Proprotein Convertase Subtilisin/Kexin Type 9 Interacts With Apolipoprotein B and Prevents Its Intracellular Degradation, Irrespective of the Low-Density Lipoprotein Receptor

Somatic Editing of Ldlr With Adeno-Associated Viral-CRISPR Is an Efficient Tool for Atherosclerosis Research

Objective- Atherosclerosis studies in Ldlr knockout mice require breeding to homozygosity and congenic status on C57BL6/J background, a process that is both time and resource intensive. We aimed to develop a new method for generating atherosclerosis through somatic deletion of Ldlr in livers of adult mice. Approach and Results- Overexpression of PCSK9 (proprotein convertase subtilisin/kexin type 9) is currently used to study atherosclerosis, which promotes degradation of LDLR (low-density lipoprotein receptor) in the liver. We sought to determine whether CRISPR/Cas9 (clustered regularly interspaced short palindromic repeats-associated 9) could also be used to generate atherosclerosis through genetic disruption of Ldlr in adult mice. We engineered adeno-associated viral (AAV) vectors expressing Staphylococcus aureus Cas9 and a guide RNA targeting the Ldlr gene (AAV-CRISPR). Both male and female mice received either (1) saline, (2) AAV-CRISPR, or (3) AAV-hPCSK9 (human PCSK9)-D374Y. A fourth group of germline Ldlr-KO mice was included for comparison. Mice were placed on a Western diet and followed for 20 weeks to assess plasma lipids, PCSK9 protein levels, atherosclerosis, and editing efficiency. Disruption of Ldlr with AAV-CRISPR was robust, resulting in severe hypercholesterolemia and atherosclerotic lesions in the aorta. AAV-hPCSK9 also produced hypercholesterolemia and atherosclerosis as expected. Notable sexual dimorphism was observed, wherein AAV-CRISPR was superior for Ldlr removal in male mice, while AAV-hPCSK9 was more effective in female mice. Conclusions- This all-in-one AAV-CRISPR vector targeting Ldlr is an effective and versatile tool to model atherosclerosis with a single injection and provides a useful alternative to the use of germline Ldlr-KO mice.

Association between plasma levels of PCSK9 and the presence of coronary artery disease in Japanese

The ability of pro-protein convertase subtilisin/kexin type 9 (PCSK9) levels to predict the presence or severity of coronary artery disease (CAD) remains controversial. The purpose of this study was to investigate these associations. We enrolled 393 patients who were clinically suspected to have CAD or who had at least one cardiac risk factor and underwent multidetector-row computed tomography coronary angiography. The presence of CAD (≥50% coronary stenosis), the number of significantly stenosed coronary vessels, and plasma levels of PCSK9 by ELISA were analyzed. Plasma PCSK9 levels (log-transformed data) were significantly associated with the presence of CAD. Next, we divided the patients into two groups (non-statin and statin groups) according to statin treatment. PCSK9 levels in the non-statin group were significantly lower than those in the statin group. There were no significant differences in PCSK9 levels between the absence and presence of CAD in the statin group. However, in the non-statin group, PCSK9 levels in patients with CAD were significantly higher than those in patients without CAD. PCSK9 levels, in addition to age, gender, BMI, DM and HDL-C, were independently associated with the presence of CAD by a multivariable analysis. In conclusion, our results demonstrated that plasma PCSK9 levels may be a marker for evaluating the presence of CAD.

Plasma PCSK9 correlates with apoB-48-containing triglyceride-rich lipoprotein production in men with insulin resistance

Intestinal triglyceride (TG)-rich lipoproteins (TRLs) are important in the pathogenesis of atherosclerosis in insulin resistance (IR). We investigated the association of plasma proprotein convertase subtilisin/kexin type 9 (PCSK9) concentrations with apoB-48-containing TRL metabolism in 148 men displaying various degrees of IR by measuring in vivo kinetics of TRL apoB-48 during a constant-fed state after a primed-constant infusion of L-[5,5,5-D3]leucine. Plasma PCSK9 concentrations positively correlated with TRL apoB-48 pool size (r = 0.31, P = 0.0002) and production rate (r = 0.24, P = 0.008) but not the fractional catabolic rate (r = -0.04, P = 0.6). Backward stepwise multiple linear regression analysis identified PCSK9 concentrations as a positive predictor of TRL apoB-48 production rate (standard β = +0.20, P = 0.007) independent of BMI, age, T2D/metformin use, dietary fat intake during the kinetic study, and fasting concentrations of TGs, insulin, glucose, LDL cholesterol, or C-reactive protein. We also assessed intestinal expression of key genes involved in chylomicron processing from duodenal samples of 71 men. Expression of PCSK9 and HMG-CoAR genes was positively associated (r = 0.43, P = 0.002). These results support PCSK9 association with intestinal secretion and plasma overaccumulation of TRL apoB-48 in men with IR.

Overexpression of transforming growth factor β induced factor homeobox 1 represses NPC1L1 and lowers markers of intestinal cholesterol absorption

BACKGROUND AND AIMS

Transforming growth factor β induced factor homeobox 1 (TGIF1) is a transcriptional repressor that limits the response to transforming growth factor ß signaling and also represses transcription independent of this pathway. Recently, we found higher serum cholesterol levels and more hepatic lipid accumulation in mice lacking Tgif1, and showed that TGIF1 can repress the expression of Soat2, the gene encoding the cholesterol esterifying enzyme acyl-Coenzyme A:cholesterol acyltransferase 2. Although there is evidence that TGIF1 plays a role in lipid metabolism, its role in this metabolic pathway is not fully characterized. Here we investigate whether overexpression of TGIF1 affects intestinal cholesterol absorption.

METHODS AND RESULTS

TGIF1 was found to repress human and mouse Niemann-Pick C1 like 1 (Npc1l1) promoter activity in intestinal Caco2 cells. We also found TGIF1 to be able to oppose the induction of the promoter activity by sterol regulatory element binding protein 2 and hepatocyte nuclear factor 1α and 4α. To validate these effects of TGIF1 in vivo, we generated transgenic mice specifically overexpressing TGIF1 in the intestine (Villin-Tgif1). We observed lower intestinal expression levels of Npc1l1 that was associated with lower expression of ATP-binding cassette transporter (Abc) a1, Abcg5, and Abcg8. Villin-Tgif1 mice fed regular chow or a high-fat diet had lower levels of markers of intestinal cholesterol absorption than wild types.

CONCLUSIONS

We suggest TGIF1 as a new player in intestinal cholesterol metabolism.

PCSK9 in cholesterol metabolism: from bench to bedside

Dyslipidemia, and specifically elevated low-density lipoprotein (LDL) cholesterol, is one of the most important cardiovascular risk factors. Statins are considered first line therapy for the primary and secondary prevention of cardiovascular disease. However, statins may not be adequate treatment for elevated circulating LDL levels and are ineffective in certain familial hypercholesterolemias. The discovery of proprotein convertase subtilisin/kexin type 9 (PCSK9), a regulatory protein that affects LDL receptors, offers a new alternative for these patients. Moreover, gain-of-function PCSK9 mutations were discovered to be the root cause of familial autosomal dominant hypercholesterolemia. Inhibition of PSCK9 reduces plasma LDL levels, even in patients for whom statins are ineffective or not tolerated. Alirocumab and evolocumab, human monoclonal antibodies that inhibit PCSK9, have been approved to lower LDL levels. While there are drawbacks to these treatments, including adverse events, administration by subcutaneous injection, and high cost, these drugs are indicated for the treatment of atherosclerotic cardiovascular disease and familial hypercholesterolemia as adjunct to diet and maximally tolerated statin therapy. PCSK9 inhibitors may work synergistically with statins to lower LDL. Novel approaches to PCSK9 inhibition are currently in development with the aim of providing safe and effective treatment options to decrease cardiovascular event burden, ideally at lower cost and with oral bioavailability.

