Molecular basis for LDL receptor recognition by PCSK9

PCSK9 as a therapeutic target of dyslipidemia

BACKGROUND

Proprotein convertase subtilisin/kexin type 9 (PCSK9), which promotes degradation of hepatic low density lipoprotein receptor (LDLR), has a role in plasma cholesterol metabolism. Its gene is associated with the development of familial hypercholesterolemia. mRNA silencing or inhibition of PCSK9-induced degradation of LDLR may be used to treat this disease.

OBJECTIVE/METHODS

To summarize approaches proposed to reduce the levels of PCSK9 and/or its activity.

RESULTS/CONCLUSIONS

mRNA knockdown approaches include the use of antisense oligonucleotides either as soluble phosphorothioates or locked nucleic acids and lipidoid nanoparticles embedded with small interfering RNAs. Passive immunization is also an option. Other strategies include inhibition of the zymogen activation of proPCSK9, or the interaction of PCSK9 with the EGF-A domain of the LDLR. The N-terminal prosegment and the C-terminal Cys-His rich domain (CHRD), are alternative targets. Annexin A2 specifically binds the CHRD and inhibits PCSK9 function, and Annexin A2 peptide mimics could pave the way for the development of novel PCSK9-inhibitory compounds.

Importance of proprotein convertase subtilisin/kexin type 9 in the hormonal and dietary regulation of rat liver low-density lipoprotein receptors

Hormonal or dietary challenge can stimulate hepatic low-density lipoprotein receptor (LDLR) expression through posttranscriptional mechanisms. We here tested whether such observations may be due to regulation of proprotein convertase subtilisin/kexin type 9 (PCSK9). Treatment with glucagon resulted in a 2-fold increase in hepatic LDLR protein expression, whereas its mRNA levels were reduced; this occurred simultaneously with a 70% reduction in PCSK9 expression. Insulin treatment resulted in responses opposite to those seen by treatment with glucagon. Furthermore, high-dose ethinylestradiol treatment reduced PCSK9 expression by half. Finally, feeding of rats with dietary cholesterol reduced PCSK9 expression, resulting in an increased number of hepatic LDLRs despite a reduction of LDLR mRNA levels. Regulation of PCSK9 occurred in part through sterol regulatory element binding protein-2, but changes in this cholesterol-controlled transcription factor could not explain all hormonal effects seen. We conclude that the hormonal and dietary regulation of hepatic LDLRs also involves posttranscriptional regulation by PCSK9. The identification of PCSK9 regulation by these various treatments is important in understanding of the physiological function of this protein and points to new targets for therapeutic treatments to increase hepatic LDLR numbers.

Peptide processing and biology in human disease

PURPOSE OF REVIEW

To describe recent advances in the processing of gastrointestinal hormones, and the consequences for human disease of mutations in the enzymes involved.

RECENT FINDINGS

Although gastrointestinal prohormones were long regarded as devoid of biological activity, recent data indicate that the prohormones for both gastrin and gastrin-releasing peptide are bioactive, through different receptors from the mature hormones. Mutations in the family of prohormone convertases responsible for the initial steps in the processing of gastrointestinal hormones are associated with several different pathophysiological conditions in humans.

SUMMARY

Human mutational studies, when taken together with the phenotypes observed in mice deficient in the prohormone convertases, emphasize the crucial importance of the processing enzymes in mammalian biology. Although the phenotypes may often be ascribed to defective production of a mature hormone or growth factor, the recognition that the precursors are independently bioactive suggests that the increased precursor concentrations may also contribute to the symptoms. The observation that the precursors often act through different receptors from the mature hormones may permit the development of precursor-selective antagonists for therapeutic use.

PCSK9: a convertase that coordinates LDL catabolism

The identification and characterization of proprotein convertase subtilisin-like/kexin type 9 (PCSK9) have provided new insights into LDL metabolism and the causal role of LDL in coronary heart disease (CHD). PCSK9 is a secreted protease that mediates degradation of the LDL receptor by interacting with the extracellular domain and targeting the receptor for degradation. Individuals with loss-of-function mutations in PCSK9 have reduced plasma levels of LDL cholesterol and are protected from CHD; these observations have validated PCSK9 as a therapeutic target and suggested new approaches for the treatment and prevention of CHD.

