Serum proprotein convertase subtilisin/kexin type 9 and cell surface low-density lipoprotein receptor: evidence for a reciprocal regulation

Current therapies for lowering lipoprotein (a)

Lipoprotein (a) [Lp(a)] is a human plasma lipoprotein with unique structural and functional characteristics. Lp(a) is an assembly of two components: a central core with apoB and an additional glycoprotein, called apo(a). Ever since the strong association between elevated levels of Lp(a) and an increased risk for CVD was recognized, interest in the therapeutic modulation of Lp(a) levels has increased. Here, the past and present therapies aiming to lower Lp(a) levels will be reviewed, demonstrating that these agents have had varying degrees of success. The next challenge will be to prove that Lp(a) lowering also leads to cardiovascular benefit in patients with elevated Lp(a) levels. Therefore, highly specific and potent Lp(a)-lowering strategies are awaited urgently.

Local effects of human PCSK9 on the atherosclerotic lesion

Proprotein convertase subtilisin/kexin type 9 (PCSK9) promotes atherosclerosis by increasing low-density lipoprotein (LDL) cholesterol levels through degradation of hepatic LDL receptor (LDLR). Studies have described the systemic effects of PCSK9 on atherosclerosis, but whether PCSK9 has local and direct effects on the plaque is unknown. To study the local effect of human PCSK9 (hPCSK9) on atherosclerotic lesion composition, independently of changes in serum cholesterol levels, we generated chimeric mice expressing hPCSK9 exclusively from macrophages, using marrow from hPCSK9 transgenic (hPCSK9tg) mice transplanted into apoE(-/-) and LDLR(-/-) mice, which were then placed on a high-fat diet (HFD) for 8 weeks. We further characterized the effect of hPCSK9 expression on the inflammatory responses in the spleen and by mouse peritoneal macrophages (MPM) in vitro. We found that MPMs from transgenic mice express both murine (m) Pcsk9 and hPCSK9 and that the latter reduces macrophage LDLR and LRP1 surface levels. We detected hPCSK9 in the serum of mice transplanted with hPCSK9tg marrow, but did not influence lipid levels or atherosclerotic lesion size. However, marrow-derived PCSK9 progressively accumulated in lesions of apoE(-/-) recipient mice, while increasing the infiltration of Ly6C(hi) inflammatory monocytes by 32% compared with controls. Expression of hPCSK9 also increased CD11b- and Ly6C(hi) -positive cell numbers in spleens of apoE(-/-) mice. In vitro, expression of hPCSK9 in LPS-stimulated macrophages increased mRNA levels of the pro-inflammatory markers Tnf and Il1b (40% and 45%, respectively) and suppressed those of the anti-inflammatory markers Il10 and Arg1 (30% and 44%, respectively). All PCSK9 effects were LDLR-dependent, as PCSK9 protein was not detected in lesions of LDLR(-/-) recipient mice and did not affect macrophage or splenocyte inflammation. In conclusion, PCSK9 directly increases atherosclerotic lesion inflammation in an LDLR-dependent but cholesterol-independent mechanism, suggesting that therapeutic PCSK9 inhibition may have vascular benefits secondary to LDL reduction.

The role of PCSK9 in intestinal lipoprotein metabolism: synergism of statin and ezetimibe

Proprotein convertase subtilisin/kexin type 9 (PCSK9) plays a major role in the regulation of lipoprotein metabolism, mostly through control of low-density lipoprotein receptor degradation. Depletion of cellular cholesterol causes a compensatory increase in plasma PCSK9 levels, which can diminish the cholesterol-lowering power of statins and may lead to the overproduction of intestinal lipoproteins, mainly thorough the up regulation of microsomal triglyceride transfer protein and the Niemann-Pick C1-like 1 protein, the target of ezetimibe. Thus, ezetimibe therapy may counter this unwanted effect of statins, providing an additional theoretical rationale for combining the effect of ezetimibe on intestinal cholesterol absorption and that of statins on cholesterol synthesis.

