel targets provided links between PCSK9 and inflammation/atherosclerosis, viral infections and cancer/metastasis. The functional activities of PCSK9, accelerated the development of novel therapies to inhibit PCSK9 functions, including small molecular inhibitors, long-term vaccines, and possibly CRISPR-based silencing of hepatic expression of PCSK9.

Summary

The future of inhibitors/silencers of PCSK9 function or expression looks bright, as these are expected to provide a modern armamentarium to treat various pathologies beyond hypercholesterolemia and its effects on atherosclerosis.

Lipid Parameters and Proprotein Convertase Subtilisin/Kexin Type 9 in Healthy Lebanese Adults

Background: High levels of non-HDL cholesterol (non-HDL-C), triglycerides (TG), lipoprotein (a) (Lp(a)), and Proprotein convertase subtilisin/kexin type 9 (PCSK9) as well as low levels of HDL-C are strongly associated with cardiovascular disease (CVD). Our study aims to estimate the prevalence of dyslipidemia and high Lp(a) in the Lebanese population and to study the relationship of these variables with gender, age, body mass index (BMI), and PCSK9. Methods: This cross-sectional study was carried out on a sample of healthy volunteers aged 18 to 65. Blood samples were drawn from volunteers for total cholesterol (TC), HDL-C, TG, PCSK9, and Lp(a) measurements. Non-HDL-C was calculated by subtracting HDL-C from TC. Results: In total, 303 volunteer subjects with an average age of 38.9 years were included in the study. Respectively, 44%, 29.8%, and 44% of men had high non-HDL-C and TG with low HDL-C versus 23.5%, 8%, and 37% in women. Non-HDL-C and TG were significantly higher in men than in women, while the reverse was observed for HDL-C (p < 0.0001 for the three comparisons). Non-HDL-C and TG were significantly correlated with age and BMI (p< 0.0001 for all correlations), while HDL-C was inversely correlated with BMI (p < 0.0001) but not with age. Abnormal Lp(a) levels (≥75 nmol/L) were found in 19.1% of the population, predominantly in women (24.1% versus 13.4% in men, p = 0.004). The median PCSK9 and its interquartile was 300 (254−382) ng/L with no gender difference (p = 0.18). None of the following factors: gender, age, BMI, non-HDL-C, HDL-C, or TG, were independently associated with Lp(a), while PCSK9 was significantly correlated with age, non-HDL-C, and TG in both men and women and inversely correlated with HDL-C in men. Dyslipidemia is very common in the Lebanese population and is associated with age, high BMI, and male sex. Lp(a) is higher in women without any correlation with the lipid profile, whereas PCSK9 is associated with non-HDL-C and TG. Further studies are needed to evaluate the potential role of Lp(a) and PCSK9 in predicting CVD in healthy populations.

Epinephelus coioides PCSK9 affect the infection of SGIV by regulating the innate immune response

Proprotein Convertase Subtilisin/Kexin Type 9 (PCSK9) in mammals is a multifunctional protein. In this study, PCSK9 of marine fish Epinephelus coioides was characterized. The full-length cDNA of E. coioides PCSK9 was 2458 bp in length containing 185 bp 5' UTR, 263 bp 3' UTR and 2010 bp open reading frame (ORF) encoding 669 amino acids with the predicted molecular weight of 71 kDa and the theoretical PI of 6.6. Similar to other members of PCSK9 family, E. coioides PCSK9 has three conserved domains: Inhibitor_ I9 super family, Peptidases_ S8_ PCSK9_ Proteinase K_ like, and PCSK9_ C-CRD super family. E. coioides PCSK9 mRNA could be detected in all the tissues examined by real-time quantitative PCR, with the highest expression in the brain, followed by skin, trunk kidney, head kidney, intestine, blood, liver, spleen, gill, muscle and heart. E. coioides PCSK9 was distributed in both the cytoplasm and nucleus. The expression of E. coioides PCSK9 was significantly upregulated during Singapore grouper iridovirus (SGIV) infection. Upregulated PCSK9 could significantly affect the activities of nuclear factor kappaB (NF-κB) promoter, SGIV-induced apoptosis, and the expressions of the key SGIV genes (ICP18, LITAT, MCP, and VP19) and the E. coioides proinflammatory factors (IL-6, IL-1β, IL-8, and TNF-α). The results illustrated that E. coioides PCSK9 might be involved in the pathogen infection by regulating the innate immune response.

20(S)-Protopanaxadiol decreases atherosclerosis in ApoE KO mice by increasing the levels of LDLR and inhibiting its binding with PCSK9

Chinese medicinal and edible plants such as Panax notoginseng and ginseng are widely used for the treatment of atherosclerosis (AS). AS is the main pathological basis of cardiac-cerebral vascular disease, which seriously threatens human health and quality of life. Low-density lipoprotein (LDL) is the main pathogenic factor of AS. The LDL receptor (LDLR) is an important protein that functions to mediate the uptake and degradation of plasma LDL. Proprotein Convertase Subtilisin/Kexin Type 9 (PCSK9) can mediate the internalization and degradation of LDLR. So, increasing the LDLR level by inhibiting PCSK9 is an important means of prevention and treatment of AS. In this study, by combining interaction technology (surface plasmon resonance, SPR) of small molecule compounds with membrane receptor proteins, cell experiments, and in vivo experiments, it is proved for the first time that 20(S)-protopanaxadiol (PPD), as a hydrolytic product of Panax notoginseng saponins in the intestinal tract, can bind to the extracellular domain of LDLR and inhibit the role of Proprotein Convertase Subtilisin/Kexin Type 9 (PCSK9) in mediating LDLR degradation. The results showed that PPD significantly reduced aortic plaques and hepatic steatosis in HFD-fed ApoE KO mice. LDLR protein levels were elevated in the liver tissues isolated from PPD-treated HFD-fed ApoE KO mice and PPD-treated HepG2 cells. Our findings demonstrated that PPD significantly increased LDLR levels and reduced AS in the HFD-fed ApoE KO mice on account of LDLR degradation being inhibited by PPD inhibiting the interaction between PCSK9 and LDLR.