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.

PCSK9: From Basic Science Discoveries to Clinical Trials

Unknown 15 years ago, PCSK9 (proprotein convertase subtilisin/kexin type 9) is now common parlance among scientists and clinicians interested in prevention and treatment of atherosclerotic cardiovascular disease. What makes this story so special is not its recent discovery nor the fact that it uncovered previously unknown biology but rather that these important scientific insights have been translated into an effective medical therapy in record time. Indeed, the translation of this discovery to novel therapeutic serves as one of the best examples of how genetic insights can be leveraged into intelligent target drug discovery. The PCSK9 saga is unfolding quickly but is far from complete. Here, we review major scientific understandings as they relate to the role of PCSK9 in lipoprotein metabolism and atherosclerotic cardiovascular disease and the impact that therapies designed to inhibit its action are having in the clinical setting.

Inclisiran for the treatment of dyslipidemia

INTRODUCTION

Dyslipidemia is one of the most important risk factors for cardiovascular disease. Insufficient reduction in LDL-C from existing therapies in patients at high risk of atherogenic cardiovascular disease is an unmet clinical need. Circulating PCSK9 causes hypercholesterolemia by reducing LDL receptors in hepatocytes. Areas covered: PCSK9 inhibition has emerged as a promising new therapeutic strategy to reduce LDL-C. Inclisiran, a novel, synthetic, siRNA molecule, inhibits PCSK9 synthesis in hepatocytes. Inclisiran targets intracellular PCSK9 synthesis specifically, resulting in a dose-dependent, long-term, significant reduction in LDL-C. Inclisiran has been well tolerated and safe, without severe adverse events so far. This review discusses current PCSK9 inhibitors and the results of phase I and II clinical trials of inclisiran. Expert opinion: Plasma PCSK9 enhances the degradation of LDL receptor, resulting in accumulation of LDL-C in the circulation. Current approaches with monoclonal antibodies sequester circulating PCSK9 but require frequent injections. Inclisiran inhibits translation of PCSK9 mRNA and thus switches off PCSK9 production and provides advantages over monoclonal antibodies with an infrequent dosing interval of twice a year to reduce LDL-C by over 50%. Ongoing studies will establish the long-term safety of inclisiran in patients with high cardiovascular risk and an elevated LDL-C.

Proprotein Convertase Subtilisin-Kexin type-9 (PCSK9) and triglyceride-rich lipoprotein metabolism: Facts and gaps

After more than a decade of intense investigation, Pro-protein Convertase Subtilisin-Kexin type 9 (PCSK9) remains a hot topic of research both at experimental and clinical level. Interestingly PCSK9 is expressed in different tissues suggesting the existence of additional function(s) beyond the modulation of the Low-Density Lipoprotein (LDL) receptor in the liver. Emerging data suggest that PCSK9 might play a role in the modulation of triglyceride-rich lipoprotein (TGRL) metabolism, mainly Very Low-Density Lipoproteins (VLDL) and their remnants. In vitro, PCSK9 affects TGRLs production by intestinal cells as well as the catabolism of LDL receptor homologous and non-homologous targets such as VLDL receptor, CD36 and ApoE2R. However, the in vivo relevance of these findings is still debated. This review aims at critically discussing the role of PCSK9 on TGRLs metabolism with a major focus on the impact of its genetic and pharmacological modulation on circulating lipids and lipoproteins beyond LDL.

PCSK9 deficiency reduces atherosclerosis, apolipoprotein B secretion, and endothelial dysfunction

Proprotein convertase subtilisin/kexin type 9 (PCSK9) interacts directly with cytoplasmic apoB and prevents its degradation via the autophagosome/lysosome pathway. This process affects VLDL and LDL production and influences atherogenesis. Here, we investigated the molecular machinery by which PCSK9 modulates autophagy and affects atherogenesis. We backcrossed Pcsk9-/- mice with atherosclerosis-prone Ldlr-/-Apobec1-/- (LDb) mice to generate Ldlr-/-Apobec1-/-Pcsk9-/- (LTp) mice. Deletion of PCSK9 resulted in decreased hepatic apoB secretion, increased autophagic flux, and decreased plasma levels of IDL and LDL particles. The LDLs from LTp mice (LTp-LDLs) were less atherogenic and contained less cholesteryl ester and phospholipids than LDb-LDLs. Moreover LTp-LDLs induced lower endothelial expression of the genes encoding TLR2, Lox-1, ICAM-1, CCL2, CCL7, IL-6, IL-1β, Beclin-1, p62, and TRAF6 Collectively, these effects were associated with substantially less atherosclerosis development (>4-fold) in LTp mice. The absence of PCSK9 in LDb mice results in decreased lipid and apoB levels, fewer atherogenic LDLs, and marked reduction of atherosclerosis. The effect on atherogenesis may be mediated in part by the effects of modified LDLs on endothelial cell receptors and proinflammatory and autophagy molecules. These findings suggest that there may be clinical benefits of PCSK9 inhibition due to mechanisms unrelated to increased LDL receptor activity.

PCSK9 in context: A contemporary review of an important biological target for the prevention and treatment of atherosclerotic cardiovascular disease

The identification of the critical role of proprotein convertase subtilisin/kexin type 9 (PCSK9) has rapidly led to the development of PCSK9 inhibition with monoclonal antibodies (mAbs). PCSK9 mAbs are already in limited clinical use and are the subject of major cardiovascular outcomes trials, which, if universally positive, could see much wider clinical application of these agents. Patients with familial hypercholesterolaemia are the most obvious candidates for these drugs, but other patients with elevated cardiovascular risk, statin intolerance or hyperlipoproteinaemia(a) may also benefit. PCSK9 mAbs, administered once or twice monthly, reduce LDL cholesterol levels by 50% to 70%, and appear to be safe and acceptable to patients over at least 2 years of treatment; however, treatment-emergent adverse effects are not always identified in clinical trials, as well-evidenced by statin myopathy. Inclisiran is a promising RNA-based therapy that promotes the degradation of PCSK9 mRNA transcripts and has similar efficacy to mAbs, but with a much longer duration of action. The cost-effectiveness and long-term safety of therapies targeted at inhibiting PCSK9 remain to be demonstrated if they are to be used widely in coronary prevention.

Association between lipoprotein (a) and proprotein convertase substilisin/kexin type 9 in patients with heterozygous familial hypercholesterolemia: A case-control study

BACKGROUND

Recent data have suggested an important role of lipoprotein (a) [Lp(a)] and proprotein convertase substilisin/kexin type 9 (PCSK9) in the development of atherosclerotic cardiovascular disease (ASCVD) in both general population and family hypercholesterolemia (FH), while the relation of Lp(a) to PCSK9 has not been examined.

OBJECTIVE

The aim of the present study was to investigate the association between plasma PCSK9 and Lp(a)in patients with heterozygous FH (HeFH).