PCSK9 Induces CD36 Degradation and Affects Long-Chain Fatty Acid Uptake and Triglyceride Metabolism in Adipocytes and in Mouse Liver

OBJECTIVE

Proprotein convertase subtilisin/kexin type 9 (PCSK9) promotes the degradation of the low-density lipoprotein receptor thereby elevating plasma low-density lipoprotein cholesterol levels and the risk of coronary heart disease. Thus, the use of PCSK9 inhibitors holds great promise to prevent heart disease. Previous work found that PCSK9 is involved in triglyceride metabolism, independently of its action on low-density lipoprotein receptor, and that other yet unidentified receptors could mediate this effect. Therefore, we assessed whether PCSK9 enhances the degradation of CD36, a major receptor involved in transport of long-chain fatty acids and triglyceride storage.

APPROACH AND RESULTS

Overexpressed or recombinant PCSK9 induced CD36 degradation in cell lines and primary adipocytes and reduced the uptake of the palmitate analog Bodipy FL C16 and oxidized low-density lipoprotein in 3T3-L1 adipocytes and hepatic HepG2 cells, respectively. Surface plasmon resonance, coimmunoprecipitation, confocal immunofluorescence microscopy, and protein degradation pathway inhibitors revealed that PCSK9 directly interacts with CD36 and targets the receptor to lysosomes through a mechanism involving the proteasome. Importantly, the level of CD36 protein was increased by >3-fold upon small interfering RNA knockdown of endogenous PCSK9 in hepatic cells and similarly increased in the liver and visceral adipose tissue of Pcsk9(-/-) mice. In Pcsk9(-/-) mice, increased hepatic CD36 was correlated with an amplified uptake of fatty acid and accumulation of triglycerides and lipid droplets.

CONCLUSIONS

Our results demonstrate an important role of PCSK9 in modulating the function of CD36 and triglyceride metabolism. PCSK9-mediated CD36 degradation may serve to limit fatty acid uptake and triglyceride accumulation in tissues, such as the liver.

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.

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.

Cross-talk between LOX-1 and PCSK9 in vascular tissues

AIMS

Lectin-like ox-LDL receptor-1 (LOX-1) plays an important role in inflammatory diseases, such as atherosclerosis. Proprotein convertase subtilisin/kexin type 9 (PCSK9) modulates LDL receptor degradation and influences serum LDL levels. The present study was designed to investigate the possible interaction between PCSK9 and LOX-1.

METHODS AND RESULTS

In the first set of experiments, human vascular endothelial cells and smooth muscle cells were studied at baseline and after lipopolysaccharide (LPS) treatment (to create an inflammatory state). Both PCSK9 and LOX-1 were strongly induced by LPS treatment. To define the role of PCSK9 in LOX-1 expression, cells were transfected with siRNA against PCSK9, which resulted in reduced LOX-1 expression and function. On the other hand, cells exposed to recombinant hPCSK9 revealed enhanced LOX-1 expression (P < 0.05). To determine whether LOX-1 also regulates PCSK9, cultured cells in which LOX-1 was knocked down by siRNA expressed less PCSK9, whereas those transfected with hLOX-1 cDNA showed increased PCSK9 expression. The second set of experiments was carried out in wild-type (WT) and gene knockout (KO; LOX-1 and PCSK9) mice; LOX-1 KO mice showed much less PCSK9 (P < 0.05 vs. WT mice). PCSK9-KO mice showed much less LOX-1 (P < 0.05 vs. WT mice). Furthermore, we observed that mitochondrial reactive oxygen species (mtROS) plays an initiating role in the LOX-1/PCSK9 interaction, since mtROS induction enhanced and its inhibition reduced the expression of both PCSK9 and LOX-1. We also found that both LOX-1 and PCSK9 regulate adhesion molecules vascular cell adhesion molecule-1 expression. Finally, oxidized low-density lipoprotein and tumour necrosis factor-α, pro-inflammatory stimuli besides LPS, regulated PCSK9 expression that is mediated by the NF-κB signalling pathway.

