PCSK9 dominant negative mutant results in increased LDL catabolic rate and familial hypobetalipoproteinemia

Unraveling Estrogen and PCSK9's Roles in Lipid Metabolism Disorders among Ovariectomized Mice

We explore the interaction between estrogen and PCSK9 and their collective impact on lipid metabolism, especially concerning the regulation of low-density lipoprotein receptor levels. Utilizing both animal and cellular models, including ovariectomized mice and HepG2 cell lines, we demonstrate that estrogen deficiency leads to a disruption in lipid metabolism, characterized by elevated levels of total cholesterol and LDL-C. The study commences with mice undergoing ovariectomy, followed by a diet regimen comprising either high-fat diet or normal feed for a four-week duration. Key assessments include analyzing lipid metabolism, measuring PCSK9 levels in the bloodstream, and evaluating hepatic low-density lipoprotein receptor expression. We will also conduct correlation analyses to understand the relationship between PCSK9 and various lipid profiles. Further, a subset of ovariectomized mice on high-fat diet will undergo treatment with either estrogen or PCSK9 inhibitor for two weeks, with a subsequent re-evaluation of the earlier mentioned parameters. Our findings reveal that estrogen inhibits PCSK9-mediated degradation of low-density lipoprotein receptor, a process crucial for maintaining lipid homeostasis. Through a series of experiments, including immunohistochemistry and western blot analysis, we establish that PCSK9 is involved in lipid metabolism disorders caused by estrogen deficiency and that estrogen regulates PCSK9 and low-density lipoprotein receptor at post-transcriptional level. The study provides a mechanism for the involvement of PCSK9 in elucidating the disorders of lipid metabolism caused by estrogen deficiency due to perimenopause and ovarian decline.

The role of PCSK9 in heart failure and other cardiovascular diseases-mechanisms of action beyond its effect on LDL cholesterol

Proprotein convertase subtilisin/kexin type-9 (PCSK9) is a protein that regulates low-density lipoprotein (LDL) cholesterol metabolism by binding to the hepatic LDL receptor (LDLR), ultimately leading to its lysosomal degradation and an increase in LDL cholesterol (LDLc) levels. Treatment strategies have been developed based on blocking PCSK9 with specific antibodies (alirocumab, evolocumab) and on blocking its production with small regulatory RNA (siRNA) (inclisiran). Clinical trials evaluating these drugs have confirmed their high efficacy in reducing serum LDLc levels and improving the prognosis in patients with atherosclerotic cardiovascular diseases. Most studies have focused on the action of PCSK9 on LDLRs and the subsequent increase in LDLc concentrations. Increasing evidence suggests that the adverse cardiovascular effects of PCSK9, particularly its atherosclerotic effects on the vascular wall, may also result from mechanisms independent of its effects on lipid metabolism. PCSK9 induces the expression of pro-inflammatory cytokines contributing to inflammation within the vascular wall and promotes apoptosis, pyroptosis, and ferroptosis of cardiomyocytes and is thus involved in the development and progression of heart failure. The elimination of PCSK9 may, therefore, not only be a treatment for hypercholesterolaemia but also for atherosclerosis and other cardiovascular diseases. The mechanisms of action of PCSK9 in the cardiovascular system are not yet fully understood. This article reviews the current understanding of the mechanisms of PCSK9 action in the cardiovascular system and its contribution to cardiovascular diseases. Knowledge of these mechanisms may contribute to the wider use of PCSK9 inhibitors in the treatment of cardiovascular diseases.

Proprotein convertase subtilisin/kexin type 9 deficiency in extrahepatic tissues: emerging considerations

Proprotein convertase subtilisin/kexin type 9 (PCSK9) is primarily secreted by hepatocytes. PCSK9 is critical in liver low-density lipoprotein receptors (LDLRs) metabolism. In addition to its hepatocellular presence, PCSK9 has also been detected in cardiac, cerebral, islet, renal, adipose, and other tissues. Once perceived primarily as a "harmful factor," PCSK9 has been a focal point for the targeted inhibition of both systemic circulation and localized tissues to treat diseases. However, PCSK9 also contributes to the maintenance of normal physiological functions in numerous extrahepatic tissues, encompassing both LDLR-dependent and -independent pathways. Consequently, PCSK9 deficiency may harm extrahepatic tissues in close association with several pathophysiological processes, such as lipid accumulation, mitochondrial impairment, insulin resistance, and abnormal neural differentiation. This review encapsulates the beneficial effects of PCSK9 on the physiological processes and potential disorders arising from PCSK9 deficiency in extrahepatic tissues. This review also provides a comprehensive analysis of the disparities between experimental and clinical research findings regarding the potential harm associated with PCSK9 deficiency. The aim is to improve the current understanding of the diverse effects of PCSK9 inhibition.

PCSK9 and Lipid Metabolism: Genetic Variants, Current Therapies, and Cardiovascular Outcomes

Proprotein convertase subtilisin/kexin type 9 (PCSK9) plays a crucial role in the modulation of lipid metabolism as a critical negative regulator of hepatic low-density lipoprotein receptor (LDLR) levels and circulating low-density lipoprotein (LDL) clearance. Numerous gain-of-function (GOF) mutations in PCSK9 have been identified as causing familial hypercholesterolemia (FH) by reducing LDLR levels, and loss-of-function (LOF) mutations associated with a hypercholesterolemia phenotype protective against atherosclerosis. PCSK9 represents an example of successful translational research resulting in the identification of PCSK9 as a major drug target for a lipid-lowering therapy. To explore the genetic constitution of PCSK9 and its biologic role, in this review, we summarize the current evidence of clinically significant PCSK9 genetic variants involved in lipid metabolism as well as emphasize the importance of PCSK9 inhibition for the improvement of cardiovascular outcomes by conducting a meta-analysis of the available data on the incidence of cardiovascular disease events.

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.