METHODS

Two hundred and fifty-five molecularly confirmed patients with HeFH were compared to 255 age- and gender-matched non-FH controls. Plasma PCSK9 and Lp(a) concentrations were measured using ELISA and immunoturbidimetric method respectively, and finally their association was assessed.

RESULTS

Both plasma PCSK9 and Lp(a) levels were significantly higher in patients with HeFH compared to control group (p<0.001). Besides, the Lp(a) concentration and percentage of Lp(a)≥300mg/L were increased by PCSK9 tertiles in HeFH group (both p<0.05) while not in control group. In partial correlation analysis, Lp(a) was associated with PCSK9 (r=0.254, p<0.001) in HeFH group but not in control, which were further confirmed by multivariable linear regression analysis. Furthermore, significant associations between Lp(a) and PCSK9 were also found in subgroups of HeFH group irrespective of definite or probable FH, with and without coronary artery disease (CAD), and with statin or not.

CONCLUSIONS

Plasma Lp(a) level was associated with PCSK9 in patients with HeFH alone, suggesting that much about the interaction of PCSK9 with Lp(a) in FH need further explorations.

New Sequencing technologies help revealing unexpected mutations in Autosomal Dominant Hypercholesterolemia

Autosomal dominant hypercholesterolemia (ADH) is characterized by elevated LDL-C levels leading to coronary heart disease. Four genes are implicated in ADH: LDLR, APOB, PCSK9 and APOE. Our aim was to identify new mutations in known genes, or in new genes implicated in ADH. Thirteen French families with ADH were recruited and studied by exome sequencing after exclusion, in their probands, of mutations in the LDLR, PCSK9 and APOE genes and fragments of exons 26 and 29 of APOB gene. We identified in one family a p.Arg50Gln mutation in the APOB gene, which occurs in a region not usually associated with ADH. Segregation and in-silico analysis suggested that this mutation is disease causing in the family. We identified in another family with the p.Ala3396Thr mutation of APOB, one patient with a severe phenotype carrying also a mutation in PCSK9: p.Arg96Cys. This is the first compound heterozygote reported with a mutation in APOB and PCSK9. Functional studies proved that the p.Arg96Cys mutation leads to increased LDL receptor degradation. This work shows that Next-Generation Sequencing (exome, genome or targeted sequencing) are powerful tools to find new mutations and identify compound heterozygotes, which will lead to better diagnosis and treatment of ADH.

Impact of protease inhibitors on circulating PCSK9 levels in HIV-infected antiretroviral-naive patients from an ongoing prospective cohort

OBJECTIVE

The study aims to assess the association between proprotein convertase subtilisin/kexin type 9 (PCSK9), a major regulator of LDL cholesterol (LDL-C) homeostasis, and HIV-related dyslipidaemia in a cohort of HIV-positive (HIV+) patients under protease inhibitors.

METHODS

Plasma PCSK9 levels were measured in 103 HIV+ patients before and after initiating protease inhibitor-based antiretroviral therapy (ART), and in 90 HIV-negative controls matched for age and sex. PCSK9 was measured by ELISA. HIV+ patients who were not virologically suppressed at follow-up or were on lipid-lowering therapy were excluded.

RESULTS

In HIV+ (median age 36 years; 77.7% men), PCSK9 levels did not increase after protease inhibitor exposure (median 14 months) (279.5 ng/ml before, 289.6 ng/ml after; P = 0.49) and were significantly elevated versus controls at all timepoints (adjusted P value before and after: <0.05). After protease inhibitor initiation, total cholesterol, LDL-C and HDL cholesterol levels increased, but LDL-C remained lower versus controls. At baseline, PCSK9 levels were positively associated with immunodeficiency and the severity of HIV disease [HIV-1 viral load (P = 0.01), CD4 T-cell count <200/μl, P = 0.002], stage C HIV disease (P = 0.0002). In protease inhibitor-treated patients, PCSK9 levels were no longer associated with HIV-related factors but with total cholesterol (P = 0.0006), LDL-C (P = 0.01), HDL cholesterol (P = 0.01), triglycerides (P = 0.05) and glycaemia (P = 0.006).

CONCLUSION

PSCK9 levels are elevated in HIV+ patients. In ART-naive patients, the relationship between PCSK9 levels and infection severity suggests an effect of HIV disease. After initiating protease inhibitor-containing ART in virologically suppressed patients, PCSK9 levels were associated with dyslipidaemia similar to controls.

Fatty Acids Consumption: The Role Metabolic Aspects Involved in Obesity and Its Associated Disorders

Obesity and its associated disorders, such as insulin resistance, dyslipidemia, metabolic inflammation, dysbiosis, and non-alcoholic hepatic steatosis, are involved in several molecular and inflammatory mechanisms that alter the metabolism. Food habit changes, such as the quality of fatty acids in the diet, are proposed to treat and prevent these disorders. Some studies demonstrated that saturated fatty acids (SFA) are considered detrimental for treating these disorders. A high fat diet rich in palmitic acid, a SFA, is associated with lower insulin sensitivity and it may also increase atherosclerosis parameters. On the other hand, a high intake of eicosapentaenoic (EPA) and docosahexaenoic (DHA) fatty acids may promote positive effects, especially on triglyceride levels and increased high-density lipoprotein (HDL) levels. Moreover, polyunsaturated fatty acids (PUFAs) and monounsaturated fatty acids (MUFAs) are effective at limiting the hepatic steatosis process through a series of biochemical events, such as reducing the markers of non-alcoholic hepatic steatosis, increasing the gene expression of lipid metabolism, decreasing lipogenic activity, and releasing adiponectin. This current review shows that the consumption of unsaturated fatty acids, MUFA, and PUFA, and especially EPA and DHA, which can be applied as food supplements, may promote effects on glucose and lipid metabolism, as well as on metabolic inflammation, gut microbiota, and hepatic metabolism.

The roles of apo(a) size, phenotype, and dominance pattern in PCSK9-inhibition-induced reduction in Lp(a) with alirocumab

An elevated level of lipoprotein (a) [Lp(a)] is a risk factor for CVD. Alirocumab, a monoclonal antibody to proprotein convertase subtilisin/kexin type 9, is reported to reduce Lp(a) levels. The relationship of Lp(a) reduction with apo(a) size polymorphism, phenotype, and dominance pattern and LDL cholesterol (LDL-C) reduction was evaluated in a pooled analysis of 155 hypercholesterolemic patients (75 with heterozygous familial hypercholesterolemia) from two clinical trials. Alirocumab significantly reduced total Lp(a) (pooled median: -21%, P = 0.0001) and allele-specific apo(a), an Lp(a) level carried by the smaller (median: -18%, P = 0.002) or the larger (median: -37%, P = 0.0005) apo(a) isoform, at week 8 versus baseline. The percent reduction in Lp(a) level with alirocumab was similar across apo(a) phenotypes (single vs. double bands) and carriers and noncarriers of a small size apo(a) (≤22 kringles). The percent reduction in LDL-C correlated significantly with the percent reduction in Lp(a) level (r = 0.407, P < 0.0001) and allele-specific apo(a) level associated with the smaller (r = 0.390, P < 0.0001) or larger (r = 0.270, P = 0.0183) apo(a) sizes. In conclusion, alirocumab-induced Lp(a) reduction was independent of apo(a) phenotypes and the presence or absence of a small size apo(a).