Therapeutic potential of herbal medicine for the management of hyperlipidemia: latest updates

Hyperlipidemia, the most common form of dyslipidemia, is the main source of cardiovascular disorders, characterized by elevated level of total cholesterol (TC), triglycerides (TG) and low-density lipoprotein cholesterol (LDL-C) with high-density lipoprotein cholesterol (HDL-C) in peripheral blood. It is caused by a defect in lipid metabolism in the surface of Apoprotein C-II or a defect in lipoprotein lipase activity as well as reported in genetic, dietary and environmental factors. Several electronic databases were investigated as information sources, including Google Scholar, PubMed, Web of Science, Scopus, ScienceDirect, SpringerLink, Semantic Scholar, MEDLINE and CNKI Scholar. The current review focused on the risk factors of dyslipidemia, synthetic medication with their side effects and different types of medicinal plants having significant potential for the management of hyperlipidemia. The management of hyperlipidemia mostly involves a constant decrease in lipid level using different remedial drugs like statin, fibrate, bile acid sequestrates and niacin. However, this extensive review suggested that the consequences of these drugs are arguable, due to their numerous adverse effects. The selected parts of herb plants are used intact or their extracts containing active phytoconstituents to regulate the lipids in blood level. It was also noted that the Chinese herbal medicine and combination therapy is promising for the lowering of hyperlipidemia. This review intends to provide a scientific base for future endeavors, such as in-depth biological and chemical investigations into previously researched topics.

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 17β-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.

Proprotein Convertase Subtilisin Kexin Type 9 (PCSK9) Beyond Lipids: The Role in Oxidative Stress and Thrombosis

Proprotein convertase subtilisin/kexin type 9 (PCSK9), mainly secreted in the liver, is a key regulator of cholesterol homeostasis inducing LDL receptors’ degradation. Beyond lipid metabolism, PCSK9 is involved in the development of atherosclerosis, promoting plaque formation in mice and human, impairing the integrity of endothelial monolayer and promoting the events that induce atherosclerosis disease progression. In addition, the PCSK9 ancillary role in the atherothrombosis process is widely debated. Indeed, recent evidence showed a regulatory effect of PCSK9 on redox system and platelet activation. In particular, the role of PCSK9 in the activation of nicotinamide adenine dinucleotide phosphate (NADPH) oxidase (Nox2) system, of MAP-kinase cascades and of CD36 and LOX-1 downstream pathways, suggests that PCSK9 may be a significant cofactor in atherothrombosis development. This evidence suggests that the serum levels of PCSK9 could represent a new biomarker for the occurrence of cardiovascular events. Finally, other evidence showed that PCSK9 inhibitors, a novel pharmacological tool introduced in clinical practice in recent years, counteracted these phenomena. In this review, we summarize the evidence concerning the role of PCSK9 in promoting oxidative-stress-related atherothrombotic process.

Caffeine blocks SREBP2-induced hepatic PCSK9 expression to enhance LDLR-mediated cholesterol clearance

Evidence suggests that caffeine (CF) reduces cardiovascular disease (CVD) risk. However, the mechanism by which this occurs has not yet been uncovered. Here, we investigated the effect of CF on the expression of two bona fide regulators of circulating low-density lipoprotein cholesterol (LDLc) levels; the proprotein convertase subtilisin/kexin type 9 (PCSK9) and the low-density lipoprotein receptor (LDLR). Following the observation that CF reduced circulating PCSK9 levels and increased hepatic LDLR expression, additional CF-derived analogs with increased potency for PCSK9 inhibition compared to CF itself were developed. The PCSK9-lowering effect of CF was subsequently confirmed in a cohort of healthy volunteers. Mechanistically, we demonstrate that CF increases hepatic endoplasmic reticulum (ER) Ca²⁺ levels to block transcriptional activation of the sterol regulatory element-binding protein 2 (SREBP2) responsible for the regulation of PCSK9, thereby increasing the expression of the LDLR and clearance of LDLc. Our findings highlight ER Ca²⁺ as a master regulator of cholesterol metabolism and identify a mechanism by which CF may protect against CVD.

Genetically Engineered Hamster Models of Dyslipidemia and Atherosclerosis

Animal models of human diseases play an extremely important role in biomedical research. Among them, mice are widely used animal models for translational research, especially because of ease of generation of genetically engineered mice. However, because of the great differences in biology between mice and humans, translation of findings to humans remains a major issue. Therefore, the exploration of models with biological and metabolic characteristics closer to those of humans has never stopped.Although pig and nonhuman primates are biologically similar to humans, their genetic engineering is technically difficult, the cost of breeding is high, and the experimental time is long. As a result, the application of these species as model animals, especially genetically engineered model animals, in biomedical research is greatly limited.In terms of lipid metabolism and cardiovascular diseases, hamsters have several characteristics different from rats and mice, but similar to those in humans. The hamster is therefore an ideal animal model for studying lipid metabolism and cardiovascular disease because of its small size and short reproduction period. However, the phenomenon of zygote division, which was unexpectedly blocked during the manipulation of hamster embryos for some unknown reasons, had plagued researchers for decades and no genetically engineered hamsters have therefore been generated as animal models of human diseases for a long time. After solving the problem of in vitro development of hamster zygotes, we successfully prepared enhanced green fluorescent protein (eGFP) transgenic hamsters by microinjection of lentiviral vectors into the zona pellucida space of zygotes. On this basis, we started the development of cardiovascular disease models using the hamster embryo culture system combined with the novel genome editing technique of clustered regularly interspaced short palindromic repeats (CRISPR )/CRISPR associated protein 9 (Cas9). In this chapter, we will introduce some of the genetically engineered hamster models with dyslipidemia and the corresponding characteristics of these models. We hope that the genetically engineered hamster models can be further recognized and complement other genetically engineered animal models such as mice, rats, and rabbits. This will lead to new avenues and pathways for the study of lipid metabolism and its related diseases.

The role of PCSK9 in inflammation, immunity, and autoimmune diseases

INTRODUCTION

Statins have pleiotropic effects, being both anti-inflammatory and immunomodulatory. Proprotein convertase subtilisin kexin 9 (PCSK9) targets the low-density lipoprotein receptor (LDLR), which increases LDL levels due to the lower expression of LDLR.

