PCSK9

Source: Wikipedia, the free encyclopedia.
Not to be confused with PKCS#9.
PCSK9
Gene ontology
Molecular function
Cellular component
Biological process
Sources:Amigo / QuickGO
Ensembl

ENSG00000169174

ENSMUSG00000044254

UniProt

Q8NBP7

Q80W65

RefSeq (mRNA)

NM_174936

NM_153565

RefSeq (protein)

NP_777596

NP_705793

Location (UCSC)Chr 1: 55.04 – 55.06 MbChr 4: 106.3 – 106.32 Mb
PubMed search[3][4]
Wikidata
View/Edit HumanView/Edit Mouse

Proprotein convertase subtilisin/kexin type 9 (PCSK9) is an

orthologs) are found across many species. As with many proteins, PCSK9 is inactive when first synthesized, because a section of peptide chains blocks their activity; proprotein convertases remove that section to activate the enzyme.[7] The PCSK9 gene also contains one of 27 loci associated with increased risk of coronary artery disease.[8]

PCSK9 is ubiquitously expressed in many tissues and cell types.

lysosomes for destruction. If PCSK9 is blocked, the LDL-LDLR complex separates during trafficking, with the LDL digested in the lysosome, but the LDLRs instead recycled back to the cell surface and so able to remove additional LDL-particles from the extracellular fluid.[10][11] Therefore, blocking PCSK9 can lower blood LDL-particle concentrations.[12][13]

PCSK9 has medical importance because it acts in lipoprotein homeostasis. Agents that block PCSK9 can lower LDL particle concentrations. The first two PCSK9 inhibitors, alirocumab and evolocumab, were approved as once every two week injections, by the U.S. Food and Drug Administration in 2015 for lowering LDL-particle concentrations when statins and other drugs were not sufficiently effective or poorly tolerated. The cost of these new medications, as of 2015, was $14,000 per year at full retail; judged of unclear cost effectiveness by some.[14] While these medications are prescribed by many physicians, the payment for prescriptions are often denied by insurance providers.[15][16][17] As a result, pharmaceutical manufacturers lowered the prices of these drugs.[18]

History

In February 2003,

nonsense mutations in the gene, thus validating PCSK9 as a biological target for drug discovery.[19][23]

In July 2015, the FDA approved the first PCSK9 Inhibitor drugs for medical use.[24]

Structure

Gene

The PCSK9 gene resides on chromosome 1 at the band 1p32.3

isoforms through alternative splicing.[27]
: Sequence & Isoform 

Protein

Crystal structure of PCSK9 (PDB: 2P4E​)[28]

PCSK9 is a member of the

peptidase S8 family.[27]
: Family & Domains 

The solved structure of PCSK9 reveals four major components in the pre-processed protein: the

C-terminal domain (residues 426–692), which is further divided into three modules.[29] The N-terminal prodomain has a flexible crystal structure and is responsible for regulating PCSK9 function by interacting with and blocking the catalytic domain, which otherwise binds the epidermal growth factor-like repeat A (EGF-A) domain of the LDLR.[29][30][31] While previous studies indicated that the C-terminal domain was uninvolved in binding LDLR,[32][33] a recent study by Du et al. demonstrated that the C-terminal domain does bind LDLR.[29] The secretion of PCSK9 is largely dependent on the autocleavage of the signal peptide and N-terminal prodomain, though the N-terminal prodomain retains its association with the catalytic domain. In particular, residues 61–70 in the N-terminal prodomain are crucial for its autoprocessing.[29]

Function

Synthesis

PCSK9 is synthesized as a soluble zymogen that undergoes autocatalytic intramolecular processing in the endoplasmic reticulum.[7] It is expressed mainly in liver, intestine, kidney, skin and the central nervous system.[34] After being processed in the ER, PCSK9 co-localizes with the protein sortilin on its way through the Golgi and trans-Golgi complex. A PCSK9-sortilin interaction is proposed to be required for cellular secretion of PCSK9.[35] In healthy humans, plasma PCSK9 levels directly correlate with plasma sortilin levels, following a diurnal rhythm similar to cholesterol synthesis.[36][37] The plasma PCSK9 concentration is higher in women compared to men, and the PCSK9 concentrations decrease with age in men but increase in women, suggesting that estrogen level most likely plays a role.[38][39] PCSK9 gene expression can be regulated by sterol-response element binding proteins (SREBP-1/2), which also controls LDLR expression.[36]