The complexity of lipoprotein (a) lowering by PCSK9 monoclonal antibodies

Since 2012, clinical trials dedicated to proprotein convertase subtilisin kexin type 9 (PCSK9) inhibition with monoclonal antibodies (mAbs) have unambiguously demonstrated robust reductions not only in low-density lipoprotein (LDL) cholesterol (LDL-C) but also in lipoprotein (a) [Lp(a)] levels. The scientific literature published prior to those studies did not provide any evidence for a link between PCSK9 and Lp(a) metabolism. More recent investigations, either in vitro or in vivo, have attempted to unravel the mechanism(s) by which PCSK9 mAbs reduce circulating Lp(a) levels, with some showing a specific implication of the LDL receptor (LDLR) in Lp(a) clearance whereas others found no significant role for the LDLR in that process. This elusive pathway appears clearly distinct from that of the widely prescribed statins that also enhance LDLR function but do not lower circulating Lp (a) levels in humans. So how does PCSK9 inhibition with mAbs reduce Lp(a)? This still remains to be established.

Lipoprotein(a) and inhibitors of proprotein convertase subtilisin/kexin type 9

Lipoprotein(a) [Lp(a)] has been identified as a risk factor for cardiovascular disease. Lp(a) levels are also high under certain clinical conditions, including familial hypercholesterolemia and high blood low-density lipoprotein (LDL) cholesterol levels. Few effective generic therapies for modulating Lp(a) have been developed. However, new therapies involving inhibitors of proprotein convertase subtilisin/kexin type 9 (PCSK9) using monoclonal antibodies have markedly reduced the blood LDL levels-and the Lp(a) levels as well. Much attention has therefore been focused on this therapy and its utility. The mechanism by which PCSK9 inhibitors reduce the Lp(a) levels remains unclear. We here describe the effects of PCSK9 inhibitors on Lp(a) and discuss potential mechanisms and perspectives of this topic.

Complex role of autophagy in regulation of hepatic lipid and lipoprotein metabolism

Discovering new therapeutic interventions to treat lipid and lipoprotein disorders is of great interest and the discovery of autophagy as a regulator of lipid metabolism has opened up new avenues for targeting modulators of this pathway. Autophagy is a degradative process that targets cellular components to the lysosome and recent studies have indicated a role for autophagy in regulating hepatic lipid metabolism (known as lipophagy) as well as lipoprotein assembly. Autophagy directly targets apolipoprotein B-100 (apoB100), the structural protein component of very low-density lipoproteins (VLDLs), and further targets lipid droplets (LDs), the cellular storage for neutral lipids. Autophagy thus plays a complex and dual role in VLDL particle assembly by regulating apoB100 degradation as well as aiding the maturation of VLDL particles by hydrolyzing lipid from LDs. The purpose of this article is to review our current understanding of molecular and cellular mechanisms mediating authophagic control of hepatic lipid biogenesis and VLDL production as well as dysregulation in insulin resistance and dyslipidemia.

Effects of PCSK9 Inhibition With Alirocumab on Lipoprotein Metabolism in Healthy Humans

Supplemental Digital Content is available in the text.

Background:

Alirocumab, a monoclonal antibody to proprotein convertase subtilisin/kexin type 9 (PCSK9), lowers plasma low-density lipoprotein (LDL) cholesterol and apolipoprotein B100 (apoB). Although studies in mice and cells have identified increased hepatic LDL receptors as the basis for LDL lowering by PCSK9 inhibitors, there have been no human studies characterizing the effects of PCSK9 inhibitors on lipoprotein metabolism. In particular, it is not known whether inhibition of PCSK9 has any effects on very low-density lipoprotein or intermediate-density lipoprotein (IDL) metabolism. Inhibition of PCSK9 also results in reductions of plasma lipoprotein (a) levels. The regulation of plasma Lp(a) levels, including the role of LDL receptors in the clearance of Lp(a), is poorly defined, and no mechanistic studies of the Lp(a) lowering by alirocumab in humans have been published to date.

Methods:

Eighteen (10 F, 8 mol/L) participants completed a placebo-controlled, 2-period study. They received 2 doses of placebo, 2 weeks apart, followed by 5 doses of 150 mg of alirocumab, 2 weeks apart. At the end of each period, fractional clearance rates (FCRs) and production rates (PRs) of apoB and apo(a) were determined. In 10 participants, postprandial triglycerides and apoB48 levels were measured.

Results:

Alirocumab reduced ultracentrifugally isolated LDL-C by 55.1%, LDL-apoB by 56.3%, and plasma Lp(a) by 18.7%. The fall in LDL-apoB was caused by an 80.4% increase in LDL-apoB FCR and a 23.9% reduction in LDL-apoB PR. The latter was due to a 46.1% increase in IDL-apoB FCR coupled with a 27.2% decrease in conversion of IDL to LDL. The FCR of apo(a) tended to increase (24.6%) without any change in apo(a) PR. Alirocumab had no effects on FCRs or PRs of very low-density lipoproteins-apoB and very low-density lipoproteins triglycerides or on postprandial plasma triglycerides or apoB48 concentrations.

Conclusions:

Alirocumab decreased LDL-C and LDL-apoB by increasing IDL- and LDL-apoB FCRs and decreasing LDL-apoB PR. These results are consistent with increases in LDL receptors available to clear IDL and LDL from blood during PCSK9 inhibition. The increase in apo(a) FCR during alirocumab treatment suggests that increased LDL receptors may also play a role in the reduction of plasma Lp(a).

Clinical Trial Registration:

URL: http://www.clinicaltrials.gov. Unique identifier: NCT01959971.

Proprotein convertase subtilisin/kexin type 9 (PCSK9) and metabolic syndrome: insights on insulin resistance, inflammation, and atherogenic dyslipidemia

Low-density lipoprotein (LDL) cholesterol plays a pivotal role in the pathogenesis of atherosclerotic cardiovascular disease (CVD). The discovery that proprotein convertase subtilisin/kexin type 9 (PCSK9) represents a key regulator pathway for hepatic LDL receptor (LDLR) degradation sheds light on new uncovered issues regarding LDL-C homeostasis. Indeed, as confirmed by phase II and III clinical trials with monoclonal antibodies, targeting PCSK9 represents the newest and most promising pharmacological tool for the treatment of hypercholesterolemia and related CVD. However, clinical, genetic, and experimental evidence indicates that PCSK9 may be either a cause or an effect in the context of metabolic syndrome (MetS), a condition comprising a cluster of risk factors including insulin resistance, obesity, hypertension, and atherogenic dyslipidemia. The latter is characterized by a triad of hypertriglyceridemia, low plasma concentrations of high-density lipoproteins, and qualitative changes in LDLs. PCSK9 levels seem to correlate with many of these lipid parameters as well as with the insulin sensitivity indices, although the molecular mechanisms behind this association are still unknown or not completely elucidated. Nevertheless, this area of research represents an important starting point for a better understanding of the physiological role of PCSK9, also considering the recent approval of new therapies involving anti-PCSK9. Thus, in the present review, we will discuss the current knowledge on the role of PCSK9 in the context of MetS, alteration of lipids, glucose homeostasis, and inflammation.