Targeting PCSK9 to tackle cardiovascular disease

Lowering blood cholesterol levels efficiently reduces the risk of developing atherosclerotic cardiovascular disease (ASCVD), including coronary artery disease (CAD), which is the main cause of death worldwide. CAD is caused by plaque formation, comprising cholesterol deposits in the coronary arteries. Proprotein convertase subtilisin kexin/type 9 (PCSK9) was discovered in the early 2000s and later identified as a key regulator of cholesterol metabolism. PCSK9 induces lysosomal degradation of the low-density lipoprotein (LDL) receptor in the liver, which is responsible for clearing LDL-cholesterol (LDL-C) from the circulation. Accordingly, gain-of-function PCSK9 mutations are causative of familial hypercholesterolemia, a severe condition with extremely high plasma cholesterol levels and increased ASCVD risk, whereas loss-of-function PCSK9 mutations are associated with very low LDL-C levels and protection against CAD. Since the discovery of PCSK9, extensive investigations in developing PCSK9 targeting therapies have been performed. The combined delineation of clear biology, genetic risk variants, and PCSK9 crystal structures have been major drivers in developing antagonistic molecules. Today, two antibody-based PCSK9 inhibitors have successfully progressed to clinical application and shown to be effective in reducing cholesterol levels and mitigating the risk of ASCVD events, including myocardial infarction, stroke, and death, without any major adverse effects. A third siRNA-based inhibitor has been FDA-approved but awaits cardiovascular outcome data. In this review, we outline the PCSK9 biology, focusing on the structure and nonsynonymous mutations reported in the PCSK9 gene and elaborate on PCSK9-lowering strategies under development. Finally, we discuss future perspectives with PCSK9 inhibition in other severe disorders beyond cardiovascular disease.

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.

Rosa26-LSL-dCas9-VPR: a versatile mouse model for tissue specific and simultaneous activation of multiple genes for drug discovery

Transgenic animals with increased or abrogated target gene expression are powerful tools for drug discovery research. Here, we developed a CRISPR-based Rosa26-LSL-dCas9-VPR mouse model for targeted induction of endogenous gene expression using different Adeno-associated virus (AAV) capsid variants for tissue-specific gRNAs delivery. To show applicability of the model, we targeted low-density lipoprotein receptor (LDLR) and proprotein convertase subtilisin/kexin type 9 (PCSK9), either individually or together. We induced up to ninefold higher expression of hepatocellular proteins. In consequence of LDLR upregulation, plasma LDL levels almost abolished, whereas upregulation of PCSK9 led to increased plasma LDL and cholesterol levels. Strikingly, simultaneous upregulation of both LDLR and PCSK9 resulted in almost unaltered LDL levels. Additionally, we used our model to achieve expression of all α₁-Antitrypsin (AAT) gene paralogues simultaneously. These results show the potential of our model as a versatile tool for optimized targeted gene expression, alone or in combination.

Inclisiran: How Widely and When Should We Use It?

PURPOSE OF REVIEW

Plasma levels of LDL cholesterol (LDL-C) are causally associated with cardiovascular risk. Reducing LDL-C results in a decreased incidence of cardiovascular events, proportionally to the absolute reduction in LDL-C. The inhibition of proprotein convertase subtilisin kexin 9 (PCSK) is a highly effective and safe approach to reducing LDL-C levels. In this review, we discuss the available data on the efficacy and safety of inclisiran, a siRNA targeting PCSK9 and propose a clinical profile for the patients who can benefit the most from this approach.

RECENT FINDINGS

Inclisiran is a small interfering RNA targeting the mRNA of PCSK9 specifically in the liver, owing to the conjugation with triantennary N-acetylgalactosamine. Randomized clinical trials have shown that inclisiran provides robust and durable reductions of PCSK9 and LDL-C levels, with a dosing schedule of once every 6 months after the initial and 3-month doses. These effects are consistent in different categories of patients, including patients with atherosclerotic cardiovascular disease and/or risk equivalent or patients with heterozygous familial hypercholesterolaemia. Ultimately the administration schedule may improve patients' compliance given also the favourable safety profile of the drug. Completion of ongoing outcome clinical trials will provide information on both the expected clinical benefit and the safety of inclisiran administered for longer.

Novel protein-truncating variant in the APOB gene may protect from coronary artery disease and adverse cardiovascular events

Background and aims

Genetic testing is still rarely used for the diagnosis of dyslipidemia, even though gene variants determining plasma lipids levels are not uncommon.

Methods

Starting from a a pilot-analysis of targeted Next Generation Sequencing (NGS) of 5 genes related to familial hypercholesterolemia (LDLR, APOB, PCSK9, HMGCR, APOE) within a cardiovascular cohort in subjects with extreme plasma concentrations of low-density lipoprotein (LDL) cholesterol, we discovered and characterized a novel point mutation in the APOB gene, which was associated with very low levels of apolipoprotein B (ApoB) and LDL cholesterol.

Results

APOB c.6943 G > T induces a premature stop codon at the level of exon 26 in the APOB gene and generates a protein which has the 51% of the mass of the wild type ApoB-100 (ApoB-51), with a truncation at the level of residue 2315. The premature stop codon occurs after the one needed for the synthesis of ApoB-48, allowing chylomicron production at intestinal level and thus avoiding potential nutritional impairments. The heterozygous carrier of APOB c.6943G > T, despite a very high-risk profile encompassing all the traditional risk factors except for dyslipidemia, had normal coronary arteries by angiography and did not report any major adverse cardiovascular event during a 20-years follow-up, thereby obtaining advantage from the gene variant as regards protection against atherosclerosis, apparently without any metabolic retaliation.

Conclusions

Our data support the use of targeted NGS in well-characterized clinical settings, as well as they indicate that.a partial block of ApoB production may be well tolerated and improve cardiovascular outcomes.

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.

Pcsk9 Deletion Promotes Murine Nonalcoholic Steatohepatitis and Hepatic Carcinogenesis: Role of Cholesterol

Proprotein convertase subtilisin/kexin type 9 (Pcsk9) binds to hepatic low-density lipoprotein receptor (LDLR) and induces its internalization and degradation. Pcsk9 inhibition increases LDLR expression by hepatocytes, which causes increased uptake of circulating LDL, thereby reducing plasma LDL-cholesterol. However, by increasing the uptake of LDL by the liver, Pcsk9 inhibition increases the exposure of the liver to cholesterol, which may result in higher risk of steatohepatitis and ever carcinogenesis. We compared Pcsk9-/- knockout (KO) mice and appropriate wild-type (WT) controls of the same strain assigned to a high-fat (15%, wt/wt) diet for 9 months supplemented with 0.25%, 0.5%, or 0.75% dietary cholesterol. Pcsk9 KO mice on a high-fat, high-cholesterol diet exhibited higher levels of hepatic free cholesterol loading and hepatic cholesterol crystallization than their WT counterparts. Pcsk9 KO mice developed crown-like structures of macrophages surrounding cholesterol crystal-containing lipid droplets and hepatocytes, exhibited higher levels of apoptosis, and developed significantly more hepatic inflammation and fibrosis consistent with fibrosing steatohepatitis, including 5-fold and 11-fold more fibrosis at 0.5% and 0.75% dietary cholesterol, respectively. When injected with diethylnitrosamine, a hepatic carcinogen, early-in-life Pcsk9 KO mice were more likely to develop liver cancer than WT mice. Conclusion: Pcsk9 KO mice on high-cholesterol diets developed increased hepatic free cholesterol and cholesterol crystals and fibrosing steatohepatitis with a higher predisposition to liver cancer compared with WT mice. Future studies should evaluate whether patients on long-term treatment with anti-PSCK9 monoclonal antibodies are at increased risk of hepatic steatosis, steatohepatitis or liver cancer, while accounting for concurrent use of statins.