CONCLUSIONS

These observations suggest that LOX-1 and PCSK9 positively influence each other's expression, especially during an inflammatory reaction. mtROS appear to be important initiators of PCSK9/LOX-1 expression.

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.

Common Proprotein Convertase Subtilisin/Kexin Type 9 (PCSK9) Epitopes Mediate Multiple Routes for Internalization and Function

Proprotein convertase subtilisin/kexin type 9 (PCSK9) is a soluble protein that directs membrane-bound receptors to lysosomes for degradation. In the most studied example of this, PCSK9 binding leads to the degradation of low density lipoprotein receptor (LDLR), significantly affecting circulating LDL-C levels. The mechanism mediating this degradation, however, is not completely understood. We show here that LDLR facilitates PCSK9 interactions with amyloid precursor like protein 2 (APLP2) at neutral pH leading to PCSK9 internalization, although direct binding between PCSK9 and LDLR is not required. Moreover, binding to APLP2 or LDLR is independently sufficient for PCSK9 endocytosis in hepatocytes, while LDL can compete with APLP2 for PCSK9 binding to indirectly mediate PCSK9 endocytosis. Finally, we show that APLP2 and LDLR are also required for the degradation of another PCSK9 target, APOER2, necessitating a general role for LDLR and APLP2 in PCSK9 function. Together, these findings provide evidence that PCSK9 has at least two endocytic epitopes that are utilized by a variety of internalization mechanisms and clarifies how PCSK9 may direct proteins to lysosomes.

Recent advances in the understanding and care of familial hypercholesterolaemia: significance of the biology and therapeutic regulation of proprotein convertase subtilisin/kexin type 9

Familial hypercholesterolaemia (FH) is an autosomal co-dominant disorder that markedly raises plasma low-density lipoprotein-cholesterol (LDL-C) concentration, causing premature atherosclerotic coronary artery disease (CAD). FH has recently come under intense focus and, although there is general consensus in recent international guidelines regarding diagnosis and treatment, there is debate about the value of genetic studies. Genetic testing can be cost-effective as part of cascade screening in dedicated centres, but the full mutation spectrum responsible for FH has not been established in many populations, and its use in primary care is not at present logistically feasible. Whether using genetic testing or not, cholesterol screening of family members of index patients with an abnormally raised LDL-C must be used to determine the need for early treatment to prevent the development of CAD. The metabolic defects in FH extend beyond LDL, and may affect triacylglycerol-rich and high-density lipoproteins, lipoprotein(a) and oxidative stress. Achievement of the recommended targets for LDL-C with current treatments is difficult, but this may be resolved by new drug therapies. Lipoprotein apheresis remains an effective treatment for severe FH and, although expensive, it costs less than the two recently introduced orphan drugs (lomitapide and mipomersen) for homozygous FH. Recent advances in understanding of the biology of proprotein convertase subtilisin/kexin type 9 (PCSK9) have further elucidated the regulation of lipoprotein metabolism and led to new drugs for effectively treating hypercholesterolaemia in FH and related conditions, as well as for treating many patients with statin intolerance. The mechanisms of action of PCSK9 inhibitors on lipoprotein metabolism and atherosclerosis, as well as their impact on cardiovascular outcomes and cost-effectiveness, remain to be established.

PCSK9 inhibition to reduce cardiovascular disease risk: recent findings from the biology of PCSK9

PURPOSE OF REVIEW

Review novel insights into the biology of proprotein convertase subtilisin/kexin 9 (PCSK9) that may explain the extreme efficiency of PCSK9 inhibition and the unexpected metabolic effects resulting from PCSK9 monoclonal antibody therapy, and may identify additional patients as target of therapy.