AREAS COVERED

Inhibition of PCSK9 by the use of antibodies represents a novel principle to lower LDL levels. LDL may have other properties than being a cholesterol carrier but is well established as a risk factor for cardiovascular disease and atherosclerosis. In atherosclerosis, the plaques are characterized by activated T cells and dendritic cells (DCs), dead cells, and OxLDL. The latter may be an important cause of the inflammation typical of atherosclerosis, by promoting a proinflammatory immune activation. This is inhibited by PCSK9 inhibition, and an anti-inflammatory type of immune activation is induced. OxLDL is raised in systemic lupus erythematosus (SLE), where both CVD and atherosclerosis are much increased compared to the general population. PCSK9 is reported to be associated with disease activity and complications in SLE. Also in other rheumatoid arthritis, PCSK9 may play a role.

EXPERT OPINION

PCSK9 has pleiotropic effects, being implicated in inflammation and immunity. Inhibition of PCSK9 is therefore interesting to study further as a potential therapy against inflammation and autoimmunity.

PCSK9 regulates the NODAL signaling pathway and cellular proliferation in hiPSCs

Proprotein convertase subtilisin kexin type 9 (PCSK9) is a key regulator of low-density lipoprotein (LDL) cholesterol metabolism and the target of lipid-lowering drugs. PCSK9 is mainly expressed in hepatocytes. Here, we show that PCSK9 is highly expressed in undifferentiated human induced pluripotent stem cells (hiPSCs). PCSK9 inhibition in hiPSCs with the use of short hairpin RNA (shRNA), CRISPR/cas9-mediated knockout, or endogenous PCSK9 loss-of-function mutation R104C/V114A unveiled its new role as a potential cell cycle regulator through the NODAL signaling pathway. In fact, PCSK9 inhibition leads to a decrease of SMAD2 phosphorylation and hiPSCs proliferation. Conversely, PCSK9 overexpression stimulates hiPSCs proliferation. PCSK9 can interfere with the NODAL pathway by regulating the expression of its endogenous inhibitor DACT2, which is involved in transforming growth factor (TGF) β-R1 lysosomal degradation. Using different PCSK9 constructs, we show that PCSK9 interacts with DACT2 through its Cys-His-rich domain (CHRD) domain. Altogether these data highlight a new role of PCSK9 in cellular proliferation and development.

The PCSK9 discovery, an inactive protease with varied functions in hypercholesterolemia, viral infections, and cancer

In 2003, the sequences of mammalian proprotein convertase subtilisin/kexin type 9 (PCSK9) were reported. Radiolabeling pulse-chase analyses demonstrated that PCSK9 was synthesized as a precursor (proPCSK9) that undergoes autocatalytic cleavage in the endoplasmic reticulum into PCSK9, which is then secreted as an inactive enzyme in complex with its inhibitory prodomain. Its high mRNA expression in liver hepatocytes and its gene localization on chromosome 1p32, a third locus associated with familial hypercholesterolemia, other than LDLR or APOB, led us to identify three patient families expressing the PCSK9 variants S127R or F216L. Although Pcsk9 and Ldlr were downregulated in mice that were fed a cholesterol-rich diet, PCSK9 overexpression led to the degradation of the LDLR. This led to the demonstration that gain-of-function and loss-of-function variations in PCSK9 modulate its bioactivity, whereby PCSK9 binds the LDLR in a nonenzymatic fashion to induce its degradation in endosomes/lysosomes. PCSK9 was also shown to play major roles in targeting other receptors for degradation, thereby regulating various processes, including hypercholesterolemia and associated atherosclerosis, vascular inflammation, viral infections, and immune checkpoint regulation in cancer. Injectable PCSK9 monoclonal antibody or siRNA is currently used in clinics worldwide to treat hypercholesterolemia and could be combined with current therapies in cancer/metastasis. In this review, we present the critical information that led to the discovery of PCSK9 and its implication in LDL-C metabolism. We further analyze the underlying functional mechanism(s) in the regulation of LDL-C, as well as the evolving novel roles of PCSK9 in both health and disease states.

Asialoglycoprotein receptor 1 is a novel PCSK9-independent ligand of liver LDLR cleaved by furin

The hepatic carbohydrate-recognizing asialoglycoprotein receptor (ASGR1) mediates the endocytosis/lysosomal degradation of desialylated glycoproteins following binding to terminal galactose/N-acetylgalactosamine. Human heterozygote carriers of ASGR1 deletions exhibit ∼34% lower risk of coronary artery disease and ∼10% to 14% reduction of non-HDL cholesterol. Since the proprotein convertase PCSK9 is a major degrader of the low-density lipoprotein receptor (LDLR), we investigated the degradation and functionality of LDLR and/or PCSK9 by endogenous/overexpressed ASGR1 using Western blot and immunofluorescence in HepG2-naïve and HepG2-PCSK9-knockout cells. ASGR1, like PCSK9, targets LDLR, and both independently interact with/enhance the degradation of the receptor. This lack of cooperativity between PCSK9 and ASGR1 was confirmed in livers of wildtype (WT) and Pcsk9^−/− mice. ASGR1 knockdown in HepG2-naïve cells significantly increased total (∼1.2-fold) and cell-surface (∼4-fold) LDLR protein. In HepG2-PCSK9-knockout cells, ASGR1 silencing led to ∼2-fold higher levels of LDLR protein and DiI (1,1′-dioctadecyl-3,3,3′,3′-tetramethylindocarbocyanine perchlorate)-LDL uptake associated with ∼9-fold increased cell-surface LDLR. Overexpression of WT-ASGR1/2 primarily reduced levels of immature non-O-glycosylated LDLR (∼110 kDa), whereas the triple Ala-mutant of Gln240/Trp244/Glu253 (characterized by loss of carbohydrate binding) reduced expression of the mature form of LDLR (∼150 kDa), suggesting that ASGR1 binds the LDLR in both a sugar-dependent and -independent fashion. The protease furin cleaves ASGR1 at the RKMK¹⁰³↓ motif into a secreted form, likely resulting in a loss of function on LDLR. Altogether, we demonstrate that LDLR is the first example of a liver-receptor ligand of ASGR1. We conclude that silencing of ASGR1 and PCSK9 may lead to higher LDL uptake by hepatocytes, thereby providing a novel approach to further reduce LDL cholesterol levels.