An Unbiased Mass Spectrometry Approach Identifies Glypican-3 as an Interactor of Proprotein Convertase Subtilisin/Kexin Type 9 (PCSK9) and Low Density Lipoprotein Receptor (LDLR) in Hepatocellular Carcinoma Cells

The mechanism of LDL receptor (LDLR) degradation mediated by the proprotein convertase subtilisin/kexin type 9 (PCSK9) has been extensively studied; however, many steps within this process remain unclear and still require characterization. Recent studies have shown that PCSK9 lacking its Cys/His-rich domain can still promote LDLR internalization, but the complex does not reach the lysosome suggesting the presence of an additional interaction partner(s). In this study we carried out an unbiased screening approach to identify PCSK9-interacting proteins in the HepG2 cells' secretome using co-immunoprecipitation combined with mass spectrometry analyses. Several interacting proteins were identified, including glypican-3 (GPC3), phospholipid transfer protein, matrilin-3, tissue factor pathway inhibitor, fibrinogen-like 1, and plasminogen activator inhibitor-1. We then validated these interactions by co-immunoprecipitation and Western blotting. Furthermore, functional validation was examined by silencing each candidate protein in HepG2 cells using short hairpin RNAs to determine their effect on LDL uptake and LDLR levels. Only GPC3 and phospholipid transfer protein silencing in HepG2 cells significantly increased LDL uptake in these cells and displayed higher total LDLR protein levels compared with control cells. Moreover, our study provides the first evidence that GPC3 can modulate the PCSK9 extracellular activity as a competitive binding partner to the LDLR in HepG2 cells.

Biology of proprotein convertase subtilisin kexin 9: beyond low-density lipoprotein cholesterol lowering

Proprotein convertase subtilisin kexin 9 (PCSK9) is a key regulator of low-density lipoprotein receptor levels and LDL-cholesterol levels. Loss-of-function mutations in PCSK9 gene are associated with hypocholesterolaemia and protection against cardiovascular disease, identifying PCSK9 inhibition as a valid therapeutic approach to manage hypercholesterolaemia and related diseases. Although PCSK9 is expressed mainly in the liver, it is present also in other tissues and organs with specific functions, raising the question of whether a pharmacological inhibition of PCSK9 to treat hypercholesterolaemia and associated cardiovascular diseases might be helpful or deleterious in non-hepatic tissues. For example, PCSK9 is expressed in the vascular wall, in the kidneys, and in the brain, where it was proposed to play a role in development, neurocognitive process, and neuronal apoptosis. A link between PCSK9 and immunity was also proposed as both sepsis and viral infections are differentially affected in the presence or absence of PCSK9. Despite the increasing number of observations, the debate on the exact roles of PCSK9 in extrahepatic tissues is still ongoing, and as very effective drugs that inhibit PCSK9 have become available to the clinician, a better understanding of the biological roles of PCSK9 is warranted.

Effects of LDL Receptor Modulation on Lymphatic Function

Atherosclerosis is driven by the accumulation of immune cells and cholesterol in the arterial wall. Although recent studies have shown that lymphatic vessels play an important role in macrophage reverse cholesterol transport, the specific underlying mechanisms of this physiological feature remain unknown. In the current report, we sought to better characterize the lymphatic dysfunction that is associated with atherosclerosis by studying the physiological and temporal origins of this impairment. First, we assessed that athero-protected Pcsk9^−/− mice exhibited improved collecting lymphatic vessel function throughout age when compared to WT mice for up to six months, while displaying enhanced expression of LDLR on lymphatic endothelial cells. Lymphatic dysfunction was present before the atherosclerotic lesion formation in a mouse model that is predisposed to develop atherosclerosis (Ldlr^−/−; hApoB100^+/+). This dysfunction was presumably associated with a defect in the collecting lymphatic vessels in a non-specific cholesterol- but LDLR-dependent manner. Treatment with a selective VEGFR-3 agonist rescued this impairment observed early in the onset of this arterial disease. We suggest that LDLR modulation is associated with early atherosclerosis-related lymphatic dysfunction, and bring forth a pleiotropic role for PCSK9 in lymphatic function. Our study unveils new potential therapeutic targets for the prevention and treatment of atherosclerosis.

PCSK9 inhibition-mediated reduction in Lp(a) with evolocumab: an analysis of 10 clinical trials and the LDL receptor's role

Lipoprotein (a) [Lp(a)] is independently associated with CVD risk. Evolocumab, a monoclonal antibody (mAb) to proprotein convertase subtilisin/kexin type 9 (PCSK9), decreases Lp(a). The potential mechanisms were assessed. A pooled analysis of Lp(a) and LDL cholesterol (LDL-C) in 3,278 patients from 10 clinical trials (eight phase 2/3; two extensions) was conducted. Within each parent study, biweekly and monthly doses of evolocumab statistically significantly reduced Lp(a) at week 12 versus control (P < 0.001 within each study); pooled median (quartile 1, quartile 3) percent reductions were 24.7% (40.0, 3.6) and 21.7% (39.9, 4.2), respectively. Reductions were maintained through week 52 of the open-label extension, and correlated with LDL-C reductions [with and without correction for Lp(a)-cholesterol] at both time points (P < 0.0001). The effect of LDL and LDL receptor (LDLR) availability on Lp(a) cell-association was measured in HepG2 cells: cell-associated LDL fluorescence was reversed by unlabeled LDL and Lp(a). Lp(a) cell-association was reduced by coincubation with LDL and PCSK9 and reversed by adding PCSK9 mAb. These studies support that reductions in Lp(a) with PCSK9 inhibition are partly due to increased LDLR-mediated uptake. In most situations, Lp(a) appears to compete poorly with LDL for LDLR binding and internalization, but when LDLR expression is increased with evolocumab, particularly in the setting of low circulating LDL, Lp(a) is reduced.

[PCSK9 - "missing link" in familial hypercholesterolemia : New therapeutic options in hypercholesterolemia and coronary artery disease]

Lowering plasma low-density lipoprotein cholesterol (LDL-C) levels to individual therapeutic goals is one of the most effective measures for the prevention of cardiovascular disease. Besides dietary measures, this can be achieved pharmaceutically by inhibition of hepatic cholesterol synthesis with statins or inhibition of intestinal cholesterol absorption (e.g., ezetimibe and bile acid sequestrants). Decisive for lowering LDL is an increased hepatic uptake of circulating LDL via an increase in LDL receptors (LDLR) in hepatic cell membranes. The formation of new LDLR and recirculation of existing LDLR play a decisive role in this process. An important modulator of LDLR is proprotein convertase subtilisin/kexin type 9 (PCSK9). In the last years genetic studies have identified several mutations in the PCSK9 gene leading to a gain of function and carriers of these mutations suffer from autosomal dominant hypercholesterolemia. In contrast, carriers of PCSK9 loss of function mutations show very low plasma LDL-C concentrations and a markedly reduced risk for coronary artery disease. These fundamental discoveries have sparked the development of a completely novel therapeutic approach to treating hypercholesterolemia. At present, inhibition of PCSK9 by monoclonal antibodies presents the most promising therapeutic approach. First human antibodies were recently approved as the first immunotherapeutic agents for the treatment of severe hypercholesterolemia and in patients with statin intolerance. An additional PCSK9 antibody is presently being studied in phase III clinical trials.

PCSK9 Association With Lipoprotein(a)

RATIONALE

Lipoprotein(a) [Lp(a)] is a highly atherogenic low-density lipoprotein-like particle characterized by the presence of apoprotein(a) [apo(a)] bound to apolipoprotein B. Proprotein convertase subtilisin/kexin type 9 (PCSK9) selectively binds low-density lipoprotein; we hypothesized that it can also be associated with Lp(a) in plasma.