The Emerging Roles of Intracellular PCSK9 and Their Implications in Endoplasmic Reticulum Stress and Metabolic Diseases

The importance of the proprotein convertase subtilisin/kexin type-9 (PCSK9) gene was quickly recognized by the scientific community as the third locus for familial hypercholesterolemia. By promoting the degradation of the low-density lipoprotein receptor (LDLR), secreted PCSK9 protein plays a vital role in the regulation of circulating cholesterol levels and cardiovascular disease risk. For this reason, the majority of published works have focused on the secreted form of PCSK9 since its initial characterization in 2003. In recent years, however, PCSK9 has been shown to play roles in a variety of cellular pathways and disease contexts in LDLR-dependent and -independent manners. This article examines the current body of literature that uncovers the intracellular and LDLR-independent roles of PCSK9 and also explores the many downstream implications in metabolic diseases.

Differential Effects of DPP-4 Inhibitors, Anagliptin and Sitagliptin, on PCSK9 Levels in Patients with Type 2 Diabetes Mellitus who are Receiving Statin Therapy

Aim: Proprotein convertase subtilisin/kexin type 9 (PCSK9) degrades the low-density lipoprotein (LDL) receptor, leading to hypercholesterolemia and cardiovascular risk. Treatment with a statin leads to a compensatory increase in circulating PCSK9 level. Anagliptin, a dipeptidyl peptidase-4 (DPP-4) inhibitor, was shown to decrease LDL cholesterol (LDL-C) levels to a greater extent than that by sitagliptin, another DPP-4 inhibitor, in the Randomized Evaluation of Anagliptin versus Sitagliptin On low-density lipoproteiN cholesterol in diabetes (REASON) trial. We investigated PCSK9 concentration in type 2 diabetes mellitus (T2DM) and the impact of treatment with anagliptin or sitagliptin on PCSK9 level as a sub-analysis of the REASON trial.

Methods: PCSK9 concentration was measured at baseline and after 52 weeks of treatment with anagliptin ( n =122) or sitagliptin ( n =128) in patients with T2DM who were receiving statin therapy. All of the included patients had been treated with a DPP-4 inhibitor prior to randomization.

Results: Baseline PCSK9 level was positively, but not significantly, correlated with LDL-C and was independently associated with platelet count and level of triglycerides. Concomitant with reduction of LDL-C, but not hemoglobin A1c (HbA1c), by anagliptin, PCSK9 level was significantly increased by treatment with sitagliptin (218±98 vs. 242±115 ng/mL, P =0.01), but not anagliptin (233±97 vs. 250±106 ng/mL, P =0.07).

Conclusions: PCSK9 level is independently associated with platelet count and level of triglycerides, but not LDL-C, in patients with T2DM. Anagliptin reduces LDL-C level independent of HbA1c control in patients with T2DM who are on statin therapy possibly by suppressing excess statin-mediated PCSK9 induction and subsequent degradation of the LDL receptor.

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.

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.

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.

New Insights Into the Regulation of Lipoprotein Metabolism by PCSK9: Lessons From Stable Isotope Tracer Studies in Human Subjects

Proprotein convertase subtilisin/kexin type 9 (PCSK9) is a convertase enzyme mostly produced by the liver. It is a key regulator of LDL metabolism because of its ability to enhance degradation of the LDL receptor. PCSK9 also regulates the metabolism of lipoprotein(a) [Lp(a)] and triglyceride-rich lipoproteins (TRLs). Its key role in modulating atherosclerotic cardiovascular disease (ASCVD) is supported by genetic studies and clinical outcome trials. Kinetic studies provide mechanistic insight into the role of PCSK9 in regulating the physiology and pathophysiology of plasma lipids and lipoproteins. Kinetic data have demonstrated that plasma PCSK9 concentration is inversely associated with the clearance of LDL in men. Gain-of-function mutations of PCSK9 markedly increase plasma LDL-cholesterol concentrations due to impaired LDL-apoB catabolism. Conversely, PCSK9 deficiency results in low LDL-cholesterol associated with enhanced LDL-apoB clearance. Inhibition of PCSK9 with monoclonal antibodies (such as evolocumab or alirocumab) lowers plasma LDL-cholesterol and apoB levels chiefly by upregulating the catabolism of LDL particles in healthy individuals. As monotherapy, PCSK9 inhibitor reduced Lp(a) concentrations by decreasing the production rate. However, as combination therapy, it reduced the plasma concentration of Lp(a) by increasing the fractional catabolism of Lp(a) particles. In statin-treated patients with high Lp(a), PCSK9 inhibition lowers plasma Lp(a) concentrations by accelerating the catabolism of Lp(a) particles. The effect of PCSK9 inhibition on TRL metabolism has been studied in healthy individuals and in patients with type 2 diabetes. These findings suggest that PCSK9 appears to play a less important role in TRL than LDL metabolism. Kinetic studies of PCSK9 inhibition therapy on lipoprotein metabolism in diverse high risk patient populations (such as familial hypercholesterolemia) and new therapeutic combination also merit further investigation.

Phenotypic Differences Between Polygenic and Monogenic Hypobetalipoproteinemia

OBJECTIVE

Primary hypobetalipoproteinemia is characterized by LDL-C (low-density lipoprotein cholesterol) concentrations below the fifth percentile. Primary hypobetalipoproteinemia mostly results from heterozygous mutations in the APOB (apolipoprotein B) and PCSK9 genes, and a polygenic origin is hypothesized in the remaining cases. Hypobetalipoproteinemia patients present an increased risk of nonalcoholic fatty liver disease and steatohepatitis. Here, we compared hepatic alterations between monogenic, polygenic, and primary hypobetalipoproteinemia of unknown cause. Approach and Results: Targeted next-generation sequencing was performed in a cohort of 111 patients with hypobetalipoproteinemia to assess monogenic and polygenic origins using an LDL-C-dedicated polygenic risk score. Forty patients (36%) had monogenic hypobetalipoproteinemia, 38 (34%) had polygenic hypobetalipoproteinemia, and 33 subjects (30%) had hypobetalipoproteinemia from an unknown cause. Patients with monogenic hypobetalipoproteinemia had lower LDL-C and apolipoprotein B plasma levels compared with those with polygenic hypobetalipoproteinemia. Liver function was assessed by hepatic ultrasonography and liver enzymes levels. Fifty-nine percent of patients with primary hypobetalipoproteinemia presented with liver steatosis, whereas 21% had increased alanine aminotransferase suggestive of liver injury. Monogenic hypobetalipoproteinemia was also associated with an increased prevalence of liver steatosis (81% versus 29%, P<0.001) and liver injury (47% versus 0%) compared with polygenic hypobetalipoproteinemia.