Structural and biochemical characterization of the wild type PCSK9-EGF(AB) complex and natural familial hypercholesterolemia mutants

PCSK9 regulates low density lipoprotein receptor (LDLR) levels and consequently is a target for the prevention of atherosclerosis and coronary heart disease. Here we studied the interaction, of LDLR EGF(A/AB) repeats with PCSK9. We show that PCSK9 binds the EGF(AB) repeats in a pH-dependent manner. Although the PCSK9 C-terminal domain is not involved in LDLR binding, PCSK9 autocleavage is required. Moreover, we report the x-ray structure of the PCSK9DeltaC-EGF(AB) complex at neutral pH. Compared with the low pH PCSK9-EGF(A) structure, the new structure revealed rearrangement of the EGF(A) His-306 side chain and disruption of the salt bridge with PCSK9 Asp-374, thus suggesting the basis for enhanced interaction at low pH. In addition, the structure of PCSK9DeltaC bound to EGF(AB)(H306Y), a mutant associated with familial hypercholesterolemia (FH), reveals that the Tyr-306 side chain forms a hydrogen bond with PCSK9 Asp-374, thus mimicking His-306 in the low pH conformation. Consistently, Tyr-306 confers increased affinity for PCSK9. Importantly, we found that although the EGF(AB)(H306Y)-PCSK9 interaction is pH-independent, LDLR(H306Y) binds PCSK9 50-fold better at low pH, suggesting that factors other than His-306 contribute to the pH dependence of PCSK9-LDLR binding. Further, we determined the structures of EGF(AB) bound to PCSK9DeltaC containing the FH-associated D374Y and D374H mutations, revealing additional interactions with EGF(A) mediated by Tyr-374/His-374 and providing a rationale for their disease phenotypes. Finally, we report the inhibitory properties of EGF repeats in a cellular assay measuring LDL uptake.

Mutation S462P in the PCSK9 gene reduces secretion of mutant PCSK9 without affecting the autocatalytic cleavage

OBJECTIVE

The normal function of proprotein convertase subtilisin/kexin type 9 (PCSK9) is to mediate degradation of the low density lipoprotein (LDL) receptors. However, the exact mechanism for this function remains to be determined. Characterization of how the naturally occurring mutations in the PCSK9 gene affect the function of PCSK9, may provide important insight into the mechanism by which PCSK9 degrades the LDL receptors.

METHODS

DNA sequencing of the 12 exons with flanking intron sequences of the PCSK9 gene was performed in 1336 unrelated hypercholesterolemic subjects. In vitro assays and bioinformatics analysis were employed to characterize the functional consequences of a novel mutation. EXPERIMENTAL RESULTS: One subject was heterozygous for the novel mutation S462P in exon 9 of the PCSK9 gene. Based upon Western blot analysis of transiently transfected HepG2 cells, S462P-PCSK9 was almost completely retained in the endoplasmic reticulum (ER) even though it did undergo autocatalytic cleavage. Thus, only trace amounts of S462P-PCSK9 were detected in the culture media of transfected cells. Flow cytometric experiments revealed that the S462P-PCSK9 mutant was unable to degrade the LDL receptors.

DISCUSSION

The markedly reduced secretion of S462P-PCSK9 makes S462P a loss-of-function mutation. Ser462 is one of the few residues in the C-terminal domain which is conserved in all known PCSK9 homologs. A hydrogen bond between the side-chain of Ser462 and the backbone of beta-strand 6 of the C-terminal domain, appears to be essential for the proper folding of the C-terminal domain. The S462P mutation is believed to disrupt the normal folding of the C-terminal domain leading to retention of the mutant protein in the ER.

Annexin A2 is a C-terminal PCSK9-binding protein that regulates endogenous low density lipoprotein receptor levels

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.

PCSK9 function and physiology

PCSK9 has exploded onto center stage plasma cholesterol metabolism, raising hopes for a new strategy to treat hypercholesterolemia. PCSK9 in a plasma protein that triggers increased degradation of the LDL receptor. Gain-of-function mutations in PCSK9 reduce LDL receptor levels in the liver, resulting in high levels of LDL cholesterol in the plasma and increased susceptibility to coronary heart disease. Loss-of-function mutations lead to higher levels of the LDL receptor, lower LDL cholesterol levels and protection from coronary heart disease. Two papers in this issue of the Journal of Lipid Research exemplify the rapid pace of progress in understanding PCSK9 molecular interactions and physiology. Dr. Shilpa Pandit and coworkers from Merck Research Laboratories describe the functional basis for the hypercholesterolemia associated with gain-of-function missense mutations in PCSK9. Dr. Jay Horton's group at UT Southwestern describe the kinetics and metabolism of PCSK9 and the impact of PCSK9 on LDL receptors in the liver and adrenal gland.