RECENT FINDINGS

For over 20 years, the practical knowledge of cholesterol metabolism has centered around cellular mechanisms, and around the idea that statin therapy is the essential step to control metabolic abnormalities for cardiovascular risk management. This view has been embraced by the recent AHA/ACC guidelines, but is being challenged by recent studies including nonstatin medications and by the development of a new class of cholesterol-lowering agents that seems destined to early US Food and Drug Administration approval. The discovery of PCSK9 - a circulating protein that regulates hepatic low-density lipoprotein (LDL) receptor and serum LDL cholesterol levels - has led to a race for its therapeutic inhibition. Recent findings on PCSK9 regulation and pleiotropic effects will help identify additional patient groups likely to benefit from the inhibitory therapy and unravel the full potential of PCSK9 inhibition therapy.

SUMMARY

Injectable human monoclonal antibodies to block the interaction between PCSK9 and LDL receptor are demonstrating extraordinary efficacy (LDL reductions of up to 70%) and almost the absence of any side-effects. A more moderate effect is seen on other lipoprotein parameters, with the exception of lipoprotein(a) levels. We describe mechanisms that can explain the effect on lipoprotein(a), predict a potential effect on postprandial triglyderides, and suggest a new category of patients for anti-PCSK9 therapy.

Targeting PCSK9 for therapeutic gains

Even though it is only a little over a decade from the discovery of proprotein convertase subtilisin/kexin type 9 (PCSK9) as a plasma protein that associates with both hypercholesterolemia and low cholesterol syndromes, a rich literature has developed describing its unique physiology and the impact of antagonism of this molecule on cholesterol metabolism for therapeutic purposes. Indeed, the PCSK9 story is unfolding rapidly, with many answers and more questions. This review summarizes the most recent data from phase II/III clinical trials of PCSK9 inhibition with the three leading antibodies, highlights the clinical significance of the ongoing studies, and suggests future areas of investigation based on recent basic science discoveries on the physiology of PCSK9.

High-fructose feeding promotes accelerated degradation of hepatic LDL receptor and hypercholesterolemia in hamsters via elevated circulating PCSK9 levels

BACKGROUND

High fructose diet (HFD) induces dyslipidemia and insulin resistance in experimental animals and humans with incomplete mechanistic understanding. By utilizing mice and hamsters as in vivo models, we investigated whether high fructose consumption affects serum PCSK9 and liver LDL receptor (LDLR) protein levels.

RESULTS

Feeding mice with an HFD increased serum cholesterol and reduced serum PCSK9 levels as compared with the mice fed a normal chow diet (NCD). In contrast to the inverse relationship in mice, serum PCSK9 and cholesterol levels were co-elevated in HFD-fed hamsters. Liver tissue analysis revealed that PCSK9 mRNA and protein levels were both reduced in mice and hamsters by HFD feeding, however, liver LDLR protein levels were markedly reduced by HFD in hamsters but not in mice. We further showed that circulating PCSK9 clearance rates were significantly lower in hamsters fed an HFD as compared with the hamsters fed NCD, providing additional evidence for the reduced hepatic LDLR function by HFD consumption. The majority of PCSK9 in hamster serum was detected as a 53 kDa N-terminus cleaved protein. By conducting in vitro studies, we demonstrate that this 53 kDa truncated hamster PCSK9 is functionally active in promoting hepatic LDLR degradation.

CONCLUSION

Our studies for the first time demonstrate that high fructose consumption increases serum PCSK9 concentrations and reduces liver LDLR protein levels in hyperlipidemic hamsters. The positive correlation between circulating cholesterol and PCSK9 and the reduction of liver LDLR protein in HFD-fed hamsters suggest that hamster is a better animal model than mouse to study the modulation of PCSK9/LDLR pathway by atherogenic diets.

Hemodynamic shear stress via ROS modulates PCSK9 expression in human vascular endothelial and smooth muscle cells and along the mouse aorta

AIMS

To investigate a possible link between hemodynamic shear stress, reactive oxygen species (ROS) generation, and proprotein convertase subtilisin/kexin type 9 (PCSK9) expression.