Proprotein convertase subtilisin/kexin Type 9 is required for Ahnak-mediated metastasis of melanoma into lung epithelial cells

Previously we demonstrated that Ahnak mediates transforming growth factor-β (TGFβ)-induced epithelial-mesenchymal transition (EMT) during tumor metastasis. It is well-known that circulating tumor cells (CTCs) invade the vasculature of adjacent target tissues before working to adapt to the host environments. Currently, the molecular mechanism by which infiltrated tumor cells interact with host cells to survive within target tissue environments is far from clear. Here, we show that Ahnak regulates tumor metastasis through PCSK9 expression. To validate the molecular function of Ahnak in metastasis, B16F10 melanoma cells were injected into WT and Ahnak knockout (Ahnak^-/-) mice. Ahnak^-/- mice were more resistant to the pulmonary metastasis of B16F10 cells compared to wild-type (WT) mice. To investigate the host function of Ahnak in recipient organs against metastasis of melanoma cells, transcriptomic analyses of primary pulmonary endothelial cells from WT or Ahnak^-/- mice in the absence or presence of TGFβ stimulation were performed. We found PCSK9, along with several other candidate genes, was involved in the invasion of melanoma cells into lung tissues. PCSK9 expression in the pulmonary artery was higher in WT mice than Ahnak^-/- mice. To evaluate the host function of PCSK9 in lung tissues during the metastasis of melanoma cells, we established lung epithelial cell-specific tamoxifen-induced PCSK9 conditional KO mice (Scgb1a1-Cre/PCSK9^fl/fl). The pulmonary metastasis of B16F10 cells in Scgb1a1-Cre/PCSK9^fl/fl mice was significantly suppressed, indicating that PCSK9 plays an important role in the metastasis of melanoma cells. Taken together, our data demonstrate that Ahnak regulates metastatic colonization through the regulation of PCSK9 expression.

Substantial PCSK9 inactivation in β-cells does not modify glucose homeostasis or insulin secretion in mice

Proprotein convertase subtilisin/kexin type 9 (PCSK9) plays an important role in cholesterol homeostasis by promoting the degradation of the LDL receptor (LDLR). PCSK9 loss-of-function mutations are associated with increased fasting plasma glucose levels and slightly elevated risk of type 2-diabetes. Considering the known detrimental effects of cholesterol accumulation in β-cell, and the widespread use of PCSK9 inhibitors to treat hypercholesterolemia, it is important to gain insight into the role of pancreatic PCSK9 in glucose homeostasis and β-cell function. We generated the first β-cell-specific KO of PCSK9 (βKO). PCSK9 mRNA and protein expression were reduced by 48% and 78% in βKO islets, respectively, indicating that β-cells constitute a major site of PCSK9 expression. In islets, loss of β-cell PCSK9 resulted in unchanged LDLR protein levels, but reduced LDLR mRNA, indicating that cholesterol internalization is enhanced and that β-cell PCSK9 promotes LDLR degradation. In contrast, whole body PCSK9 KO mice exhibited 2-fold higher LDLR protein levels in islets and a stable expression of cholesterogenic genes. Whole body KO and βKO mice presented normal glucose tolerance, insulin release in response to glucose load and insulin sensitivity. Ex vivo glucose-stimulated insulin secretion in presence or absence of fatty acids was similar in WT and KO islets. Like KO mice, individuals carrying loss-of-function PCSK9 variants may be protected from cholesterol-induced toxicity due to reduced circulating cholesterol levels. Using both whole body KO or βKO models, our data demonstrate that PCSK9 deletion in mouse does not have any toxic effect on β-cell function and glucose homeostasis.

APOE gene variants in primary dyslipidemia

Apolipoprotein E (apoE) is a major apolipoprotein involved in lipoprotein metabolism. It is a polymorphic protein and different isoforms are associated with variations in lipid and lipoprotein levels and thus cardiovascular risk. The isoform apoE4 is associated with an increase in LDL-cholesterol levels and thus a higher cardiovascular risk compared to apoE3. Whereas, apoE2 is associated with a mild decrease in LDL-cholesterol levels. In the presence of other risk factors, apoE2 homozygotes could develop type III hyperlipoproteinemia (familial dysbetalipoproteinemia or FD), an atherogenic disorder characterized by an accumulation of remnants of triglyceride-rich lipoproteins. Several rare APOE gene variants were reported in different types of dyslipidemias including FD, familial combined hyperlipidemia (FCH), lipoprotein glomerulopathy and bona fide autosomal dominant hypercholesterolemia (ADH). ADH is characterized by elevated LDL-cholesterol levels leading to coronary heart disease, and due to molecular alterations in three main genes: LDLR, APOB and PCSK9. The identification of the APOE-p.Leu167del variant as the causative molecular element in two different ADH families, paved the way to considering APOE as a candidate gene for ADH. Due to non mendelian interacting factors, common genetic and environmental factors and perhaps epigenetics, clinical presentation of lipid disorders associated with APOE variants often strongly overlap. More studies are needed to determine the spectrum of APOE implication in each of the diseases, notably ADH, in order to improve clinical and genetic diagnosis, prognosis and patient management. The purpose of this review is to comment on these APOE variants and on the molecular and clinical overlaps between dyslipidemias.

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.

Circulating Proprotein Convertase Subtilisin/Kexin Type 9 Levels and Cardiometabolic Risk Factors: A Population-Based Cohort Study

Aims: To evaluate the prospective association of circulating PCSK9 levels with the cardiometabolic risk profiles (high LDL-cholesterol, high triglycerides, low HDL-cholesterol, hypertension, type 2 diabetes, and metabolic syndrome).

Methods: A population-based prospective study was conducted among 7,104 Chinese individuals (age 56.2 ± 7.5 years; 32.0% men). Circulating PCSK9 levels were measured using ELISA.

