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

Elevated low-density lipoprotein cholesterol (LDLC) levels in the plasma is the most important causative factor of atherosclerosis and associated ischemic cardiovascular diseases. The LDL receptor (LDLR) is the preferential pathway through which LDLs are cleared from the circulation. LDLs bound to the LDLR are internalized into clathrin-coated pits and subsequently undergo lysosomal degradation, whereas the LDLR is recycled back to the plasma membrane. See accompanying article on page 1333 Familial hypercholesterolemia (FH) is an autosomal dominant disorder associated with elevated LDL levels and premature coronary heart disease. FH is caused primarily by mutations of the LDLR or of apolipoprotein B100 (apoB100), the protein component of LDL that interacts with the LDLR. In 2003, “gain of function” mutations on a newly identified gene, proprotein convertase subtilisin/kexin type 9 ( PCSK9), were associated with FH. In 2005, a causative association was established between “loss of function” mutations in PCSK9 and low LDLC levels in 2% of the African-American population. The coronary heart disease risk in these individuals was reduced by 88%. As a result of these landmark studies (reviewed in Reference 1), PCSK9 became the subject of intensive research to discover the underlying mechanisms. PCSK9 is a serine protease mainly expressed in the liver and the intestine. It acts by reducing the amount of LDLR in hepatocytes. This was demonstrated in vitro and in mouse models …

Proprotein convertase subtilisin/kexin type 9 (PCSK9) is a secreted protein that regulates the expression of LDL receptor (LDLR) protein. Gain-of-function mutations in PCSK9 cause hypercholesterolemia, and loss-of-function mutations result in lower plasma LDL-cholesterol. Here, we investigate the kinetics and metabolism of circulating PCSK9 relative to tissue levels of LDLRs. The administration of recombinant human PCSK9 (32 microg) to mice by a single injection reduced hepatic LDLRs by approximately 90% within 60 min, and the receptor levels returned to normal within 6 h. The half-life of the PCSK9 was estimated to be approximately 5 min. Continuous infusion of PCSK9 (32 microg/h) into wild-type mice caused a approximately 90% reduction in hepatic LDLRs within 2 h and no associated change in the level of LDLR in the adrenals. Parallel studies were performed using a catalytically inactive form of PCSK9, PCSK9(S386A), and similar results were obtained. Infusion of PCSK9(D374Y), a gain-of-function mutation, resulted in accelerated clearance of the mutant PCSK9 and a greater reduction in hepatic LDLRs. Combined, these data suggest that exogenously administrated PCSK9 in plasma preferentially reduces LDLR protein levels in liver at concentrations found in human plasma and that PCSK9's action on the LDLR is not dependent on catalytic activity in vivo.

Furin-cleaved Proprotein Convertase Subtilisin/Kexin Type 9 (PCSK9) Is Active and Modulates Low Density Lipoprotein Receptor and Serum Cholesterol Levels

GRP94 Regulates Circulating Cholesterol Levels through Blockade of PCSK9-Induced LDLR Degradation.

The role of the C-terminal domain of PCSK9 and SEC24 isoforms in PCSK9 secretion

The proprotein convertase subtilisin/kexin-type 9 (PCSK9), which promotes degradation of the hepatic low density lipoprotein receptor (LDLR), is now recognized as a major player in plasma cholesterol metabolism. Several gain-of-function mutations in PCSK9 cause hypercholesterolemia and premature atherosclerosis, and thus, inhibition of PCSK9-induced degradation of the LDLR may be used to treat this deadly disease. Herein, we discovered an endogenous PCSK9 binding partner by Far Western blotting, co-immunoprecipitation, and pull-down assays. Following two-dimensional gel electrophoresis and mass spectrometry analysis, we demonstrated that PCSK9 binds to a approximately 33-kDa protein identified as annexin A2 (AnxA2) but not to the closely related annexin A1. Furthermore, our functional LDLR assays and small hairpin RNA studies show that AnxA2 and the AnxA2.p11 complex could prevent PCSK9-directed LDLR degradation in HuH7, HepG2, and Chinese hamster ovary cells. Immunocytochemistry revealed that PCSK9 and AnxA2 co-localize at the cell surface, indicating a possible competition with the LDLR. Structure-function analyses demonstrated that the C-terminal cysteine-histidine-rich domain of PCSK9 interacts specifically with the N-terminal repeat R1 of AnxA2. Mutational analysis of this 70-amino acid-long repeat indicated that the RRTKK81 sequence of AnxA2 is implicated in this binding because its mutation to AATAA81 prevents its interaction with PCSK9. To our knowledge, this work constitutes the first to show that PCSK9 activity on LDLR can be regulated by an endogenous inhibitor. The identification of the minimal inhibitory sequence of AnxA2 should pave the way toward the development of PCSK9 inhibitory lead molecules for the treatment of hypercholesterolemia.