RESULTS

Using a parallel-plate flow chamber, we observed that PCSK9 expression in vascular smooth muscle cells (SMCs) and endothelial cells (ECs) reached maximal value at low shear stress (3-6 dynes/cm(2)), and then began to decline with an increase in shear stress. PCSK9 expression increased when cells were treated with lipopolysaccharide. PCSK9 expression was always greater in SMCs than in ECs. ROS generation followed the same pattern as PCSK9 expression. Aortic branching and aorta-iliac bifurcation regions of mouse aorta that express low shear stress were also found to have greater PCSK9 expression (vs. other regions). To determine a relationship between ROS and PCSK9 expression, ECs and SMCs were treated with ROS inhibitors diphenylene-iodonium chloride and apocynin, and both markedly reduced PCSK9 expression. Relationship between PCSK9 and ROS was further studied in p47(phox) and gp91(phox) knockout mice; both mice strains revealed low PCSK9 levels in serum and mRNA levels in aorta-iliac bifurcation regions (vs. wild-type mice). Other studies showed that ROS and NF-κB activation plays a bridging role in PCSK9 expression via lectin-like oxidized low-density lipoprotein receptor-1 (LOX-1).

INNOVATION

Low shear stress induces PCSK9 expression, which is mediated by NADPH oxidase-dependent ROS production.

CONCLUSIONS

This study provides evidence that low shear stress enhances PCSK9 expression in concert with ROS generation in vascular ECs and SMCs. ROS seem to regulate PCSK9 expression. We propose that PCSK9-ROS interaction may be important in the development of atherosclerosis in arterial channels with low shear stress.

Impacts of ezetimibe on PCSK9 in rats: study on the expression in different organs and the potential mechanisms

Background

Previous studies including our group have indicated the effects of ezetimibe on increased plasma proprotein convertase subtilisin/kexin type 9 (PCSK9) concentration, while the rapid expression in different organs and the potential molecular mechanisms for this impact have not been carefully evaluated.

Methods

Thirty rats were randomly divided into two groups (n = 15 for each), which were orally administrated with ezetimibe (10 mg/kg/day) or normal saline. Blood samples were obtained at day 3 after orally administration, and the PCSK9 levels were determined by ELISA. We further analyzed the mRNA expression of PCSK9, low-density lipoprotein receptor (LDLR), sterol regulator element-binding protein 2 (SREBP2), and hepatocyte nuclear factor 1 alpha (HNF-1α) by real-time PCR, as well as the protein expression by western blot, in liver, intestine and kidney respectively.

Results

Ezetimibe significantly increased plasma PCSK9 levels compared with control group, while there was no significant difference between the two groups with regard to lipid profile at day 3. Moreover, ezetimibe remarkably increased the expression of PCSK9, LDLR, SREBP2 and HNF-1α in liver. Enhanced expression of PCSK9, LDLR and SREBP2 protein were found in intestine and kidney, while no changes in the expression of HNF-1α were observed in intestine and kidney of rats with ezetimibe treatment.

Conclusions

The data demonstrated that ezetimibe increased PCSK9 expression through the SREBP2 and HNF-1α pathways in different organs, subsequently resulting in elevated plasma PCSK9 levels prior to the alterations of lipid profile in rats.

Anti-PCSK9 antibody pharmacokinetics and low-density lipoprotein-cholesterol pharmacodynamics in nonhuman primates are antigen affinity-dependent and exhibit limited sensitivity to neonatal Fc receptor-binding enhancement