Results: Circulating PCSK9 levels were higher in women than men (286.7 ± 90.1 vs. 276.1 ± 86.4 ng/ml, p < 0.001). And circulating PCSK9 was positively correlated with LDL-cholesterol, total cholesterol, and triglycerides both in men and women (all p < 0.001). The positive correlation between PCSK9 and waist circumference, fasting glucose, insulin resistance, systolic blood pressure, diastolic blood pressure and C-reactive protein (all p < 0.01) was observed in women only. According to Cox regression analysis, circulating PCSK9 was positively associated with incidence of high LDL-cholesterol both in men (HR 1.33, 95% CI 1.09–1.65, p < 0.001) and women (HR 1.36, 95% CI 1.12–1.69, p < 0.001). Moreover, PCSK9 was significantly associated with incident high triglycerides (HR 1.31, 95% CI 1.13–1.72, p < 0.001), hypertension (HR 1.28, 95% CI 1.08–1.53, p = 0.011), type 2 diabetes (HR 1.34, 95% CI 1.09–1.76, p = 0.005), and metabolic syndrome (HR 1.30, 95% CI 1.11–1.65, p = 0.009) per SD change in women only. No statistically significant association was observed between circulating PCSK9 and incidence of low HDL-cholesterol (p > 0.1).

Conclusions: Elevated circulating PCSK9 was significantly associated with cardiometabolic risk factors and independently contributed to the prediction of cardiometabolic risks in women.

PCSK9 Functions in Atherosclerosis Are Not Limited to Plasmatic LDL-Cholesterol Regulation

The relevance of PCSK9 in atherosclerosis progression is demonstrated by the benefits observed in patients that have followed PCSK9-targeted therapies. The impact of these therapies is attributed to the plasma lipid-lowering effect induced when LDLR hepatic expression levels are recovered after the suppression of soluble PCSK9. Different studies show that PCSK9 is involved in other mechanisms that take place at different stages during atherosclerosis development. Indeed, PCSK9 regulates the expression of key receptors expressed in macrophages that contribute to lipid-loading, foam cell formation and atherosclerotic plaque formation. PCSK9 is also a regulator of vascular inflammation and its expression correlates with pro-inflammatory cytokines release, inflammatory cell recruitment and plaque destabilization. Furthermore, anti-PCSK9 approaches have demonstrated that by inhibiting PCSK9 activity, the progression of atherosclerotic disease is diminished. PCSK9 also modulates thrombosis by modifying platelets steady-state, leukocyte recruitment and clot formation. In this review we evaluate recent findings on PCSK9 functions in cardiovascular diseases beyond LDL-cholesterol plasma levels regulation.

Furin Expression in Patients With Psoriasis-A Patient Cohort Endangered to SARS-COV2?

Background: SARS-Cov2 has raised concerns among dermatologists regarding psoriasis and its respective treatments. Comorbidities, which induce the expression of the proprotease furin have been associated with severe course of COVID-19. Furin and angiotensin converting enzyme 2 (ACE2) play a major role in viral host cell entry of SARS-Cov2.

Objective: To evaluate mRNA expression of Furin and ACE2 from blood cells in psoriasis patients, and whether systemic or topical treatment reduces expression levels.

Methods: This observational translational study analyzed blood samples from patients from a clinical trial and samples retrieved from the biobank of the Psoriasis Registry Austria (PsoRA). Furin and ACE2 expression levels were analyzed prior to as well as 3 and 12–24 months after start of biologic treatment with either ustekinumab or secukinumab. Additionally, the study analyzed expression levels prior to, 6 days after start of dithranol treatment and 4–6 weeks after end of dithranol treatment.

Results: Furin mRNA expression was significantly increased at baseline in the biologic (4.9 ± 2.6 fold, p < 0.0001) and in the dithranol group (2.7 ± 1.4 fold, p < 0.001) compared to controls. There was a trend for arthritis patients to express more furin than patients with psoriatic skin involvement only (5.26 ± 2.30 vs. 3.48 ± 2.27, p = 0.078). Analyzing furin mRNA expression after treatment initiation with secukinumab or ustekinumab revealed a normalization of levels after 3 and 12 to 24 months. Similar findings were obtained for patients treated with dithranol, with significantly decreased expression levels 6 days after start of dithranol treatment and also at follow-up, (4–6 weeks after dithranol treatment had been terminated). ACE2 expression levels did not differ from controls at any timepoint, regardless of biologic or topical treatment.

Conclusion: Significantly overexpressed levels of furin were observed in untreated patients, and, thus, these patients may be at risk for infection and a severe course of COVID-19. However, the data indicate that successful therapeutic intervention in psoriasis, by systemic biologic or topical treatment, can efficiently reduce furin levels in blood cells, possibly limiting the risk of psoriasis patients for a severe COVID-19 course.

Clinical Trial Registration:ClinicalTrials.gov, identifier NCT02752672.

Loss of Furin in β-Cells Induces an mTORC1-ATF4 Anabolic Pathway That Leads to β-Cell Dysfunction

FURIN is a proprotein convertase (PC) responsible for proteolytic activation of a wide array of precursor proteins within the secretory pathway. It maps to the PRC1 locus, a type 2 diabetes susceptibility locus, but its specific role in pancreatic β-cells is largely unknown. The aim of this study was to determine the role of FURIN in glucose homeostasis. We show that FURIN is highly expressed in human islets, whereas PCs that potentially could provide redundancy are expressed at considerably lower levels. β-cell-specific Furin knockout (βFurKO) mice are glucose intolerant as a result of smaller islets with lower insulin content and abnormal dense-core secretory granule morphology. mRNA expression analysis and differential proteomics on βFurKO islets revealed activation of activating transcription factor 4 (ATF4), which was mediated by mammalian target of rapamycin C1 (mTORC1). βFurKO cells show impaired cleavage or shedding of vacuolar-type ATPase (V-ATPase) subunits Ac45 and prorenin receptor, respectively, and impaired lysosomal acidification. Blocking V-ATPase pharmacologically in β-cells increased mTORC1 activity, suggesting involvement of the V-ATPase proton pump in the phenotype. Taken together, these results suggest a model of mTORC1-ATF4 hyperactivation and impaired lysosomal acidification in β-cells lacking Furin, causing β-cell dysfunction.