OBJECTIVE

Characterize the association of PCSK9 and Lp(a) in 39 subjects with high Lp(a) levels (range 39-320 mg/dL) and in transgenic mice expressing either human apo(a) only or human Lp(a) (via coexpression of human apo(a) and human apolipoprotein B).

METHODS AND RESULTS

We show that PCSK9 is physically associated with Lp(a) in vivo using 3 different approaches: (1) analysis of Lp(a) fractions isolated by ultracentrifugation; (2) immunoprecipitation of plasma using antibodies to PCSK9 and immunodetection of apo(a); (3) ELISA quantification of Lp(a)-associated PCSK9. Plasma PCSK9 levels correlated with Lp(a) levels, but not with the number of kringle IV-2 repeats. PCSK9 did not bind to apo(a) only, and the association of PCSK9 with Lp(a) was not affected by the loss of the apo(a) region responsible for binding oxidized phospholipids. Preferential association of PCSK9 with Lp(a) versus low-density lipoprotein (1.7-fold increase) was seen in subjects with high Lp(a) and normal low-density lipoprotein. Finally, Lp(a)-associated PCSK9 levels directly correlated with plasma Lp(a) levels but not with total plasma PCSK9 levels.

CONCLUSIONS

Our results show, for the first time, that plasma PCSK9 is found in association with Lp(a) particles in humans with high Lp(a) levels and in mice carrying human Lp(a). Lp(a)-bound PCSK9 may be pursued as a biomarker for cardiovascular risk.

Targeting PCSK9 for therapeutic gains: Have we addressed all the concerns?

Proprotein Convertase Subtilisin/Kexin Type 9 (PCSK9) regulates the expression of low-density lipoprotein (LDL)-receptors, through reducing their recycling by binding to the receptor along with LDL and targeting it for lysosomal destruction. PCSK9 also enhances the degradation of very-low-density-lipoprotein receptor (VLDLR) and lipoprotein receptor-related protein 1 (LRP-1) in a LDL-receptor independent manner. This role in lipid homeostasis presents PCSK9 as an attractive target for the therapeutic management of familial hypercholesterolemia as well as other refractory dyslipidaemias. However, PCSK9 mediates multifarious functions independent of its role in lipid homeostasis, which can be grouped under "pleiotropic functions" of the protein. This includes PCSK9's role in: trafficking of epithelial sodium channel; hepatic regeneration; pancreatic integrity and glucose homeostasis; antiviral activity; antimalarial activity; regulation of different cell signalling pathways; cortical neural differentiation; neuronal apoptosis and Alzheimer's disease. The question that needs to be investigated in depth is "How will the pleotropic functions of PCSK9, be affected by the therapeutic intervention of the protease's LDL-receptor lowering activity?" In this review, we appraise the different lipid lowering strategies targeting PCSK9 in light of the protein's different pleiotropic functions. Additionally, we delineate the key areas that require further examination, to ensure the long-term safety of the above lipid-lowering strategies.

Proprotein convertase subtilisin/kexin type 9 inhibition: a new therapeutic mechanism for reducing cardiovascular disease risk

Proprotein convertase subtilisin/kexin type 9 (PCSK9) plays an important role in the regulation of cholesterol homeostasis. By binding to hepatic low-density lipoprotein (LDL) receptors and promoting their lysosomal degradation, PCSK9 reduces LDL uptake, leading to an increase in LDL cholesterol concentrations. Gain-of-function mutations in PCSK9 associated with high LDL cholesterol and premature cardiovascular disease have been causally implicated in the pathophysiology of autosomal-dominant familial hypercholesterolemia. In contrast, the more commonly expressed loss-of-function mutations in PCSK9 are associated with reduced LDL cholesterol and cardiovascular disease risk. The development of therapeutic approaches that inhibit PCSK9 function has therefore attracted considerable attention from clinicians and the pharmaceutical industry for the management of hypercholesterolemia and its associated cardiovascular disease risk. This review summarizes the effects of PCSK9 on hepatic and intestinal lipid metabolism and the more recently explored functions of PCSK9 in extrahepatic tissues. Therapeutic approaches that prevent interaction of PCSK9 with hepatic LDL receptors (monoclonal antibodies, mimetic peptides), inhibit PCSK9 synthesis in the endoplasmic reticulum (antisense oligonucleotides, siRNAs), and interfere with PCSK9 function (small molecules) are also described. Finally, clinical trials testing the safety and efficacy of monoclonal antibodies to PCSK9 are reviewed. These have shown dose-dependent decreases in LDL cholesterol (44%-65%), apolipoprotein B (48%-59%), and lipoprotein(a) (27%-50%) without major adverse effects in various high-risk patient categories, including those with statin intolerance. Initial reports from 2 of these trials have indicated the expected reduction in cardiovascular events. Hence, inhibition of PCSK9 holds considerable promise as a therapeutic option for decreasing cardiovascular disease risk.

Suppressor of Cytokine Signaling-3 (SOCS-3) Induces Proprotein Convertase Subtilisin Kexin Type 9 (PCSK9) Expression in Hepatic HepG2 Cell Line

The suppressor of cytokine signaling (SOCS) proteins are negative regulators of the JAK/STAT pathway activated by proinflammatory cytokines, including the tumor necrosis factor-α (TNF-α). SOCS3 is also implicated in hypertriglyceridemia associated to insulin resistance. Proprotein convertase subtilisin kexin type 9 (PCSK9) levels are frequently found to be positively correlated to insulin resistance and plasma very low density lipoprotein (VLDL) triglycerides concentrations. The present study aimed to investigate the possible role of TNF-α and JAK/STAT pathway on de novo lipogenesis and PCSK9 expression in HepG2 cells. TNF-α induced both SOCS3 and PCSK9 in a concentration-dependent manner. This effect was inhibited by transfection with siRNA anti-STAT3, suggesting the involvement of the JAK/STAT pathway. Retroviral overexpression of SOCS3 in HepG2 cells (HepG2(SOCS3)) strongly inhibited STAT3 phosphorylation and induced PCSK9 mRNA and protein, with no effect on its promoter activity and mRNA stability. Consistently, siRNA anti-SOCS3 reduced PCSK9 mRNA levels, whereas an opposite effect was observed with siRNA anti-STAT3. In addition, HepG2(SOCS3) express higher mRNA levels of key enzymes involved in the de novo lipogenesis, such as fatty-acid synthase, stearoyl-CoA desaturase (SCD)-1, and apoB. These responses were associated with a significant increase of SCD-1 protein, activation of sterol regulatory element-binding protein-1c (SREBP-1), accumulation of cellular triglycerides, and secretion of apoB. HepG2(SOCS3) show lower phosphorylation levels of insulin receptor substrate 1 (IRS-1) Tyr(896) and Akt Ser(473) in response to insulin. Finally, insulin stimulation produced an additive effect with SOCS3 overexpression, further inducing PCSK9, SREBP-1, fatty acid synthase, and apoB mRNA. In conclusion, our data candidate PCSK9 as a gene involved in lipid metabolism regulated by proinflammatory cytokine TNF-α in a SOCS3-dependent manner.