Proprotein convertase subtilisin/kexin type 9 (PCSK9) is associated with familial autosomal dominant hypercholesterolemia and is a natural inhibitor of the LDL receptor (LDLr). PCSK9 is degraded by other proprotein convertases: PC5/6A and furin. Both PCSK9 and the LDLr are up-regulated by the hypocholesterolemic statins. Thus, inhibitors or repressors of PCSK9 should amplify their beneficial effects. In the present study, we showed that PPARalpha activation counteracts PCSK9 induction by statins by repressing PCSK9 promoter activity and by increasing PC5/6A and furin expression. Quantification of mRNA and protein levels showed that various fibrates decreased PCSK9 and increased PC5/6A and furin expression. Fenofibric acid (FA) reduced PCSK9 protein content in immortalized human hepatocytes (IHH) as well as its cellular secretion. FA suppressed PCSK9 induction by statins or by the liver X receptor agonist TO901317. PCSK9 repression is occurring at the promoter level. We showed that PC5/6A and furin fibrate-mediated up-regulation is PPARalpha-dependent. As a functional test, we observed that FA increased by 30% the effect of pravastatin on the LDLr activity in vitro. In conclusion, fibrates simultaneously decreased PCSK9 expression while increasing PC5/6A and furin expression, indicating a broad action of PPARalpha activation in proprotein convertase-mediated lipid homeostasis. Moreover, this study validates the functional relevance of a combined therapy associating PCSK9 repressors and statins.

BackgroundCirculating proprotein convertase subtilisin/kexin type 9 (PCSK9) is a crucial regulator of cholesterol metabolism. Loss-of-function variants in PCSK9 are associated with lower levels of circulating low-density lipoprotein cholesterol (LDL-C) and reduced cardiovascular disease (CVD) risk, while gain-of-function variants correlate with elevated LDL-C concentrations and increased CVD risk. This study investigated whether genetically determined LDL-C levels, proxied by four PCSK9 genetic variants, influence common carotid artery atherosclerosis.MethodsThe analysis included 3040 European participants (mean age 64.2±5.4 years; 45.8% men) at high cardiovascular risk from the IMPROVE Study, alongside 49 088 individuals of white British ancestry (mean age 55.2±7.6 years; 47.9% men) from the UK Biobank (UKB). Ultrasonographic measurements of common carotid intima-media thickness (CC-IMTmean, CC-IMTmax, CC-IMTmean-max) were obtained. Four lipid-level affecting genetic variants in the PCSK9 locus were selected for analysis, both individually and in a standardised Lipid-Lowering Allelic Score (LLAS), to assess their effects on LDL-C and PCSK9 levels in the IMPROVE cohort and on ultrasonographic measures in both IMPROVE and UKB.ResultsIn the IMPROVE cohort, PCSK9 variants (rs11206510, rs2479409, rs11591147, rs11583680) exhibited expected effect directions, although not all statistically significant, on LDL-C and PCSK9 levels. The LLAS was negatively correlated with CC-IMTmean, CC-IMTmax and CC-IMTmean-max among women in IMPROVE, and among men and overall in UKB (all p<0.05). Effect sizes were comparable between cohorts.ConclusionsGenetic variants in the PCSK9 locus influence LDL-C levels and CC-IMT, in keeping with proven benefits of PCSK9 inhibitors on atherosclerotic cardiovascular events.

Development of Novel DNA-Encoded PCSK9 Monoclonal Antibodies as Lipid-Lowering Therapeutics.