Low levels of PCSK9 are associated with remission in patients with rheumatoid arthritis treated with anti-TNF-α: potential underlying mechanisms

Background

Proprotein convertase subtilisin kexin 9 (PCSK9) targets the LDL-receptor (LDLR) which raises LDL-levels. In addition, PCSK9 has proinflammatory immunological effects. Here, we investigate the role of PCSK9 in relation to the inflammatory activity in patients with rheumatoid arthritis (RA).

Methods

PCSK9-levels were determined at baseline by ELISA in 160 patients with RA not previously treated with biologics. The patients started anti-TNF-α (adalimumab, infliximab, or etanercept) treatment and were followed-up for 1 year. Disease activity was determined by DAS28.

Effects of PCSK9 on cytokine production from macrophages of healthy individuals and synoviocytes from RA patients and inhibition by anti-PCSK9 antibodies were studied in supernatants by ELISA.

Results

A significantly lower level of PCSK9 at baseline, p = 0.035, was observed in patients who reached remission within 1 year, defined as DAS28 < 2.6, compared to those not in remission. At 12 months of TNF-α antagonist treatment, the mean DAS28 was reduced but was significantly greater in patients with highest quartile PCSK9 (Q4) compared to those at lowest PCSK9 (Q1) in both crude (p = 0.01) and adjusted analysis (p = 0.004).

In vitro, PCSK9 induced TNF-alpha and IL-1beta in macrophages and monocyte chemoattractant protein-1 (MCP1) in synoviocytes. These effects were inhibited by anti-PCSK9 antibodies.

Conclusions

Low levels of PCSK9 at baseline are associated with being DAS28-responder to anti-TNF-α treatment in RA. An underlying cause could be that PCSK9 stimulates the production of proinflammatory cytokines from macrophages and synoviocytes, effects inhibited by anti-PCSK9 antibodies. PCSK9 could thus play an immunological role in RA.

Association of circulating proprotein convertase subtilisin/kexin type 9 levels and the risk of incident type 2 diabetes in subjects with prediabetes: a population-based cohort study

BACKGROUND

Proprotein convertase subtilisin/kexin type 9 (PCSK9) regulates cholesterol metabolism by targeting the low-density lipoprotein receptor. Recent studies have shown that circulating PCSK9 is associated with glucose homeostasis and insulin resistance. The aim of this study was to examine the association of circulating PCSK9 levels and risk for the development of type 2 diabetes in individuals with prediabetes.

METHODS

A population-based prospective study was conducted among 4205 Chinese subjects with prediabetes (average age 56.1 ± 7.5 years). Incident type 2 diabetes was diagnosed according to 2010 American Diabetes Association criteria. Circulating PCSK9 levels were measured using a commercially available enzyme-linked immunosorbent assay (ELISA). The association of circulating PCSK9 levels with the risk of incident type 2 diabetes was assessed by Cox regression analysis.

RESULTS

During a median follow-up period of 3.1 years, 568 subjects developed type 2 diabetes. Baseline circulating PCSK9 levels were significantly higher in female subjects developing incident type 2 diabetes than in those not developing incident type 2 diabetes (p < 0.001). In female subjects, the risk of incident type 2 diabetes was significantly higher in the highest PCSK9 quartile group (hazard ratio 2.16; 95% confidence interval 1.16-4.04) than in the lowest quartile group after adjustments for age, body mass index, waist circumference, C-reactive protein, γ-glutamyltransferase, triglycerides, low-density lipoprotein cholesterol, systolic blood pressure, and homeostatic model assessment of insulin resistance score. No significant association was observed between PCSK9 and incident type 2 diabetes in male subjects.

CONCLUSION

Elevated circulating PCSK9 levels are associated with an increased incidence of type 2 diabetes in female subjects with prediabetes.

An Update on the Role of PCSK9 in Atherosclerosis

Atherosclerosis is initiated by functional changes in the endothelium accompanied by accumulation, oxidation, and glycation of LDL-cholesterol in the inner layer of the arterial wall and continues with the expression of adhesion molecules and release of chemoattractants. PCSK9 is a proprotein convertase that increases circulating LDL levels by directing hepatic LDL receptors into lysosomes for degradation. The effects of PCSK9 on hepatic LDL receptors and contribution to atherosclerosis via the induction of hyperlipidemia are well defined. Monoclonal PCSK9 antibodies that block the effects of PCSK9 on LDL receptors demonstrated beneficial results in cardiovascular outcome trials. In recent years, extrahepatic functions of PCSK9, particularly its direct effects on atherosclerotic plaques have received increasing attention. Experimental trials have revealed that PCSK9 plays a significant role in every step of atherosclerotic plaque formation. It contributes to foam cell formation by increasing the uptake of LDL by macrophages via scavenger receptors and inhibiting cholesterol efflux from macrophages. It induces the expression of inflammatory cytokines, adhesion molecules, and chemoattractants, thereby increasing monocyte recruitment, inflammatory cell adhesion, and inflammation at the atherosclerotic vascular wall. Moreover, low shear stress is associated with increased PCSK9 expression. PCSK9 may induce endothelial cell apoptosis and autophagy and stimulate the differentiation of smooth muscle cells from the contractile phenotype to synthetic phenotype. Increasing evidence indicates that PCSK9 is a molecular target in the development of novel approaches toward the prevention and treatment of atherosclerosis. This review focuses on the molecular roles of PCSK9 in atherosclerotic plaque formation.