CONCLUSIONS

This study highlights the importance of genetic diagnosis in the clinical care of primary hypobetalipoproteinemia patients. It shows for the first time that a polygenic origin of hypobetalipoproteinemia is associated with a lower risk of liver steatosis and liver injury versus monogenic hypobetalipoproteinemia. Thus, polygenic risk score is a useful tool to establish a more personalized follow-up of primary hypobetalipoproteinemia patients.

PCSK9 Variants in Familial Hypercholesterolemia: A Comprehensive Synopsis

Autosomal dominant familial hypercholesterolemia (FH) affects approximately 1/250, individuals and potentially leads to elevated blood cholesterol and a significantly increased risk of atherosclerosis. Along with improvements in detection and the increased early diagnosis and treatment, the serious burden of FH on families and society has become increasingly apparent. Since FH is strongly associated with proprotein convertase subtilisin/kexin type 9 (PCSK9), increasing numbers of studies have focused on finding effective diagnostic and therapeutic methods based on PCSK9. At present, as PCSK9 is one of the main pathogenic FH genes, its contribution to FH deserves more explorative research.

The history of proprotein convertase subtilisin kexin-9 inhibitors and their role in the treatment of cardiovascular disease

A consensus has formed based on epidemiological studies and clinical trials that intervention to reduce low density lipoprotein cholesterol (LDL-C) will reduce cardiovascular disease (CVD) events. This has progressively reduced the thresholds for intervention and targets for treatment. Whist statins are sufficient for many people in primary prevention, they only partially achieve the newer targets of secondary prevention for established CVD. Increasing use of statins has highlighted that 1–2% cannot tolerate these drugs. Other cholesterol-lowering drugs such as ezetimibe add to the benefits of statins but have limited efficacy. The discovery of activating mutations in proprotein convertase subtilisin kexin-9 (PCSK9) as a cause of familial hypercholesterolaemia while inactivating mutations lower LDL-C led to the idea to develop PCSK9 inhibitors as drugs. This article reviews the history of lipid-lowering therapies, the discovery of PCSK9 and the development of PCSK9 inhibitors. It reviews the key trials of the current antibody-based drugs and how these have influenced new guidelines. It also reviews the controversy caused by their cost and the increasing application of health economics to determine the optimum strategy for implementation of novel therapeutic pathways and surveys other options for targeting PCSK9 as well as other LDL-C lowering compounds in late development.

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.

Circulating PCSK9 is associated with liver biomarkers and hepatic steatosis

BACKGROUND

In parallel to the increasing prevalence of metabolic syndrome, the prevalence of hepatic steatosis has also increased dramatically worldwide. Hepatic steatosis is a major risk factor of hepatic cirrhosis, cardiovascular disease and type 2 diabetes. Circulating levels of proprotein convertase subtilisin/kexin type 9 (PCSK9) have been positively associated with the metabolic syndrome. However, the association between PCSK9 and the liver function is still controversial.

OBJECTIVE

The objective of this study is to investigate the association between circulating PCSK9 levels and the presence of hepatic steatosis, as well as with liver biomarkers in a cohort of healthy individuals.

METHODS

Total PCSK9 levels were measured by an in-house ELISA using a polyclonal antibody. Plasma albumin, alkaline phosphatase, ALT, AST, total bilirubin and GGT were measured in 698 individuals using the COBAS system. The presence of hepatic steatosis was assessed using ultrasound liver scans.

RESULTS

In a multiple regression model adjusted for age, sex, insulin resistance, body mass index and alcohol use, circulating PCSK9 level was positively associated with albumin (β = 0.102, P = 0.008), alkaline phosphatase (β = 0.201, P < 0.0001), ALT (β = 0.238, P < 0.0001), AST (β = 0.120, P = 0.003) and GGT (β = 0.103, P = 0.007) and negatively associated with total bilirubin (β = -0.150, P < 0.0001). Tertile of circulating PCSK9 was also associated with hepatic steatosis (OR 1.48, 95% CI 1.05-2.08, P = 0.02).

CONCLUSION

Our data suggest a strong association between PCSK9 and liver biomarkers as well as hepatic steatosis. Further studies are needed to explore the role of PCSK9 on hepatic function.

VLDL (Very-Low-Density Lipoprotein)-Apo E (Apolipoprotein E) May Influence Lp(a) (Lipoprotein [a]) Synthesis or Assembly

Objective:

To clarify the association between PCSK9 (proprotein convertase subtilisin/kexin type 9) and Lp(a) (lipoprotein [a]), we studied Lp(a) kinetics in patients with loss-of-function and gain-of-function PCSK9 mutations and in patients in whom extended-release niacin reduced Lp(a) and PCSK9 concentrations.

Approach and Results:

Six healthy controls, 9 heterozygous patients with familial hypercholesterolemia (5 with low-density lipoprotein receptor [ LDLR ] mutations and 4 with PCSK9 gain-of-function mutations) and 3 patients with heterozygous dominant-negative PCSK9 loss-of-function mutations were included in the preliminary study. Eight patients were enrolled in a second study assessing the effects of 2 g/day extended-release niacin. Apolipoprotein kinetics in VLDL (very-low-density lipoprotein), LDL (low-density lipoprotein), and Lp(a) were studied using stable isotope techniques. Plasma Lp(a) concentrations were increased in PCSK9 -gain-of-function and familial hypercholesterolemia- LDLR groups compared with controls and PCSK9 -loss-of-function groups (14±12 versus 5±4 mg/dL; P =0.04), but no change was observed in Lp(a) fractional catabolic rate. Subjects with PCSK9 -loss-of-function mutations displayed reduced apoE (apolipoprotein E) concentrations associated with a VLDL-apoE absolute production rate reduction. Lp(a) and VLDL-apoE absolute production rates were correlated ( r =0.50; P <0.05). ApoE-to-apolipoprotein (a) molar ratios in Lp(a) increased with plasma Lp(a) ( r =0.96; P <0.001) but not with PCSK9 levels. Extended-release niacin-induced reductions in Lp(a) and VLDL-apoE absolute production rate were correlated ( r =0.83; P =0.015). In contrast, PCSK9 reduction (−35%; P =0.008) was only correlated with that of VLDL-apoE absolute production rate ( r =0.79; P =0.028).