Structural requirements for PCSK9-mediated degradation of the low-density lipoprotein receptor

Proprotein convertase subtilisin/kexin type 9 (PCSK9) is a secreted protein that controls plasma LDL cholesterol levels by posttranslational regulation of the LDL receptor (LDLR). Previously, we showed that PCSK9 binds specifically to an EGF-like repeat (EGF-A) in LDLR and reroutes the receptor from endosomes to lysosomes rather than to the cell surface. Here, we defined the regions in LDLR and PCSK9 that are required for receptor degradation and examined the relationship between PCSK9 binding and LDLR conformation. Addition of PCSK9 to cultured hepatocytes promoted degradation of WT LDLR and of receptors lacking up to four ligand binding domains, EGF-B or the clustered O-linked sugar region. In contrast, LDLRs lacking the entire ligand binding domain or the beta-propeller domain failed to be degraded, although they bound and internalized PCSK9. Using gel filtration chromatography, we assessed the effects of PCSK9 binding on an acid-dependent conformational change that happens in the extracellular domain of the LDLR. Although PCSK9 prevented the reduction in hydrodynamic radius of the receptor that occurs at a reduced pH, the effect was not sufficient for LDLR degradation. A truncated version of PCSK9 containing the prodomain and the catalytic domain, but not the C-terminal domain, bound the receptor but did not stimulate LDLR degradation. Thus, domains in both the LDLR and PCSK9 that are not required for binding (or internalization) are essential for PCSK9-mediated degradation of the LDLR.

Proprotein convertase subtilisin/kexin type 9 (PCSK9): hepatocyte-specific low-density lipoprotein receptor degradation and critical role in mouse liver regeneration

The gene encoding the proprotein convertase subtilisin/kexin type 9 (PCSK9) is linked to familial hypercholesterolemia, as are those of the low-density lipoprotein receptor (LDLR) and apolipoprotein B. PCSK9 enhances LDLR degradation, resulting in low-density lipoprotein accumulation in plasma. To analyze the role of hepatic PCSK9, total and hepatocyte-specific knockout mice were generated. They exhibit 42% and 27% less circulating cholesterol, respectively, showing that liver PCSK9 was responsible for two thirds of the phenotype. We also demonstrated that, in liver, PCSK9 is exclusively expressed in hepatocytes, representing the main source of circulating PCSK9. The data suggest that local but not circulating PCSK9 regulates cholesterol levels. Although transgenic mice overexpressing high levels of liver and circulating PCSK9 led to the almost complete disappearance of the hepatic LDLR, they did not recapitulate the plasma cholesterol levels observed in LDLR-deficient mice. Single LDLR or double LDLR/PCSK9 knockout mice exhibited similar cholesterol profiles, indicating that PCSK9 regulates cholesterol homeostasis exclusively through the LDLR. Finally, the regenerating liver of PCSK9-deficient mice exhibited necrotic lesions, which were prevented by a high-cholesterol diet. However, lipid accumulation in hepatocytes of these mice was markedly reduced under both chow and high-cholesterol diets, revealing that PCSK9 deficiency confers resistance to liver steatosis.

CONCLUSION

Although PCSK9 is a target for controlling hypercholesterolemia, our data indicate that upon hepatic damage, patients lacking PCSK9 could be at risk.

PCSK9 binds to multiple receptors and can be functionally inhibited by an EGF-A peptide

Proprotein convertase subtilisin/kexin type 9 (PCSK9) binds to low density lipoprotein receptor (LDLR) and induces its internalization and degradation. PCSK9 binding to LDLR is mediated through the LDLR epidermal growth factor-like repeat A (EGF-A) domain. We show for the first time that an EGF-A peptide inhibits PCSK9-mediated degradation of LDLR in HepG2 cells. In addition to LDLR, we show that PCSK9 also binds directly to ApoER2 and mouse VLDLR. Importantly, binding of PCSK9 to either LDLR or mouse VLDLR was effectively inhibited by EGF-A while binding to ApoER2 was less affected. In contrast, LDL receptor-associated protein (RAP), which interacts with LDL receptor repeat type A (LA) domains, inhibited PCSK9 binding to ApoER2 with greater efficacy than either LDLR or mVLDLR. These data demonstrate that while PCSK9 binds several receptors via its EGF-A binding domain, additional contacts with other receptor domains are also involved.