Proprotein convertase subtilisin/kexin type 9 (PCSK9) has emerged as an attractive therapeutic target for cardiovascular disease. Monoclonal antibodies (mAbs) that bind PCSK9 and prevent PCSK9:low-density lipoprotein receptor complex formation reduce serum low-density lipoprotein-cholesterol (LDL-C) in vivo. PCSK9-mediated lysosomal degradation of bound mAb, however, dramatically reduces mAb exposure and limits duration of effect. Administration of high-affinity mAb1:PCSK9 complex (1:2) to mice resulted in significantly lower mAb1 exposure compared with mAb1 dosed alone in normal mice or in PCSK9 knockout mice lacking antigen. To identify mAb-binding characteristics that minimize lysosomal disposition, the pharmacokinetic behavior of four mAbs representing a diverse range of PCSK9-binding affinities at neutral (serum) and acidic (endosomal) pH was evaluated in cynomolgus monkeys. Results revealed an inverse correlation between affinity and both mAb exposure and duration of LDL-C lowering. High-affinity mAb1 exhibited the lowest exposure and shortest duration of action (6 days), whereas mAb2 displayed prolonged exposure and LDL-C reduction (51 days) as a consequence of lower affinity and pH-sensitive PCSK9 binding. mAbs with shorter endosomal PCSK9:mAb complex dissociation half-lives (<20 seconds) produced optimal exposure-response profiles. Interestingly, incorporation of previously reported Fc-region amino acid substitutions or novel loop-insertion peptides that enhance in vitro neonatal Fc receptor binding, led to only modest pharmacokinetic improvements for mAbs with pH-dependent PCSK9 binding, with only limited augmentation of pharmacodynamic activity relative to native mAbs. A pivotal role for PCSK9 in mAb clearance was demonstrated, more broadly suggesting that therapeutic mAb-binding characteristics require optimization based on target pharmacology.

PMP22 is critical for actin-mediated cellular functions and for establishing lipid rafts

Haploinsufficiency of peripheral myelin protein 22 (PMP22) causes hereditary neuropathy with liability to pressure palsies, a peripheral nerve lesion induced by minimal trauma or compression. As PMP22 is localized to cholesterol-enriched membrane domains that are closely linked with the underlying actin network, we asked whether the myelin instability associated with PMP22 deficiency could be mediated by involvement of the protein in actin-dependent cellular functions and/or lipid raft integrity. In peripheral nerves and cells from mice with PMP22 deletion, we assessed the organization of filamentous actin (F-actin), and actin-dependent cellular functions. Using in vitro models, we discovered that, in the absence of PMP22, the migration and adhesion capacity of Schwann cells and fibroblasts are similarly impaired. Furthermore, PMP22-deficient Schwann cells produce shortened myelin internodes, and display compressed axial cell length and collapsed lamellipodia. During early postnatal development, F-actin-enriched Schmidt-Lanterman incisures do not form properly in nerves from PMP22(-/-) mice, and the expression and localization of molecules associated with uncompacted myelin domains and lipid rafts, including flotillin-1, cholesterol, and GM1 ganglioside, are altered. In addition, we identified changes in the levels and distribution of cholesterol and ApoE when PMP22 is absent. Significantly, cholesterol supplementation of the culture medium corrects the elongation and migration deficits of PMP22(-/-) Schwann cells, suggesting that the observed functional impairments are directly linked with cholesterol deficiency of the plasma membrane. Our findings support a novel role for PMP22 in the linkage of the actin cytoskeleton with the plasma membrane, likely through regulating the cholesterol content of lipid rafts.

Molecular and cellular function of the proprotein convertase subtilisin/kexin type 9 (PCSK9)

The proprotein convertase subtilisin/kexin type 9 (PCSK9) has emerged as a promising treatment target to lower serum cholesterol, a major risk factor of cardiovascular diseases. Gain-of-function mutations of PCSK9 are associated with hypercholesterolemia and increased risk of cardiovascular events. Conversely, loss-of-function mutations cause low-plasma LDL-C levels and a reduction of cardiovascular risk without known unwanted effects on individual health. Experimental studies have revealed that PCSK9 reduces the hepatic uptake of LDL-C by increasing the endosomal and lysosomal degradation of LDL receptors (LDLR). A number of clinical studies have demonstrated that inhibition of PCSK9 alone and in addition to statins potently reduces serum LDL-C concentrations. This review summarizes the current data on the regulation of PCSK9, its molecular function in lipid homeostasis and the emerging evidence on the extra-hepatic effects of PCSK9.