Loss-of-function PCSK9 Mutants Evade the Unfolded Protein Response Sensor, GRP78, and Fail to Induce Endoplasmic Reticulum Stress when Retained

The discovery in 2003 that variants in the PCSK9 (proprotein convertase subtilisin/kexin type 9) gene cause autosomal dominant hypercholesterolemia revealed a new aspect of LDL cholesterol (LDL-C)2 metabolism (1). PCSK9, a protein of 692 amino acid residues produced at high concentrations in the liver, kidney, and intestine, has profound effects on LDL-C concentrations (2). Both gain-of-function and loss-of-function PCSK9 variants that cause higher and lower LDL-C concentrations, respectively, have been described, and variants in PCSK9 contribute to population variation in LDL-C values (3). The mechanism by which PCSK9 affects LDL-C concentrations involves direct binding of the protein to epidermal growth factor–like repeat A in the extracellular domain of the LDL receptor (LDLR), thereby accelerating degradation of the receptor (4). PCSK9 is a secreted protein that circulates in the blood. Several ELISA assays have been developed to measure PCSK9 concentrations (5, 6) and relate them to various biological correlates and differences in response to therapy, which may have implications for the therapeutic targeting of this protein. The interactions of statins with PCSK9 are of great interest, not only because statins are the predominant therapeutic agents used to decrease LDL-C but also because both the LDLR and PCSK9 share a common transcriptional regulator, sterol regulator element–binding protein 2 (SREBP-2) (7). Statin exposure produces an increase in the concentrations of both LDLR and PCSK9 mRNAs. The upregulation of PCSK9, which promotes the degradation of the LDLR, serves as a counterregulatory molecular brake on LDL-C lowering. The study described by Awan et al. (8) in …

The proprotein convertase subtilisin/kexin type 9 (PCSK9) regulates lipid metabolism, inflammation and haemostasis. Pathogenic gain-of-function variants in the PCSK9 gene are causative of autosomal-dominant form of familial hypercholesterolemia, while several PCSK9 alleles have been associated with elevated levels of low-density lipoprotein cholesterol (LDL-C) and an increased risk of cardiovascular disease. Elevated lipoprotein(a) (Lp(a)) levels, regardless of LDL-C levels, as well as functional and morphological changes in the arterial vessel wall predict future cardiovascular events. In our study, we aimed to identify whether treatment with statins nullifies the effect of four gain-of-function gene variants in PCSK9 on lipoproteins and arterial wall properties in high-risk post-myocardial infarction patients with severely elevated Lp(a) levels. We included 68 patients after myocardial infarction with elevated Lp(a) levels on maximal statin therapy. Biochemical analysis of lipid parameters and total PCSK9 levels were performed. Arterial wall properties were measured by ultrasound. Genotyping was performed for four gain-of-function, i.e. rs11206510, rs2479409, rs2479408 and rs1711503, and one intergenic, rs11591147 PCSK9 single nucleotide polymorphisms. The results showed no association between studied PCSK9 gene variants and lipid parameters, PCSK9 levels and arterial wall properties in our patient cohort. Clinical, biochemical and arterial wall parameters did not differ between the group with lower compared to group with higher number of PCSK9 alleles. Our results suggest that in high-risk patients after myocardial infarction with increased Lp(a) levels treated with statins the studied PCSK9 gain-of-function gene variants are associated neither with lipoproteins nor with arterial wall properties.

PCSK9 inhibition is an effective therapy to reduce LDL cholesterol (LDL-C) and cardiovascular events. A recent study shows that one or two doses of inclisiran, a long-acting synthetic small-interfering RNA (siRNA) that selectively targets hepatic PCSK9, causes a sustained reduction of plasma LDL-C for up to 6 months. Pending further studies of safety and efficacy, this may represent an important addition to the armamentarium for inhibiting PCSK9.

Familial hypercholesterolemia (FH) is an autosomal dominant disease caused by pathogenic variants in genes encoding for LDL receptor (LDLR), Apolipoprotein B and Proprotein convertase subtilisin/kexin type 9 (PCSK9). Among PCSK9 variants, only Gain-of- Function (GOF) variants lead to FH. Greater attention should be paid to the classification of variants as pathogenic. Two hundred sixty nine patients with a clinical suspect of FH were screened for variants in LDLR and the patients without pathogenic variants were screened for variants in PCSK9 and APOB. Functional characterization of PCSK9 variants was performed by assessment of protein secretion, of LDLR activity in presence of PCSK9 variant proteins as well as of the LDLR affinity of the PCSK9 variants. Among 81 patients without pathogenic variants in LDLR, 7 PCSK9 heterozygotes were found, 4 of whom were carriers of variants whose role in FH pathogenesis is still unknown. Functional characterization revealed that two variants (p.(Ser636Arg) and p.(Arg357Cys)) were GOF variants. In Conclusions, we demonstrated a GOF effect of 2 PCSK9 variants that can be considered as FH-causative variants. The study highlights the important role played by functional characterization in integrating diagnostic procedures when the pathogenicity of new variants has not been previously demonstrated.

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