Circulating Rather Than Intestinal PCSK9 (Proprotein Convertase Subtilisin Kexin Type 9) Regulates Postprandial Lipemia in Mice

OBJECTIVE

Increased postprandial lipemia (PPL) is an independent risk factor for atherosclerotic cardiovascular diseases. PCSK9 (Proprotein convertase subtilisin kexin type 9) is an endogenous inhibitor of the LDLR (low-density lipoprotein receptor) pathway. We previously showed that PCSK9 inhibition in mice reduces PPL. However, the relative contribution of intracellular intestinal PCSK9 or liver-derived circulating PCSK9 to this effect is still unclear. Approach and Results: To address this issue, we generated the first intestine-specific Pcsk9-deficient (i-Pcsk9^-/-) mouse model. PPL was measured in i-Pcsk9^-/- as well as in wild-type and streptozotocin-induced diabetic mice following treatment with a PCSK9 monoclonal antibody (alirocumab). Blocking the circulating form of PCSK9 with alirocumab significantly reduced PPL, while overexpressing human PCSK9 in the liver of full Pcsk9^-/- mice had the opposite effect. Alirocumab regulated PPL in a LDLR-dependent manner as this effect was abolished in Ldlr^-/- mice. In contrast, i-Pcsk9^-/- mice did not exhibit alterations in plasma lipid parameters nor in PPL. Finally, PPL was highly exacerbated by streptozotocin-induced diabetes mellitus in Pcsk9^+/+ but not in Pcsk9^-/- mice, an effect that was mimicked by the use of alirocumab in streptozotocin-treated Pcsk9^+/+ mice.

CONCLUSIONS

Taken together, our data demonstrate that PPL is significantly altered by full but not intestinal PCSK9 deficiency. Treatment with a PCSK9 monoclonal antibody mimics the effect of PCSK9 deficiency on PPL suggesting that circulating PCSK9 rather than intestinal PCSK9 is a critical regulator of PPL. These data validate the clinical relevance of PCSK9 inhibitors to reduce PPL, especially in patients with type 2 diabetes mellitus.

Proprotein convertase subtilisin kexin 9 is associated with disease activity and is implicated in immune activation in systemic lupus erythematosus

BACKGROUND

Low-density lipoprotein (LDL) levels are increased by proprotein convertase subtilisin kexin 9 (PCSK9) which targets the LDL receptor. We recently reported that PCSK9 ameliorates dendritic cell (DC) activation by oxidized LDL (OxLDL), which is abundant in atherosclerotic plaques and is also associated with cardiovascular disease (CVD) in systemic lupus erythematosus (SLE). Here, we investigated the role of PCSK9 in SLE.

METHODS

PCSK9 levels were determined by ELISA among SLE patients (N = 109) and age- and sex-matched population-based controls (N = 91). Common carotid intima-media thickness (IMT) and plaque occurrence were determined by B-mode ultrasound. Plaques were graded by echogenicity. Human peripheral blood monocytes from SLE patients or controls were differentiated into DCs. The effects of PCSK9 and its inhibition by silencing were studied.

RESULTS

PCSK9 levels were non-significantly higher among SLE-patients compared to controls but significantly associated with SLE disease activity, as determined by the Systemic Lupus Activity Measure (p = 0.020) or the SLE Disease Activity Index (p = 0.0178). There was no association between PCSK9 levels and atherosclerosis as determined by IMT, prevalence of plaques or echolucent (potentially vulnerable) plaques. PCSK9 levels were significantly associated with CVD among SLE patients but not after adjusting for age. OxLDL induced PCSK9 in DCs and DC maturation with increased expression of CD86 and HLA-DR. The effects were significantly stronger in DCs from SLE patients than from controls. Silencing of PCSK9 abolished OxLDL-induced DC maturation.

CONCLUSIONS

PCSK9 is associated with disease activity in SLE. One underlying cause could be OxLDL promoting DC activation which depends on PCSK9. OxLDL induces PCSK9 - an effect which is higher among SLE patients. PCSK9 could play an unexpected immunological role in SLE.

NLRP3 inflammasome via IL-1β regulates PCSK9 secretion

Background: Both PCSK9 and NLRP3 inflammasome play important roles in atherogenesis. This study was designed to test the hypothesis that NLRP3 inflammasome via IL-1β induces PCSK9 secretion. The inter-twined relationship between NLRP3 inflammasome, IL-1β and PCSK9 may be relevant in atherogenesis.

Methods: We studied NLRP3 inflammasome-mediated PCSK9 secretion in mouse peritoneal macrophages and in a variety of tissues, such as liver, kidney and small intestine. Macrophages were derived from wild-type (WT) and a variety of gene deletion mice to define the mechanistic basis of NLRP3 inflammasome -mediated PCSK9 secretion. Additional studies were performed in high-fat diet fed mice.

Results: We observed that NLRP3 and its downstream signals ASC, Caspase-1, IL-18, and IL-1β all participate in PCSK9 secretion. IL-1β seems to be more important than IL-18 in the induction of PCSK9 secretion. Further, there appears to be significant involvement of MAPKs in this process. Lastly, we observed that mice fed high fat diet have high expression of NLRP3 and a greater secretion of PCSK9 than mice fed a standard diet, and this increased secretion of PCSK9 in high fat diet-fed mice was attenuated in IL-1β^-/- mice.

Conclusions: This study based on extensive in vitro and in vivo data provides evidence that NLRP3 inflammasome via IL-1β plays an important role in determining PCSK9 secretion, particularly in the presence of high-fat diet.

Novel strategies to target proprotein convertase subtilisin kexin 9: beyond monoclonal antibodies

Since the discovery of the role of proprotein convertase subtilisin kexin 9 (PCSK9) in the regulation of low-density lipoprotein cholesterol (LDL-C) in 2003, a paradigm shift in the treatment of hypercholesterolaemia has occurred. The PCSK9 secreted into the circulation is a major downregulator of the low-density lipoprotein receptor (LDLR) protein, as it chaperones it to endosomes/lysosomes for degradation. Humans with loss-of-function of PCSK9 exhibit exceedingly low levels of LDL-C and are protected from atherosclerosis. As a consequence, innovative strategies to modulate the levels of PCSK9 have been developed. Since 2015 inhibitory monoclonal antibodies (evolocumab and alirocumab) are commercially available. When subcutaneously injected every 2-4 weeks, they trigger a ∼60% LDL-C lowering and a 15% reduction in the risk of cardiovascular events. Another promising approach consists of a liver-targetable specific PCSK9 siRNA which results in ∼50-60% LDL-C lowering that lasts up to 6 months (Phases II-III clinical trials). Other strategies under consideration include: (i) antibodies targeting the C-terminal domain of PCSK9, thereby inhibiting the trafficking of PCSK9-LDLR to lysosomes; (ii) small molecules that either prevent PCSK9 binding to the LDLR, its trafficking to lysosomes or its secretion from cells; (iii) complete silencing of PCSK9 by CRISPR-Cas9 strategies; (iv) PCSK9 vaccines that inhibit the activity of circulating PCSK9. Time will tell whether other strategies can be as potent and safe as monoclonal antibodies to lower LDL-C levels.