Macrophage apoAI protects against dyslipidemia-induced dermatitis and atherosclerosis without affecting HDL

Tissue cholesterol accumulation, macrophage infiltration, and inflammation are features of atherosclerosis and some forms of dermatitis. HDL and its main protein, apoAI, are acceptors of excess cholesterol from macrophages; this process inhibits tissue inflammation. Recent epidemiologic and clinical trial evidence questions the role of HDL and its manipulation in cardiovascular disease. We investigated the effect of ectopic macrophage apoAI expression on atherosclerosis and dermatitis induced by the combination of hypercholesterolemia and absence of HDL in mice. Hematopoietic progenitor cells were transduced to express human apoAI and transplanted into lethally irradiated LDL receptor(-/-)/apoAI(-/-) mice, which were then placed on a high-fat diet for 16 weeks. Macrophage apoAI expression reduced aortic CD4(+) T-cell levels (-39.8%), lesion size (-25%), and necrotic core area (-31.6%), without affecting serum HDL or aortic macrophage levels. Macrophage apoAI reduced skin cholesterol by 39.8%, restored skin morphology, and reduced skin CD4(+) T-cell levels. Macrophage apoAI also reduced CD4(+) T-cell levels (-32.9%) in skin-draining lymph nodes but had no effect on other T cells, B cells, dendritic cells, or macrophages compared with control transplanted mice. Thus, macrophage apoAI expression protects against atherosclerosis and dermatitis by reducing cholesterol accumulation and regulating CD4(+) T-cell levels, without affecting serum HDL or tissue macrophage levels.

Removal of plasma mature and furin-cleaved proprotein convertase subtilisin/kexin 9 by low-density lipoprotein-apheresis in familial hypercholesterolemia: development and application of a new assay for PCSK9

CONTEXT

Proprotein convertase subtilisin/kexin 9 (PCSK9) is known to be a good target to decrease LDL cholesterol (LDL-C) and two forms of PCSK9, mature and furin-cleaved PCSK9, circulate in blood. However, it has not been clarified whether and how the levels of each PCSK9 are affected by LDL-apheresis (LDL-A) treatment, a standard therapy in patients with severe forms of familial hypercholesterolemia (FH).

OBJECTIVE

Our objective was to investigate the differences in LDL-A-induced reduction of mature and furin-cleaved PCSK9 between homozygous and heterozygous FH, and between dextran sulfate (DS) cellulose adsorption and double membrane (DM) columns and to clarify the mechanism of their removal.

DESIGN

A sandwich ELISA to measure two forms of PCSK9s using monoclonal antibodies was developed. Using the ELISA, PCSK9 levels were quantified before and after LDL-A with DS columns in 7 homozygous and 11 heterozygous FH patients. A crossover study between the two column types was performed. The profiles of PCSK9s were analyzed after fractionation by gel filtration chromatography. Immunoprecipitation of apolipoprotein B (apoB) in FH plasma was performed.

RESULTS

Both mature and furin-cleaved PCSK9s were significantly decreased by 55-56% in FH homozygotes after a single LDL-A treatment with DS columns, and by 46-48% or 48-56% in FH heterozygotes after treatment with DS or DM columns. The reduction ratios of LDL-C were strongly correlated with that of PCSK9 in both FH homozygotes and heterozygotes. In addition, more than 80% of plasma PCSK9s were in the apoB-deficient fraction and a significant portion of mature PCSK9 was bound to apoB, as shown by immunoprecipitation.

CONCLUSIONS

Both mature and furin-cleaved PCSK9s were removed by LDL-A in homozygous and heterozygous FH either by binding to apoB or by other mechanisms. The ELISA method to measure both forms of plasma PCSK9 would be useful for investigating physiological or pathological roles of PCSK9.