Mechanism of low density lipoprotein (LDL) release in the endosome: implications of the stability and Ca2+ affinity of the fifth binding module of the LDL receptor

Uptake of low density lipoproteins (LDL) by their receptor, LDLR, is the primary mechanism by which cells incorporate cholesterol from plasma. Mutations in LDLR lead to familial hypercholesterolemia, a common disease affecting 1 in 500 of the human population. LDLR is a modular protein that uses several small repeats to bind LDL. The repeats contain around 40 residues, including three disulfide bonds and a calcium ion. Repeat 5 (LR5) is critical for LDL and beta-migrating very low density lipoprotein binding. Based on the crystal structure of LDLR at endosomal pH (but close to extracellular calcium concentration), LR5 has been proposed to bind to the epidermal growth factor (EGF) precursor domain of LDLR in the endosome, thus releasing the LDL particles previously bound in extracellular conditions. We report here the conformational stability of LR5 as a function of temperature and calcium concentration under both extracellular and endosomal pH conditions. The repeat was very stable when it bore a bound calcium ion but was severely destabilized in the absence of calcium and even further destabilized at acidic versus neutral pH. The temperature and calcium concentration dependence of LR5 stability clearly indicate that under endosomal conditions the unfolded conformation of the repeat is largely dominant. We thus propose a new mechanism for LDL release in the endosome in which calcium depletion and decreased stability at acidic pH drives LR5 unfolding, which triggers LDL release from the receptor. Subsequent binding of LR5 to the EGF precursor domain, if it takes place at low calcium concentrations, would contribute to a further shifting of the equilibrium toward dissociation.

PCSK9 is phosphorylated by a Golgi casein kinase-like kinase ex vivo and circulates as a phosphoprotein in humans

Proprotein convertase subtilisin/kexin 9 (PCSK9) is a secreted glycoprotein that regulates the degradation of the low-density lipoprotein receptor. Single nucleotide polymorphisms in its gene associate with both hypercholesterolemia and hypocholesterolemia, and studies have shown a significant reduction in the risk of coronary heart disease for 'loss-of-function' PCSK9 carriers. Previously, we reported that proPCSK9 undergoes autocatalytic processing of its prodomain in the endoplasmic reticulum and that its inhibitory prosegment remains associated with it following secretion. Herein, we used a combination of mass spectrometry and radiolabeling to report that PCSK9 is phosphorylated at two sites: Ser47 in its propeptide and Ser688 in its C-terminal domain. Site-directed mutagenesis suggested that a Golgi casein kinase-like kinase is responsible for PCSK9 phosphorylation, based on the consensus site, SXE/S(p). PCSK9 phosphorylation was cell-type specific and occurs physiologically because human plasma PCSK9 is phosphorylated. Interestingly, we show that the naturally occurring 'loss-of-function' variant PCSK9(R46L) exhibits significantly decreased propeptide phosphorylation in the Huh7 liver cell line by 34% (P < 0.0001). PCSK9(R46L) and the engineered, unphosphorylated variant PCSK9(E49A) are cleaved following Ser47, suggesting that phosphorylation protects the propeptide against proteolysis. Phosphorylation may therefore play an important regulatory role in PCSK9 function. These findings will be important for the future design of PCSK9 inhibitors.

Plasma PCSK9 preferentially reduces liver LDL receptors in mice

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.

Functional analysis of sites within PCSK9 responsible for hypercholesterolemia

Mutations within proprotein convertase subtilisin/kexin type 9 (PCSK9) are associated with dominant forms of familial hypercholesterolemia. PCSK9 binds the LDL receptor (LDLR), and addition of PCSK9 to cells promotes degradation of LDLR. PCSK9 mutant proteins associated with hypercholesterolemia (S127R and D374Y) are more potent in decreasing LDL uptake than is wild-type PCSK9. To better understand the mechanism by which mutations at the Ser127 and Asp374 residues of PCSK9 influence PCSK9 function, a limited vertical scanning mutagenesis was performed at both sites. S127R and S127K proteins were more potent in decreasing LDL uptake than was wild-type PCSK9, and each D374 mutant tested was more potent in reducing LDL uptake when the proteins were added exogenously to cells. The potencies of D374 mutants in lowering LDL uptake correlated with their ability to interact with LDLR in vitro. Combining S127R and D374Y was also found to have an additive effect in enhancing PCSK9's ability to reduce LDL uptake. Modeling of PCSK9 S127 and D374 mutations indicates that mutations that enhance PCSK9 function stabilize or destabilize the protein, respectively. In conclusion, these results suggest a model in which mutations at Ser127 and Asp374 residues modulate PCSK9's ability to regulate LDLR function through distinct mechanisms.