The associations between proprotein convertase subtilisin/kexin type 9 E670G polymorphism and the risk of coronary artery disease and serum lipid levels: a meta-analysis

BACKGROUND

Studies had investigated the associations between proprotein convertase subtilisin/kexin type 9 (PCSK9) E670G polymorphism and coronary artery disease (CAD) and lipid levels, but the results were controversial. Thus, we performed this meta-analysis to investigate the association between PCSK9 E670G polymorphism and lipid levels and the susceptibility to CAD.

METHODS

All relevant articles according to the inclusion criteria were retrieved and included in the present meta-analysis. Odds ratios (ORs) with 95 % confidence interval (CI) were used to analyze the strength of the association between PCSK9 E670G polymorphism and the susceptibility to CAD. At the same time, the pooled standardized mean difference (SMD) with 95 % CI was used for the meta-analysis of PCSK9 E670G polymorphism and lipid levels. The publication bias was examined by using Begg's funnel plots and Egger's test.

RESULTS

A total of seventeen studies met the inclusion criteria. For CAD association, the pooled effects indicated that the G allele carriers had higher risk of CAD than non-carriers in dominant genetic model (OR:1.601, 95 % CI: 1.314-1.951, P < 0.001), as well as in allelic genetic model (OR: 1.546, 95 % CI: 1.301-1.838, P < 0.001). When the subgroup analysis stratified by ethnicity and HWE was performed, the positive result existed in most of the subgroups. For lipid levels association, the pooled effects indicated that the G allele carriers had higher TC and LDL-C levels than the non-carriers (for TC, SMD: 0.126, 95 % CI: 0.023-0.229, P = 0.016; for LDL-C, SMD: 0.170, 95 % CI: 0.053-0.287, P = 0.004, respectively). There was no difference in the levels of TG and HDL-C between the G carriers and the non-carriers in the whole population (SMD: 0.031, 95 % CI: -0.048-0.110, P = 0.440; SMD: -0.123, 95 % CI: -0.251-0.006, P = 0.061, respectively). When the studies were stratified by ethnicity and type of study, the G carriers had higher TC levels than the non-carriers (SMD: 0.126, 95 % CI: 0.014-0.238, P = 0.027) in the non-Asian subgroup. The similar results existed in cohort subgroup. The association between PCSK9 E670G polymorphism and LDL-C levels was significant in all subgroups. Meanwhile, the G carriers had higher TG levels than the non-carriers (SMD: 0.113, 95 % CI: 0.012-0.214, P = 0.028) in the case-control subgroup. AG + GG genotypes had lower HDL-C levels than AA genotype in Asian subgroup (SMD: -0.224, 95 % CI: -0.423- -0.025, P = 0.027) and in case-control subgroup (SMD: -0.257, 95 % CI: -0.467--0.048, P = 0.016).

CONCLUSIONS

The present meta-analysis concluded that PCSK9 E670G polymorphism was associated with CAD risk and lipid levels.

The contribution of PCSK9 levels to the phenotypic severity of familial hypercholesterolemia is independent of LDL receptor genotype

Autosomal dominant familial hypercholesterolemia (FH) is caused by genetic mutations in the LDL receptor (LDLR), its ligand apolipoprotein (apo) B, or proprotein convertase subtilisin/kexin type 9 (PCSK9). Although PCSK9 levels have been shown to correlate with LDL-cholesterol (LDL-C) levels in FH, the extent to which PCSK9 levels modulate the phenotypic severity of this disease independent of LDLR genotype remains to be clarified.

OBJECTIVE

To assess the relationship between LDLR genotype and the plasma levels of PCSK9, LDL-C, and lipoprotein (a) (Lp(a)) in a large cohort of genetically defined FH heterozygotes (HeFH).

METHODS

A total of 292 HeFH carrying one of the nine French-Canadian mutations in the LDLR gene were recruited. The cohort included 226 carriers of a negative-receptor (NR) mutation and 66 carriers of a defective-receptor (DR) LDLR gene mutation. Fifty-six control subjects, who were matched with the HeFH subjects based on gender and body mass index, were also recruited.

RESULTS

PCSK9 levels were higher in the HeFH group than in the control group (317.9±107.1 ng/mL vs. 203.3±59.8 ng/mL; P<0.0001). The strength of the association between PCSK9 and LDL-C levels was similar among controls (r=0.37; P=0.005) and HeFH (r=0.31; P<0.0001). Furthermore, a multiple linear regression analysis revealed that the positive correlation between PCSK9 and LDL-C levels remained significant after adjusting for LDLR genotype in the HeFH group.

CONCLUSION

These results suggested that the contribution of PCSK9 levels to the phenotypic severity in FH heterozygotes is independent of LDLR genotype.

Hepatic S1P deficiency lowers plasma cholesterol levels in apoB-containing lipoproteins when LDLR function is compromised

Background

Site-1 protease (S1P) is the key enzyme required for activation of the sterol regulatory element binding proteins (SREBPs) that govern lipid synthesis. While S1P has been speculated to influence plasma apoB-containing lipoprotein (Blp) metabolism, there has been little investigative work. LDL receptor (LDLR) is the major receptor for clearing plasma LDL cholesterol (LDL-c). Proprotein convertase subtilisin kexin type 9 (PCSK9) modulates LDL-c through post-translational degradation of the LDLR.

Methods

A hepatic-specific knockdown (KD) of S1P was achieved using floxed S1P mouse models (S1P^f/f and LDLR^-/-S1P^f/f) and hepatic expression of Cre recombinase. Lipids were measured in total plasma and size fractionated plasma using colorimetric assays. Realtime polymerase chain reaction, western blotting and ELISA were used to determine hepatic expression of key genes/protein. Plasmid mediated overexpression and siRNA mediated knockdown of genes were performed in mouse primary hepatocytes to determine the mechanistic basis of PCSK9 gene regulation.

Results

A hepatic-specific KD of S1P resulted in a 45 % and 38 % reduction in plasma total cholesterol and triglyceride levels, respectively. Hepatic S1P KD had a minimal effect on plasma Blp cholesterol (Blp-c) in S1P^f/f mice, despite significantly reducing VLDL secretion. Notably, hepatic S1P KD decreased the LDL receptor (LDLR) mRNA expression by 50 %. However, the reduction in LDLR protein levels was less than that of mRNA expression, especially under fed conditions. Further assessment of hepatic S1P deficiency revealed that it increased LDLR protein stability in vivo. Mechanistically, hepatic S1P KD was shown to decrease the liver and plasma levels of the protein proprotein convertase subtilisin/kexin type 9 (PCSK9), which degrades LDLR protein. This effect was more prominent in the fed condition and sufficient to account for the discordance in LDLR mRNA and protein levels. Furthermore, hepatic S1P was shown to regulate PCSK9 expression through activation of the SREBPs. In the LDLR^-/- background, hepatic S1P KD significantly reduced Blp-c levels.

Conclusion

Hepatic S1P is a physiological modulator of plasma Blp metabolism through its regulation of LDLR and PCSK9. Hepatic S1P is a valid target for lowering plasma Blp-c levels in the situation where LDLR function is compromised.