Methotrexate Decreases the Level of PCSK9-A Novel Indicator of the Risk of Proatherogenic Lipid Profile in Psoriasis. The Preliminary Data

Background: Proprotein convertase subtilisin/kexin type 9 (PCSK9) exerts an important role in inflammatory processes, lipids homeostasis, and cardiometabolic disorders that are closely associated with psoriasis. The aim of the study was to analyze the clinical and diagnostic value of serum PCSK9 concentrations and their connections with disease severity, inflammation, metabolic syndrome, and impact of systemic therapies in psoriatic patients. The study enrolled thirty-five patients with active plaque-type psoriasis and eighteen healthy volunteers served as controls. Blood samples were obtained before and after 12 weeks of treatment with methotrexate or acitretin. Serum PCSK9 concentrations were measured by the ELISA (Enzyme-Linked Immunosorbent Assay) commercial kits. Morphological and biochemical parameters were assayed using routine laboratory techniques. Psoriatic patients showed significantly elevated levels of PCSK9 compared to controls (p < 0.01), mostly in patients with a mild and moderate course of psoriasis. PCSK9 concentrations correlated positively with BMI and triglyceride levels (p < 0.05). Interestingly, PCSK9 had a strong negative correlation with low-density lipoprotein levels and total cholesterol (p < 0.05). Three months of monotherapy with methotrexate significantly reduced PCSK9 level (p < 0.05), on the contrary, the acitretin group showed a further increase of PCSK9 levels (p < 0.05). PCSK9 seems to be a novel marker of psoriasis and a putative explanation of lipid disturbances, which are common in patients with psoriasis and are vital for the further developing of metabolic syndrome. Methotrexate should be considered as a treatment of choice in patients with an elevated PCSK9 concentration.

EGF-like and Other Disulfide-rich Microdomains as Therapeutic Scaffolds

Disulfide bonds typically introduce conformational constraints into peptides and proteins, conferring improved biopharmaceutical properties and greater therapeutic potential. In our opinion, disulfide-rich microdomains from proteins are potentially a rich and under-explored source of drug leads. A survey of the UniProt protein database shows that these domains are widely distributed throughout the plant and animal kingdoms, with the EGF-like domain being the most abundant of these domains. EGF-like domains exhibit large diversity in their disulfide bond topologies and calcium binding modes, which we classify in detail here. We found that many EGF-like domains are associated with disease phenotypes, and the interactions they mediate are potential therapeutic targets. Indeed, EGF-based therapeutic leads have been identified, and we further propose that these domains can be optimized to expand their therapeutic potential using chemical design strategies. This Review highlights the potential of disulfide-rich microdomains as future peptide therapeutics.

Cholesterol-Lowering Agents

Loss-of-function variants in PCSK9 (proprotein convertase subtilisin-kexin type 9) are associated with lower lifetime risk of atherosclerotic cardiovascular disease) events. Confirmation of these genetic observations in large, prospective clinical trials in participants with atherosclerotic cardiovascular disease has provided guidance on risk stratification and enhanced our knowledge on hitherto unresolved and contentious issues concerning the efficacy and safety of markedly lowering LDL-C (low-density lipoprotein cholesterol). PCSK9 has a broad repertoire of molecular effects. Furthermore, clinical trials with PCSK9 inhibitors demonstrate that reductions in atherosclerotic cardiovascular disease events are more effective in patients with recent myocardial infarction, multiple myocardial infarctions, multivessel coronary artery disease, and lower extremity arterial disease. The potent LDL-C lowering efficacy of PCSK9 inhibitors provides the opportunity for more aggressive LDL-lowering strategies in high-risk patients with atherosclerotic cardiovascular disease and supports the notion that there is no lower limit for LDL-C. Aggressive LDL-C lowering with fully human PCSK9 monoclonal antibodies has been associated by a safety profile superior to that of other classes of LDL-lowering agents. These clinical trials provide evidence that LDL lowering with PCSK9 inhibitors is an effective therapy for lowering cardiovascular events in high-risk patients with LDL-C levels ≥70 mg/dL on maximally tolerated oral therapies, including statins and ezetimibe.

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)

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.

The Arg499His gain-of-function mutation in the C-terminal domain of PCSK9

BACKGROUND AND AIMS

Familial hypercholesterolemia (FH) is a monogenic disease characterized by high levels of low-density lipoprotein cholesterol and premature atherosclerotic cardiovascular disease. FH is caused by loss of function mutations in genes encoding LDL receptor (LDLR), and Apolipoprotein B (APOB) or gain of function (GOF) mutations in proprotein convertase subtilisin/kexin type 9 (PCSK9). In this study, we identified a novel variant in PCSK9, p.(Arg499His), located in the C-terminal domain, in two unrelated FH patients from Spain and Italy.

METHODS

We studied familial segregation and determined variant activity in vitro.

RESULTS

We determined PCSK9 expression, secretion and activity of the variant in transfected HEK293 cells; extracellular activity of the recombinant p.(Arg499His) PCSK9 variant in HEK 293 and HepG2 cells; PCSK9 affinity to the LDL receptor at neutral and acidic pH; the mechanism of action of the p.(Arg499His) PCSK9 variant by co-transfection with a soluble construct of the LDL receptor and by determining total PCSK9 intracellular accumulation when endosomal acidification is impaired and when an excess of soluble LDLr is present in the culture medium. Our results show high LDL-C concentrations and FH phenotype in p.(Arg499His) carriers. In vitro functional characterization shows that p.(Arg499His) PCSK9 variant causes a reduction in LDLr expression and LDL uptake. An intracellular activity for this variant is also shown when blocking the activity of secreted PCSK9 and by inhibiting endosomal acidification.

CONCLUSIONS

We demonstrated that p.(Arg499His) PCSK9 variant causes a direct intracellular degradation of LDLr therefore causing FH by reducing LDLr availability.