Conclusions:

VLDL-apoE production could determine Lp(a) production and/or assembly. As PCSK9 inhibitors reduce plasma apoE and Lp(a) concentrations, apoE could be the link between PCSK9 and Lp(a).

A Healthy Family of Familial Hypobetalipoproteinemia Caused by a Protein-truncating Variant in the PCSK9 Gene

We present the first case of a Japanese patient with familial hypobetalipoproteinemia (FHBL) caused by a protein-truncating variant in the proprotein convertase subtilisin/kexin type 9 (PCSK9) gene. A 34-year-old woman was referred to our hospital due to her low low-density lipoprotein (LDL)-cholesterolemia (34 mg/dL). She did not have any secondary causes of hypobetalipoproteinemia. Her father and her younger sister also exhibited low LDL cholesterol levels. We identified a protein-truncating variant in the PCSK9 gene (c.1090_1091del/p.Pro364ArgfsTer62) among them. None of them exhibited atherosclerotic cardiovascular diseases nor any other complications associated with low LDL cholesterol, including fatty liver, neurocognitive disorders, or cerebral hemorrhaging.

Rare Diseases Related with Lipoprotein Metabolism

Rare diseases are gathering increasing attention in last few years, not only for its effects on innovation scientific research, but also for its propounding influence on common diseases. One of the most famous milestones made by Michael Brown and Joseph Goldstein in metabolism field is the discovery of the defective gene in familial hypercholesterolemia, a rare human genetic disease manifested with extreme high level of serum cholesterol (Goldstein JL, Brown MS, Proc Natl Acad Sci USA 70:2804-2808, 1973; Brown MS, Dana SE, Goldstein JL, J Biol Chem 249:789-796, 1974). Follow-up work including decoding the gene function, mapping-related pathways, and screening therapeutic targets are all based on the primary finding (Goldstein JL, Brown MS Arterioscler Thromb Vasc Biol 29:431-438, 2009). A series of succession win the two brilliant scientists the 1985 Nobel Prize, and bring about statins widely used for lipid management and decreasing cardiovascular disease risks. Translating the clinical extreme phenotypes into laboratory bench work has turned out to be the first important step in the paradigm conducting translational and precise medical research. Here we review the main categories of rare disorders related with lipoprotein metabolism, aiming to strengthen the notion that human rare inheritable genetic diseases would be the window to know ourselves better, to treat someone more efficiently, and to lead a healthy life longer. Few rare diseases related with lipoprotein metabolism were clustered into six sections based on changes in lipid profile, namely, hyper- or hypocholesterolemia, hypo- or hyperalphalipoproteinemia, abetalipoproteinemia, hypobetalipoproteinemia, and sphingolipid metabolism diseases. Each section consists of a brief introduction, followed by a summary of well-known disease-causing genes in one table, and supplemented with one or two diseases as example for detailed description. Here we aimed to raise more attention on rare lipoprotein metabolism diseases, calling for more work from basic research and clinical trials.

Type 2 diabetes-associated single nucleotide polymorphism in Sorcs1 gene results in alternative processing of the Sorcs1 protein in INS1 β-cells

A threonine-to-Isoleucine (Thr₅₂Ile) mutation within the pro-domain of the Sorcs1 gene was positionally cloned as the gene underlying a quantitative trait locus that affects fasting insulin levels in mice. In humans, genome-wide association studies and linkage studies have shown that SORCS1 is associated with diabetes and all of diabetes complications. We have recently shown that deletion of Sorcs1 in mice made obese with the leptin^ob mutation results in diabetes and an insulin granule stability defect. This present study investigates the functional consequence of the Sorcs1 Thr₅₂Ile mutation in the rat INS1 β-cell line expressing either the wildtype or mutant Sorcs1 allele. We find that Sorcs1 Thr₅₂Ile mutation is associated with increased basal insulin secretion, reduced glucose-stimulated insulin secretion and decreased insulin content in INS1 cells. Moreover, expression of Thr₅₂Ile causes differential processing of the Sorcs1 protein resulting in the formation of an additional 90 kDa mutant form of the protein. The mutant form of the protein is localised to the ER, retains its pro-domain, and concurrently reduces expression of the functional mature 130 kDa Sorcs1 protein. These findings provide a mechanistic clue to why this specific allelic variation in Sorcs1 was associated with reduced insulin levels and type 2 diabetes.

PCSK9 inhibition as a novel therapeutic target for alcoholic liver disease

Alcoholic liver disease (ALD) causes significant morbidity and mortality, and pharmacological treatment options are limited. In this study, we evaluated the PCSK9 inhibitor alirocumab, a monoclonal antibody that robustly reduces low-density lipoprotein cholesterol (LDL-C), for the treatment of ALD using a rat model of chronic alcohol exposure. Alirocumab (50 mg/kg) or vehicle was administered weekly for 6 weeks to rats receiving a 12% alcohol liquid diet or an isocaloric control diet. At the end of the alcohol exposure protocol, serum and liver samples were obtained for molecular characterization and histopathological analysis. PCSK9 inhibition with alirocumab attenuated alcohol-induced hepatic triglyceride accumulation through regulation of lipid metabolism (mRNA expression of modulators of fatty acid synthesis (FAS) and catabolism (PPARα and CPT1)), hepatocellular injury (ALT), hepatic inflammation (mRNA expression of pro-inflammatory cytokines/chemokines (TNFa, IL-1β, IL-22, IL-33, IL-17α, IL-2, MIP-2, and MCP-1), and neutrophil infiltration (myeloperoxidase staining)). Alirocumab treatment also attenuated alcohol-induced PCSK9 mRNA elevation and upregulated LDL-receptor (LDL-R) via modulation of the transcription factors (SREBP-1, SREBP-2, and E2F1) in liver. We demonstrated that chronic anti-PCSK9 treatment using the monoclonal antibody alirocumab attenuated alcohol-induced steatohepatitis in the rat model. Given the large unmet clinical need for effective and novel treatments for ALD, anti-PCSK9 treatment with the monoclonal antibody that spares liver metabolism is a viable new therapeutic possibility. Future studies are needed to elucidate the exact role of PCSK9 in ALD and alcohol use disorder (AUD) and to evaluate efficacy and safety of anti-PCSK9 treatment in clinical populations with ALD/AUD.

Anti-PCSK9 treatment: is ultra-low low-density lipoprotein cholesterol always good?