Cholesterol homeostasis

As a negative post-translational regulator of the low-density lipoprotein receptor (LDLR), PCSK9 plays a major role in cholesterol homeostasis. Upon binding of low-density lipoprotein (LDL) cholesterol to its receptor, the resulting LDLR-LDL complex is internalized. When exposed to the acidic environment within the resulting endosome LDLR adopts a hairpin conformation.[40] This conformational change in turn induces the dissociation of the LDL-LDLR complex, allowing LDLR to be recycled back to the plasma membrane. Binding of PCSK9 binds to cell surface LDLR (through the LDLR EGF-A domain) also induces LDLR internalization. However, unlike LDL binding, PCSK9 prevents LDLR from undergoing a conformational change. This inhibition redirects LDLR to a lysosome where it is degraded.[40] Thus, PCSK9 lowers cell surface expression of LDLR and thereby decreases metabolism of LDL-particles, which in turn may lead to hypercholesterolemia.[41] PCSK9 also plays an important role in triglyceride-rich apoB lipoprotein production in small intestine and postprandial lipemia.[42][43][44]

Skin and inflammation

ApoB lipoprotein, PCSK9, and the genes involved in cholesterol synthesis are highly expressed in the epidermis.[45][46] The cutaneous expression of PCSK9 is likely important for proper skin barrier formation as ceramides, free fatty acids, and cholesterol are the three major components of the epidermal lipid barrier.[47] Matching its function in cholesterol homeostasis, there is a gradient of PCSK9 expression in the epidermis. PCSK9 is selectively expressed in basal and spinous layer keratinocytes with little to no expression in granular layer keratinocytes.[45] In contrast to basal layer keratinocytes, granular layer keratinocytes release large amounts of cholesterol and other lipids to form a lipid rich "mortar" in the intracellular space between keratinocytes.[47] In addition to its likely role in epidermal lipid barrier formation, PCSK9 has also been linked to skin inflammation. For example, genetic variants of PCSK9 have been linked psoriasis,[45] and knockdown expression of PCSK9 in keratinocytes results in increase expression of IL-36G and other keratinocyte-derived inflammatory mediators.[45]

Other functions of PCSK9

PCSK9 may also have a role in the differentiation of cortical neurons.[5]

Clinical significance

Variants of PCSK9 can reduce or increase circulating cholesterol. LDL-particles are removed from the blood when they bind to LDLR on the surface of cells, including liver cells, and are taken inside the cells. When PCSK9 binds to an LDLR, the receptor is destroyed along with the LDL particle. PCSK9 degrades LDLR by preventing the hairpin conformational change of LDLR.[48] If PCSK9 does not bind, the receptor will return to the surface of the cell and can continue to remove LDL-particles from the bloodstream.[49]

Other variants are associated with a rare autosomal dominant familial hypercholesterolemia (HCHOLA3).[50][21][51] The mutations increase its protease activity, reducing LDLR levels and preventing the uptake of cholesterol into the cells.[21]

In humans, PCSK9 was initially discovered as a

smooth muscle cells, and macrophages, with a local effect that can regulate vascular homeostasis and atherosclerosis.[54][55][56] Accordingly, it is now very clear that PCSK9 has pro-atherosclerotic effects and regulates lipoprotein synthesis.[57]

As PCSK9 binds to LDLR, which prevents the removal of

LDL-particles from the blood plasma, several studies have determined the potential use of PCSK9 inhibitors in the treatment of hyperlipoproteinemia (commonly called hypercholesterolemia).[14][53][58][59][60][61][62][63] Furthermore, loss-of-function mutations in the PCSK9 gene result in lower levels of LDL and protection against cardiovascular disease.[57][64][65]

In addition to its lipoprotein synthetic and pro-atherosclerotic effects, PCSK9 is involved in glucose metabolism and obesity,[66] regulation of re-absorption of sodium in the kidney which is relevant in hypertension.[67][68] Furthermore, PCSK9 may be involved in bacterial or viral infections and sepsis.[69][70][71] In the brain the role of PCSK9 is still controversial and may be either pro-apoptotic or protective in the development of the nervous system.[5] PCSK9 levels have been detected in the cerebrospinal fluid at a 50-60 times lower level than in serum.[72]

Clinical marker

A multi-locus genetic risk score study based on a combination of 27 loci including the PCSK9 gene, identified individuals at increased risk for both incident and recurrent coronary artery disease events, as well as an enhanced clinical benefit from statin therapy. The study was based on a community cohort study (the Malmo Diet and Cancer study) and four additional randomized controlled trials of primary prevention cohorts (JUPITER and ASCOT) and secondary prevention cohorts (CARE and PROVE IT-TIMI 22).[8]

