Proprotein convertase subtilisin/kexin type 9 (PCSK9), a member of the proteinase K subfamily of subtilases, promotes internalization
and degradation of low density lipoprotein receptors (LDLRs) after binding the receptor on the surface of hepatocytes. PCSK9
has autocatalytic activity that releases the prodomain at the N terminus of the protein. The prodomain remains tightly associated
with the catalytic domain as the complex transits the secretory pathway. It is not known whether enzymatic activity is required
for the LDLR-reducing effects of PCSK9. Here we expressed the prodomain together with a catalytically inactive protease domain
in cells and purified the protein from the medium. The ability of the catalytically inactive PCSK9 to bind and degrade LDLRs
when added to culture medium of human hepatoma HepG2 cells at physiological concentrations was similar to that seen using
wild-type protein. Similarly, a catalytic-dead version of a gain-of-function mutant, PCSK9(D374Y), showed no loss of activity
compared with a catalytically active counterpart; both proteins displayed ∼10-fold increased activity in degradation of cell
surface LDLRs compared with wild-type PCSK9. We conclude that the ability of PCSK9 to degrade LDLRs is independent of catalytic
activity and suggest that PCSK9 functions as a chaperone to prevent LDLR recycling and/or to target LDLRs for lysosomal degradation.
"The attached prodomain typically undergoes a secondary cleavage event to relieve its inhibitory effect; however, the prodomain of secreted PCSK9 remains intact and tightly bound in the catalytic site, rendering the mature enzyme catalytically inert . Accordingly, PCSK9 acts as a molecular chaperone to direct LDLR degradation in cultured hepatic cells and mouse liver [15,16]. Mechanistic studies have identified two separate routes of LDLR degradation induced by PCSK9 (Fig. 1). "
[Show abstract][Hide abstract] ABSTRACT: Purpose of review
Proprotein convertase subtilisin/kexin type-9 (PCSK9) binds to LDL receptor (LDLR) and targets it for lysosomal degradation in cells. Decreased hepatic clearance of plasma LDL-cholesterol is the primary gauge of PCSK9 activity in humans; however, PCSK9's evolutionary role may extend to other lipoprotein classes and processes. This review highlights studies that are providing novel insights into physiological regulation of PCSK9 transcription and plasma PCSK9 activity.
Recent studies indicate that circulating PCSK9 binds to apolipoprotein B100 on LDL particles, which in turn inhibits PCSK9's ability to bind to cell surface LDLRs. Negative feedback of secreted PCSK9 activity by LDL could serve to increase plasma excursion of triglyceride-rich lipoproteins and monitor lipoprotein remodeling. Recent findings have identified hepatocyte nuclear factor-1α as a key transcriptional regulator that cooperates with sterol regulatory element-binding protein-2 to control PCSK9 expression in hepatocytes in response to nutritional and hormonal inputs, as well as acute inflammation.
PCSK9 is an established target for cholesterol-lowering therapies. Further study of PCSK9 regulatory mechanisms may identify additional control points for pharmacological inhibition of PCSK9-mediated LDLR degradation. PCSK9 function could reflect ancient roles in the fasting-feeding cycle and in linking lipoprotein metabolism with innate immunity.
Current Opinion in Lipidology 08/2014; 25(5). DOI:10.1097/MOL.0000000000000114 · 5.66 Impact Factor
"Unlike other convertases, the N-terminal prosegment of PCSK9 is not released after a second autocatalytic cleavage but remains tightly associated with the catalytic domain, rendering PCSK9 enzymatically inactive.33,42 In hepatocytes, PCSK9 is also prone to proteolytic cleavage by proprotein convertases, which can be localized in the Golgi, endosomes, and at the plasma membrane.43–46 "
[Show abstract][Hide abstract] ABSTRACT: Proprotein convertase subtilisin/kexin type 9 (PCSK9) directly binds to the epidermal growth factor-like repeat A domain of low-density lipoprotein receptor and induces its degradation, thereby controlling circulating low-density lipoprotein cholesterol (LDL-C) concentration. Heterozygous loss-of-function mutations in PCSK9 can decrease the incidence of coronary heart disease by up to 88%, owing to lifelong reduction of LDL-C. Moreover, two subjects with PCSK9 loss-of-function mutations on both alleles, resulting in a total absence of functional PCSK9, were found to have extremely low circulating LDL-C levels without other apparent abnormalities. Accordingly, PCSK9 could represent a safe and effective pharmacological target to increase clearance of LDL-C and to reduce the risk of coronary heart disease. Recent clinical trials using anti-PCSK9 monoclonal antibodies that block the PCSK9:low-density lipoprotein receptor interaction were shown to considerably reduce LDL-C levels by up to 65% when given alone and by up to 72% in patients already receiving statin therapy. In this review, we will discuss how major scientific breakthroughs in PCSK9 cell biology have led to the development of new and forthcoming LDL-C-lowering pharmacological agents.
Drug Design, Development and Therapy 10/2013; 7:1135-1148. DOI:10.2147/DDDT.S36984 · 3.03 Impact Factor
"The proprotein convertase subtilisin/kexin 9 (PCSK9) enhances the degradation of the LDLR, and is well-established as a gene associated with familial hypercholesterolemia, along with LDLR, APOB
– and very recently APOE
. By an as yet unknown mechanism(s), and independent of its enzymatic activity , PCSK9 enhances the degradation of cell surface LDLR – in endosomes/lysosomes , resulting in increased circulating LDL cholesterol (LDLc). In fact, Pcsk9−/− mice exhibit higher levels of LDLR in liver and ∼42% less circulating total cholesterol, with a ∼80% drop in LDLc , , emphasizing the therapeutic potential of a PCSK9 inhibitor/silencer . "
[Show abstract][Hide abstract] ABSTRACT: Elevated LDL-cholesterol (LDLc) levels are a major risk factor for cardiovascular disease and atherosclerosis. LDLc is cleared from circulation by the LDL receptor (LDLR). Proprotein convertase subtilisin/kexin 9 (PCSK9) enhances the degradation of the LDLR in endosomes/lysosomes, resulting in increased circulating LDLc. PCSK9 can also mediate the degradation of LDLR lacking its cytosolic tail, suggesting the presence of as yet undefined lysosomal-targeting factor(s). Herein, we confirm this, and also eliminate a role for the transmembrane-domain of the LDLR in mediating its PCSK9-induced internalization and degradation. Recent findings from our laboratory also suggest a role for PCSK9 in enhancing tumor metastasis. We show herein that while the LDLR is insensitive to PCSK9 in murine B16F1 melanoma cells, PCSK9 is able to induce degradation of the low density lipoprotein receptor-related protein 1 (LRP-1), suggesting distinct targeting mechanisms for these receptors. Furthermore, PCSK9 is still capable of acting upon the LDLR in CHO 13-5-1 cells lacking LRP-1. Conversely, PCSK9 also acts on LRP-1 in the absence of the LDLR in CHO-A7 cells, where re-introduction of the LDLR leads to reduced PCSK9-mediated degradation of LRP-1. Thus, while PCSK9 is capable of inducing degradation of LRP-1, the latter is not an essential factor for LDLR regulation, but the LDLR effectively competes with LRP-1 for PCSK9 activity. Identification of PCSK9 targets should allow a better understanding of the consequences of PCSK9 inhibition for lowering LDLc and tumor metastasis.
PLoS ONE 05/2013; 8(5):e64145. DOI:10.1371/journal.pone.0064145 · 3.23 Impact Factor
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