Anti-PCSK9 (proprotein convertase subtilisin kexin 9) monoclonal antibodies (Mab) are novel, potent lipid-lowering drugs. They demonstrated to improve the lipid profile in high cardiovascular risk patients. Anti-PCSK9 Mab inhibit the targeted low-density lipoprotein (LDL)-receptor degradation induced by PCSK9 protein and are able to reduce LDL cholesterol (LDL-C) levels on top of conventional lipid-lowering therapy. Though these drugs proved to be very safe in the short-term, little is known about the possible long-term effects, due to the short period of their marketing. The genetic low cholesterol syndromes (LCS) represent the natural models of the lipid-lowering anti-PCSK9 therapy, and a valuable opportunity to predict the long-term effects of these drugs. By looking at the clinical features of such models, we could be able to foresee possible drug-induced side effects. In the present review, the correspondences and discordances between the side effects of anti-PCSK9 therapy and the corresponding LCS models will be examined in the attempt to forecast possible long-term consequences of these novel lipid-lowering agents.

N-Glycosylation Defects in Humans Lower Low-Density Lipoprotein Cholesterol Through Increased Low-Density Lipoprotein Receptor Expression

BACKGROUND

The importance of protein glycosylation in regulating lipid metabolism is becoming increasingly apparent. We set out to further investigate this by studying patients with type I congenital disorders of glycosylation (CDGs) with defective N-glycosylation.

METHODS

We studied 29 patients with the 2 most prevalent types of type I CDG, ALG6 (asparagine-linked glycosylation protein 6)-deficiency CDG and PMM2 (phosphomannomutase 2)-deficiency CDG, and 23 first- and second-degree relatives with a heterozygous mutation and measured plasma cholesterol levels. Low-density lipoprotein (LDL) metabolism was studied in 3 cell models-gene silencing in HepG2 cells, patient fibroblasts, and patient hepatocyte-like cells derived from induced pluripotent stem cells-by measuring apolipoprotein B production and secretion, LDL receptor expression and membrane abundance, and LDL particle uptake. Furthermore, SREBP2 (sterol regulatory element-binding protein 2) protein expression and activation and endoplasmic reticulum stress markers were studied.

RESULTS

We report hypobetalipoproteinemia (LDL cholesterol [LDL-C] and apolipoprotein B below the fifth percentile) in a large cohort of patients with type I CDG (mean age, 9 years), together with reduced LDL-C and apolipoprotein B in clinically unaffected heterozygous relatives (mean age, 46 years), compared with 2 separate sets of age- and sex-matched control subjects. ALG6 and PMM2 deficiency led to markedly increased LDL uptake as a result of increased cell surface LDL receptor abundance. Mechanistically, this outcome was driven by increased SREBP2 protein expression accompanied by amplified target gene expression, resulting in higher LDL receptor protein levels. Endoplasmic reticulum stress was not found to be a major mediator.

CONCLUSIONS

Our study establishes N-glycosylation as an important regulator of LDL metabolism. Given that LDL-C was also reduced in a group of clinically unaffected heterozygotes, we propose that increasing LDL receptor-mediated cholesterol clearance by targeting N-glycosylation in the LDL pathway may represent a novel therapeutic strategy to reduce LDL-C and cardiovascular disease.

Changes in circulating pro-protein convertase subtilisin/kexin type 9 levels - experimental and clinical approaches with lipid-lowering agents

Regulation of pro-protein convertase subtilisin/kexin type 9 (PCSK9) by drugs has led to the development of a still small number of agents with powerful activity on low-density lipoprotein cholesterol levels, associated with a significant reduction of cardiovascular events in patients in secondary prevention. The Further Cardiovascular Outcomes Research with PCSK9 Inhibition in Subjects with Elevated Risk (FOURIER) and Evaluation of Cardiovascular Outcomes After an Acute Coronary Syndrome During Treatment With Alirocumab (ODYSSEY OUTCOMES) studies, with the two available PCSK9 antagonists, i.e. evolocumab and alirocumab, both reported a 15% reduction in major adverse cardiovascular events. Regulation of PCSK9 expression is dependent upon a number of factors, partly genetic and partly associated to a complex transcriptional system, mainly controlled by sterol regulatory element binding proteins. PCSK9 is further regulated by concomitant drug treatments, particularly by statins, enhancing PCSK9 secretion but decreasing its stimulatory phosphorylated form (S688). These complex transcriptional mechanisms lead to variable circulating levels making clinical measurements of plasma PCSK9 for cardiovascular risk assessment a debated matter. Determination of total PCSK9 levels may provide a diagnostic tool for explaining an apparent resistance to PCSK9 inhibitors, thus indicating the need for other approaches. Newer agents targeting PCSK9 are in clinical development with a major interest in those with a longer duration of action, e.g. RNA silencing, allowing optimal patient compliance. Interest has been expanded to areas not only limited to low-density lipoprotein cholesterol reduction but also investigating other non-lipid pathways raising cardiovascular risk, in particular inflammation associated to raised high-sensitivity C-reactive protein levels, not significantly affected by the present PCSK9 antagonists.

Inhibiting PCSK9 - biology beyond LDL control

Clinical trials have unequivocally shown that inhibition of proprotein convertase subtilisin/kexin type 9 (PCSK9) efficaciously and safely prevents cardiovascular events by lowering levels of LDL cholesterol. PCSK9 in the circulation is derived mainly from the liver, but the protein is also expressed in the pancreas, the kidney, the intestine and the central nervous system. Although PCSK9 modulates cholesterol metabolism by regulating LDL receptor expression in the liver, in vitro and in vivo studies have suggested that PCSK9 is involved in various other physiological processes. Although therapeutic PCSK9 inhibition could theoretically have undesired effects by interfering with these non-cholesterol-related processes, studies of individuals with genetically determined reduced PCSK9 function and clinical trials of PCSK9 inhibitors have not revealed clinically meaningful adverse consequences of almost completely eradicating PCSK9 from the circulation. The clinical implications of PCSK9 functions beyond lipid metabolism in terms of wanted or unwanted effects of therapeutic PCSK9 inhibition therefore appear to be limited. The objective of this Review is to describe the physiological role of PCSK9 beyond the LDL receptor to provide a rational basis for monitoring the effects of PCSK9 inhibition as these drugs gain traction in the clinic.