New developments in atherosclerosis: clinical potential of PCSK9 inhibition

Pro-protein convertase subtilisin/kexin type 9 (PCSK9) is a secreted 692-amino acid protein that binds surface low-density lipoprotein (LDL) receptor (LDLR) and targets it toward lysosomal degradation. As a consequence, the number of LDLRs at the cell surface is decreased, and LDL-cholesterol (LDL-C) clearance is reduced, a phenomenon that is magnified by gain-of-function mutations of PCSK9. In contrast, loss-of-function mutations of PCSK9 result in increased surface LDLR and improved LDL-C clearance. This provides the rationale for targeting PCSK9 in hypercholesterolemic subjects as a means to lower LDL-C levels. Monoclonal antibodies (mAbs) against PCSK9 that block its interaction with the LDLR have been developed in the past decade. Two companies have recently received the approval for their anti-PCSK9 mAbs by the US Food and Drug Administration (FDA) and the European Medicines Agency (EMA) Regeneron/Sanofi, with alirocumab (commercial name – PRALUENT^®) and, Amgen with evolocumab (commercial name – Repatha™). The introduction of anti-PCSK9 mAbs will provide an alternative therapeutic strategy to address many of the unmet needs of current lipid-lowering therapies, such as inability to achieve goal LDL-C level, or intolerance and aversion to statins. This review will focus on the kinetics of PCSK9, pharmacokinetics and pharmacodynamics of anti-PCSK9 mAbs, and recent data linking PCSK9 and anti-PCSK9 mAbs to cardiovascular events. Moreover, it will highlight the unanswered questions that still need to be addressed in order to understand the physiologic function, kinetics, and dynamics of PCSK9.

Plasma Membrane Tetraspanin CD81 Complexes with Proprotein Convertase Subtilisin/Kexin Type 9 (PCSK9) and Low Density Lipoprotein Receptor (LDLR), and Its Levels Are Reduced by PCSK9

Proprotein convertase subtilisin/kexin type 9 (PCSK9) is an important factor in plasma cholesterol regulation through modulation of low density lipoprotein receptor (LDLR) levels. Naturally occurring mutations can lead to hyper- or hypocholesterolemia in human. Recently, we reported that PCSK9 was also able to modulate CD81 in Huh7 cells. In the present study, several gain-of-function and loss-of-function mutants as well as engineered mutants of PCSK9 were compared for their ability to modulate the cell surface expression of LDLR and CD81. Although PCSK9 gain-of-function D374Y enhanced the degradation both receptors, D374H and D129N seemed to only reduce LDLR levels. In contrast, mutations in the C-terminal hinge-cysteine-histidine-rich domain segment primarily affected the PCSK9-induced CD81 degradation. Furthermore, when C-terminally fused to an ACE2 transmembrane anchor, the secretory N-terminal catalytic or hinge-cysteine-histidine-rich domain domains of PCSK9 were able to reduce CD81 and LDLR levels. These data confirm that PCSK9 reduces CD81 levels via an intracellular pathway as reported for LDLR. Using immunocytochemistry, a proximity ligation assay, and co-immunoprecipitation, we found that the cell surface level of PCSK9 was enhanced upon overexpression of CD81 and that both PCSK9 and LDLR interact with this tetraspanin protein. Interestingly, using CHO-A7 cells lacking LDLR expression, we revealed that LDLR was not required for the degradation of CD81 by PCSK9, but its presence strengthened the PCSK9 effect.

PCSK9 and triglyceride-rich lipoprotein metabolism

Pro-protein convertase subtilisin-kexin 9 (PCSK9) is known to affect low-density lipoprotein (LDL) metabolism, but there are indications from several lines of research that it may also influence the metabolism of other lipoproteins, especially triglyceride-rich lipoproteins (TRL). This review summarizes the current data on this possible role of PCSK9. A link between PCSK9 and TRL has been suggested through the demonstration of (1) a correlation between plasma PCSK9 and triglyceride (TG) levels in health and disease, (2) a correlation between plasma PCSK9 and markers of carbohydrate metabolism, which is closely related to TG metabolism, (3) an effect of TG-lowering fibrate therapy on plasma PCSK9 levels, (4) an effect of PCSK9 on postprandial lipemia, (5) an effect of PCSK9 on adipose tissue biology, (6) an effect of PCSK9 on apolipoprotein B production from the liver and intestines, (7) an effect of PCSK9 on receptors other than low density lipoprotein receptor (LDLR) that are involved in TRL metabolism, and (8) an effect of anti-PCSK9 therapy on serum TG levels. The underlying mechanisms are unclear but starting to emerge.

Alirocumab: PCSK9 inhibitor for LDL cholesterol reduction

The proof of concept that proprotein convertase subtilisin/kexin type 9 (PCSK9) inhibition affects cholesterol levels was first established after the demonstration that PCSK9 loss-of-function mutations result in a significant drop in circulating LDL cholesterol levels. Subsequent studies revealed that PCSK9 binds the epidermal growth factor precursor homology domain-A on the surface LDL Receptor (LDLR) and directs LDLR and PCSK9 for lysosomal degradation. Alirocumab (also known as SAR236553/REGN727) is a monoclonal antibody that binds circulating PCSK9 and blocks its interactions with surface LDLR. Alirocumab clinical trials with different doses on different administration schedules were shown to significantly reduce LDL cholesterol both as a mono-therapy and in combination with statins or ezetimibe. Although there is great potential for anti-PCSK9 therapies in the management of cholesterol metabolism, there is no clear evidence yet that blocking PCSK9 reduces cardiovascular disease outcome. This is being investigated in ongoing Phase III clinical trials with alirocumab.

Comparison of two low-density lipoprotein apheresis systems in patients with homozygous familial hypercholesterolemia

Low-density lipoprotein (LDL) apheresis (LA) is a reliable method to decrease LDL-C concentrations and remains the gold standard therapy in homozygous familial hypercholesterolemia (HoFH). The objective of this study was to compare the efficacy of two LA systems [heparin-induced extracorporeal LDL precipitation (HELP) vs. dextran sulfate adsorption (DS) on the reduction of lipids, inflammatory markers, and adhesion molecules in a sample of genetically defined HoFH subjects (n = 9)]. Fasting blood samples were collected before and after LA. All subjects served as their own control and were first treated with the HELP system then with DS in this single sequence study. Compared with HELP, DS led to significantly greater reductions in total cholesterol (-63.3% vs. -59.9%; P = 0.05), LDL-C (-70.5% vs. -63.0%; P = 0.02), CRP (-75.3% vs. -48.8%; P < 0.0001), and TNF-α (-23.7% vs. +14.7%; P = 0.003). Reductions in the plasma levels of PCSK9 (-45.3% vs. -63.4%; P = 0.31), lipoprotein (a) (-70.6% vs. -65.0%; P = 0.30), E-selectin (-16.6% vs. -18.3%; P = 0.65), ICAM-1 (-4.0 vs. 5.6%; P = 0.56), and VCAM-1 (8.3% vs. -1.8%; P = 0.08) were not different between the two systems. For the same volume of filtered plasma (3,000 mL), however, HELP led to greater reductions in plasma apoB (-63.1% vs. -58.3%; P = 0.04), HDL-C (-20.6% vs. -6.5%; P = 0.003), and PCSK9 (-63.4% vs. -28.5%; P = 0.02) levels. These results suggest that both LA systems are effective in reducing plasma lipids and inflammatory markers in HoFH. Compared with HELP, greater reductions in lipid levels and inflammatory markers were achieved with DS, most likely because this method allows for a larger plasma volume to be filtered. J. Clin. Apheresis 31:359-367, 2016. © 2015 Wiley Periodicals, Inc.