Inhibitors

Under normal conditions, PCSK9 binds to the LDL-LDLR-complex and directs both to the lysosome for degradation.[73]
PCSK9-inhibitors that prevent the association between PCSK9 and the LDLR mean that when LDLR is internalised, it releases the LDL before reaching the lysosome and is instead recycled to the cell surface to be available for binding again.[73]

Several studies have determined the potential use of PCSK9 inhibitors in the treatment of hyperlipoproteinemia (commonly called hypercholesterolemia).[14][53] Furthermore, loss-of-function mutations in the PCSK9 gene result in lower levels of LDL and protection against cardiovascular disease.[57]

PCSK9 inhibitor drugs are now approved by the FDA to treat familial hypercholesterolemia.[15]

As a drug target

Drugs can inhibit PCSK9, leading to lowered circulating LDL particle concentrations. Since LDL particle concentrations are thought by many experts to be a driver of

RNAi therapeutic inclisiran.[79] PCSK9 inhibitors are promising therapeutics for the treatment of people who exhibit statin intolerance, or as a way to bypass frequent dosage of statins for higher LDL concentration reduction.[80][81]

A review published in 2015 concluded that these agents, when used in patients with high LDL-particle concentrations (thus at greatly elevated risk for cardiovascular disease) seem to be safe and effective at reducing all-cause mortality, cardiovascular mortality, and heart attacks.[82] However a 2020 review concluded that while PCSK9 inhibitor treatment provides additional benefits beyond maximally tolerated statin therapy in high-risk individuals,[83] PCSK9 inhibitor use probably produces little or no difference in mortality.[84]

FDA for treatment of hypercholesterolemia, notably the genetic condition heterozygous familial hypercholesterolemia which causes high cholesterol levels and heart attacks at a young age.[20] These drugs were later approved by the FDA for the reduction of cardiovascular events including a reduction in all-cause mortality.[85]

In a meta-analysis involving data from 3 randomized controlled trials, early initiation of PCSK9 inhibitors within 72 hours of acute coronary event along with high dose statin was associated with a more rapid decline in cholesterol levels 4 weeks after the cardiac event, which translated into a significant reduction hospital readmission post-acute cardiac event.[86]

Warning

An FDA warning in March 2014 about possible cognitive adverse effects of PCSK9 inhibition caused concern, as the FDA asked companies to include neurocognitive testing into their

Phase III clinical trials.[87]

Monoclonal antibodies

A number of

monoclonal antibodies that bind to and inhibit PCSK9 near the catalytic domain were in clinical trials as of 2014. These include evolocumab (Amgen), bococizumab (Pfizer), and alirocumab (Sanofi/Regeneron Pharmaceuticals).[73] As of July 2015, the EU approved these drugs including Evolocumab/Amgen according to Medscape news agency report. A meta-analysis of 24 clinical trials has shown that monoclonal antibodies against PCSK9 can reduce cholesterol, cardiac events and all-cause mortality.[82] The most recent guidelines for cholesterol management from the American Heart Association and American College of Cardiology now provide guidance for when PCSK9 inhibitors should be considered, particularly focusing on cases in which maximally tolerated statin and ezetimibe fail to achieve goal LDL reduction.[88]

A possible side effect of the monoclonal antibody might be irritation at the injection site. Before the infusions, participants received oral corticosteroids, histamine receptor blockers, and acetaminophen to reduce the risk of infusion-related reactions, which by themselves will cause several side effects.[89]

Peptide mimics

Peptides that mimick the EGFA domain of the LDLR that binds to PCSK9 have been developed to inhibit PCSK9.[90]

Gene silencing

The PCSK9

mRNA levels in mice.[92][93] Initial clinical trials showed positive results of ALN-PCS, which acts by means of RNA interference.[78][94][95]

In 2021, scientists demonstrated that

Macaca fascicularis monkeys for months by 60% via knockdown of PCSK9 in the liver.[96][97]

In 2023, a clinical trial demonstrated that

heterozygous familial hypercholesterolemia.[98][99]

Vaccination

A vaccine that targets PCSK9 has been developed to treat high LDL-particle concentrations. The vaccine uses a VLP (

antibodies that bound to circulating PCSK9. Vaccination was associated with significant reductions in total cholesterol, free cholesterol, phospholipids, and triglycerides.[100]

Naturally occurring inhibitors

The plant alkaloid berberine inhibits the transcription of the PCSK9 gene in immortalized human hepatocytes in vitro,[101] and lowers serum PCSK9 in mice and hamsters in vivo.[102] It has been speculated[102] that this action contributes to the ability of berberine to lower serum cholesterol.[103] Annexin A2, an endogenous protein, is a natural inhibitor of PCSK9 activity.[104]

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Further reading

External links
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