The effects of proprotein convertase subtilisin/kexin type 9 inhibitors on lipid metabolism and cardiovascular function

Low density lipoprotein cholesterol (LDL-C) is a well-established risk factor for cardiovascular disease. Although there are several developed lipid lowering drugs such as statins and fenofibrates, many patients do not achieve an adequate response. Recently, proprotein convertase subtilisin/kexin type 9 (PCSK9) inhibitors have been developed as a new therapeutic strategy for cholesterol regulation. PCSK9 binds to low density lipoprotein receptors (LDLR) and initiates LDLR degradation, elevating LDL-C. Therefore, PCSK9 inhibition could exert beneficial effects on cardiovascular disease outcomes. This review comprehensively summarizes and discusses the effects of PCSK9 inhibitors on lipid metabolism and cardiovascular function comparatively with current lipid lowering drugs. This review also details essential information regarding the cardiovascular benefits of PCSK9 inhibition which could encourage further clinical studies.

Prevalence of hypobetalipoproteinemia and related psychiatric characteristics in a psychiatric population: results from the retrospective HYPOPSY Study

BACKGROUND

Hypobetalipoproteinemia (HBL) is defined by plasma concentrations of LDL-cholesterol (LDL-C) lower than the fifth percentile for age and sex. Several psychiatric symptoms have been reported in association with HBL. The objective was to assess the prevalence of primary HBL in patients hospitalized in a psychiatric population and to better characterize the related psychiatric disorders.

METHODS

HYPOPSY is a retrospective study including 839 adults hospitalized in the Psychiatry department of Nantes University Hospital during the year 2014, except patients with eating disorders. The prevalence of primary HBL was defined by a plasma LDL-C concentration ≤ 50 mg/dL. Secondary causes of HBL were excluded after a review of medical records (n=2). Related-psychiatric disorders in patients with and without HBL were recorded using the ICD-10 classification.

RESULTS

Twenty cases of primary HBL (mean [SD] LDL-C: 42 [7] mg/dL) were diagnosed, leading to a prevalence of 2.39%. In comparison, the prevalence of HBL in a healthy control population was 0.57%. Psychiatric patients with HBL were characterized by a higher frequency of schizophrenia (p=0.044), hetero-aggression (p=0.015) and pervasive and specific developmental disorders (including autism) (p=0.011).

CONCLUSIONS

The prevalence of HBL is 4-fold higher in psychiatric than in general population. More specifically, some statistically significant associations were found between low LDL-C concentrations and schizophrenia, autism and hetero-aggression. These data reinforce the hypothesis for a link between genetically low LDL-C levels and psychiatric disorders.

Pleiotropic effects of proprotein convertase subtilisin/kexin type 9 inhibitors?

PURPOSE OF REVIEW

Current data suggest that proprotein convertase subtilisin/kexin type 9 (PCSK9) inhibitors may affect many metabolic pathways beyond lowering LDL cholesterol. The aim of the present manuscript is to present these so-called pleiotropic effects of PCSK9 inhibitors.

RECENT FINDINGS

PCSK9 may affect the activity of other receptors beyond LDL receptors (LDLR), such as cluster of differentiation 36 (CD36), very-low-density-lipoprotein (VLDL) receptors, apolipoprotein (Apo) E receptors, LDLR-related protein 1 (LRP-1) and ATP-Binding Cassette Transporter (ABCA1). Thus, a role of PCSK9 in the development of atherosclerosis, in vascular wall inflammation and in platelet function has been suggested. Additionally, PCSK9 inhibitors may affect lipid variables beyond LDL cholesterol, carbohydrate variables, as well as they may affect brain and kidney function. Additionally, a controversial role of PCSK9 in sepsis, hepatitis C infection and Alzheimer's disease has been suggested.

SUMMARY

These possible pleiotropic effects of PCSK9 inhibitors need further research, as they may affect cardiovascular risk and provide further insights in the development of atherosclerosis and other diseases such as Alzheimer's disease or chronic viral infection and sepsis.

Beyond LDL: What Role for PCSK9 in Triglyceride-Rich Lipoprotein Metabolism?

Elevated plasma triglyceride (TG) levels are an independent risk factor for cardiovascular disease (CVD). Proprotein convertase subtilisin-kexin 9 (PCSK9) - a protein therapeutically targeted to lower plasma cholesterol levels - might regulate plasma TG-rich lipoprotein (TRL) levels. We provide a timely and critical review of the current evidence for a role of PCSK9 in TRL metabolism by assessing the impact of PCSK9 gene variants, by reviewing recent clinical data with PCSK9 inhibitors, and by describing the potential mechanisms by which PCSK9 might regulate TRL metabolism. We conclude that the impact of PCSK9 on TRL metabolism is relatively modest, especially compared to its impact on cholesterol metabolism.

Poststatin era in atherosclerosis management: lessons from epidemiologic and genetic studies

PURPOSE OF REVIEW

Cardiovascular diseases (CVD) are the leading cause of death worldwide with over 17 million deaths every year and represent a major public health challenge. The last decade has seen the emergence of novel antiatherogenic therapies.

RECENT FINDINGS

Despite intensive lipid and blood pressure interventions, the burden of CVD is expected to markedly progress because of the global aging of the population and increasing exposure to detrimental lifestyle-related risk. Epidemiologic and genetic studies helped to better apprehend the biology of atherosclerosis and allowed pharmaceutical innovation and recent translational successes. This includes the development of novel lipid and glucose-lowering therapies and the leverage of anti-inflammatory therapies.

SUMMARY

Here, we discuss promises and expectations of emerging scientific and pharmaceutical innovations and translational successes to meet the global therapeutic demand.

Lipid Lowering Therapy and Circulating PCSK9 Concentration

Hypercholesterolemia, particularly an increase in low-density lipoprotein cholesterol (LDL-C) levels, contributes substantially to the development of coronary artery disease and the risk for cardiovascular events. As the first-line pharmacotherapy, statins have been shown to reduce both LDL-C levels and cardiovascular events. However, despite intensive statin therapy, a sizable proportion of statin-treated patients are unable to achieve the recommended target LDL-C levels, and not all patients can avoid future cardiovascular events. Proprotein convertase subtilisin/kexin type 9 (PCSK9) plays a key role in cholesterol homeostasis by enhancing the degradation of hepatic low-density lipoprotein receptor (LDLR). Owing to its importance in lipid metabolism, PCSK9 has emerged as a novel pharmacological target for lowering LDL-C levels. In this review, the potential role of circulating PCSK9 as a new biomarker of lipid metabolism is described. Next, previous studies evaluating the effects of lipid-modifying pharmacological agents, particularly statins, on circulating PCSK9 concentrations are summarized. Statins decrease hepatic intracellular cholesterol, resulting in increased LDLRs as well as increased PCSK9 protein. There is a clear dose-response effect of statin treatment on PCSK9 level, as increasing doses of statins also increase the level of circulating PCSK9. Finally, the available therapeutic strategies to inhibit PCSK9 are present. Monoclonal antibodies against PCSK9, in combination with statins, are one of the most promising and novel approaches to achieve further reduction of LDL-C levels and reduce the risk of cardiovascular events.

Evolocumab for the treatment of hypercholesterolemia

INTRODUCTION

Evolocumab is a fully human monoclonal immunoglobulin G2 directed against human proprotein convertase subtilisin/kexin type 9 (PCSK9). It is administered by subcutaneous injection every 2 weeks or once monthly. Area covered: Herein, the authors discuss the rationale for inhibiting PCSK9 and describe the pharmacodynamics, pharmacokinetics and clinical trials with evolocumab. Evolocumab reduces low density lipoprotein cholesterol (LDL-C) levels by 50 to 60% in most patients with and without background treatment with statins or other lipid lowering agents. The safety profile appears satisfactory from the completed clinical studies and concerns regarding the risk of neurocognitive events have largely been dispelled. Expert opinion: The reduction of LDL-C with evolocumab to previously unattainable levels has resulted in a reduction in the composite cardiovascular event endpoint in the FOURIER trial and this is likely to impact on future lipid management guidelines. The clinical outcome data and excellent tolerability profile clearly support the use of evolocumab in patients at high cardiovascular risk, including those with heterozygous or homozygous familial hypercholesterolemia, who are unable to achieve LDL-C targets with statins with or without other lipid-lowering drugs. The high cost of evolocumab will restrict its use, however.

Safety of Very Low Low-Density Lipoprotein Cholesterol Levels With Alirocumab: Pooled Data From Randomized Trials

BACKGROUND

Proprotein convertase subtilisin/kexin type 9 monoclonal antibodies can reduce low-density lipoprotein cholesterol (LDL-C) to very low levels when added to background lipid-lowering therapy.

OBJECTIVES

The safety of alirocumab was evaluated in patients with at least 2 consecutive LDL-C values <25 or <15 mg/dl in the ODYSSEY program, with follow-up as long as 104 weeks.

METHODS

Pooled data from 14 trials were analyzed (double-blind treatment 8 to 104 weeks; n = 3,340 alirocumab, n = 1,894 control [placebo or ezetimibe]; representing 4,029 [alirocumab] and 2,114 [control] double-blind patient-years' exposure).

RESULTS

In alirocumab-treated patients, 839 (25.1%) achieved 2 consecutive LDL-C values <25 mg/dl, and 314 (9.4%) achieved <15 mg/dl. Baseline LDL-C was lower (mean 100.3 vs. 134.3 mg/dl) in patients with LDL-C <25 versus ≥25 mg/dl. Similar rates of adverse events occurred in patients achieving LDL-C <25 and <15 mg/dl (72.7% and 71.7%, respectively), compared with 76.6% in those who did not achieve LDL-C <25 mg/dl. Neurological and neurocognitive events were similar among the 3 groups. In a propensity score analysis, the rate of cataracts was higher in patients with LDL-C <25 mg/dl (2.6%) versus ≥25 mg/dl (0.8%; hazard ratio: 3.40; 95% confidence interval: 1.58 to 7.35). However, no difference in cataract incidence was observed between pooled alirocumab and control groups.

CONCLUSIONS

LDL-C levels <25 or <15 mg/dl on alirocumab were not associated with an increase in overall treatment-emergent adverse event rates or neurocognitive events, although cataract incidence appeared to be increased in the group achieving LDL-C levels <25 mg/dl. (Pooled analyses of already reported trials; NCT01288443, NCT01288469, NCT01266876, NCT01812707, NCT01507831, NCT01617655, NCT01623115, NCT01709500, NCT01644175, NCT01644188, NCT01730040, NCT01730053, NCT01644474, and NCT01709513).

Can LDL cholesterol be too low? Possible risks of extremely low levels

Following the continuous accumulation of evidence supporting the beneficial role of reducing low-density lipoprotein cholesterol (LDL-C) levels in the treatment and prevention of atherosclerotic cardiovascular disease and its complications, therapeutic possibilities now exist to lower LDL-C to very low levels, similar to or even lower than those seen in newborns and nonhuman species. In addition to the important task of evaluating potential side effects of such treatments, the question arises whether extremely low LDL-C levels per se may provoke adverse effects in humans. In this review, we summarize information from studies of human cellular and organ physiology, phenotypic characterization of rare genetic diseases of lipid metabolism, and experience from clinical trials. Specifically, we emphasize the importance of the robustness of the regulatory systems that maintain balanced fluxes and levels of cholesterol at both cellular and organismal levels. Even at extremely low LDL-C levels, critical capacities of steroid hormone and bile acid production are preserved, and the presence of a cholesterol blood-brain barrier protects cells in the central nervous system. Apparent relationships sometimes reported between less pronounced low LDL-C levels and disease states such as cancer, depression, infectious disease and others can generally be explained as secondary phenomena. Drug-related side effects including an increased propensity for development of type 2 diabetes occur during statin treatment, whilst further evaluation of more potent LDL-lowering treatments such as PCSK9 inhibitors is needed. Experience from the recently reported and ongoing large event-driven trials are of great interest, and further evaluation including careful analysis of cognitive functions will be important.

Complexity of mechanisms among human proprotein convertase subtilisin-kexin type 9 variants

PURPOSE OF REVIEW

There are many reports of human variants in proprotein convertase subtilisin-kexin type 9 (PCSK9) that are either gain-of-function (GOF) or loss-of-function (LOF), with downstream effects on LDL cholesterol and cardiovascular disease (CVD) risk. However, data on particular mechanisms have only been minimally curated.

RECENT FINDINGS

GOF variants are individually ultrarare, affect all domains of the protein, act to reduce LDL receptor expression through several mechanisms, are a minor cause of familial hypercholesterolemia, have been reported mainly within families, have variable LDL cholesterol-raising effects, and are associated with increased CVD risk mainly through observational studies in families and small cohorts. In contrast, LOF variants can be either ultrarare mutations or relatively more common polymorphisms seen in populations, affect all domains of the protein, act to increase LDL receptor expression through several mechanisms, have variable LDL cholesterol-lowering effects, and have been associated with decreased CVD risk mainly through Mendelian randomization studies in epidemiologic populations.

SUMMARY

There is considerable complexity underlying the clinical concept of both LOF and GOF variants of PCSK9. But despite the underlying mechanistic heterogeneity, altered PCSK9 secretion or function is ultimately correlated with plasma LDL cholesterol level, which is also the driver of CVD outcomes.