On the function and homeostasis of PCSK9: reciprocal interaction with LDLR and additional lipid effects

Proprotein convertase subtilisin kexin type 9 (PCSK9) is a circulatory ligand that terminates the lifecycle of the low-density lipoprotein (LDL) receptor (LDLR) thus affecting plasma LDL-cholesterol (LDL-C) levels. Recent evidence shows that in addition to the straightforward mechanism of action, there are more complex interactions between PCSK9, LDLR and plasma lipoprotein levels, including: (a) the presence of both parallel and reciprocal regulation of surface LDLR and plasma PCSK9; (b) a correlation between PCSK9 and LDL-C levels dependent not only on the fact that PCSK9 removes hepatic LDLR, but also due to the fact that up to 40% of plasma PCSK9 is physically associated with LDL; and (c) an association between plasma PCSK9 production and the assembly and secretion of triglyceride-rich lipoproteins. The effect of PCSK9 on LDLR is being successfully utilized toward the development of anti-PCSK9 therapies to reduce plasma LDL-C levels. Current biochemical research has uncovered additional mechanisms of action and interacting partners for PCSK9, and this opens the way for a more thorough understanding of the regulation, metabolism, and effects of this interesting protein.

PCSK9: A key factor modulating atherosclerosis

Coronary artery disease (CAD) due to obstructive atherosclerosis is a leading cause of death and has been recognized as a worldwide health threat. Measures to decrease low-density lipoprotein cholesterol (LDL-C) levels are the cornerstone in the management of patients with atherosclerotic cardiovascular disease, particularly those with CAD, for over two decades. Proprotein convertase subtilisin/kexin type 9 (PCSK9), a newly recognized protein, plays a key role in cholesterol homeostasis by enhancing degradation of hepatic LDL receptor (LDLR). Interestingly, PCSK9 is also involved in the inflammatory process. Plasma PCSK9 and lipid or nonlipid cardiovascular risk factors are correlated, and the associations between PCSK9 with cardiovascular health and disease make this protein worthy of attention for the treatment of hyperlipidemia and atherosclerosis. Here, we provide an overview of the physiological role of PCSK9, which contributes to atherosclerosis, and provide data on PCSK9 as a novel pharmacological target. Clinical evidence shows that PCSK9 inhibition is as promising as statins as a target to treat CAD. The efficacy of these drugs may potentially enable effective CAD prophylaxis for more patients.

Proprotein Convertase Subtilisin/Kexin Type 9 (PCSK9): Lessons Learned from Patients with Hypercholesterolemia

BACKGROUND

Identification of the proprotein convertase subtilisin/kexin type 9 (PCSK9) as the third gene causing familial hypercholesterolemia (FH) and understanding its complex biology has led to the discovery of a novel class of therapeutic agents.

CONTENT

PCSK9 undergoes autocatalytic cleavage in the endoplasmic reticulum and enters the secretory pathway. The PCSK9 gene is under the regulatory control of sterol receptor binding proteins 1 and 2. Statins increase PCSK9 and this may modulate the response to this class of medications. In plasma, PCSK9 binds to the epidermal growth factor–like domain of the LDL receptor (LDL-R) on the cell and, once incorporated in the late endosomal pathway, directs the LDL-R toward lysosomal degradation rather than recycling to the plasma membrane. Thus, gain-of-function PCSK9 mutations lead to an FH phenotype, whereas loss-of-function mutations are associated with increased LDL-R–mediated endocytosis of LDL particles and lower LDL cholesterol in plasma. Inhibition of PCSK9 is thus an attractive therapeutic target. Presently, this is achieved by using monoclonal antibodies for allosteric inhibition of the PCSK9–LDL-R interaction. Phase 2 and 3 clinical trials in patients with moderate and severe hypercholesterolemia (including FH) show that this approach is safe and highly efficacious to lower LDL-C and lipoprotein(a).

SUMMARY

PCSK9 has other biological roles observed in vitro and in animal studies, including viral entry into the cell, insulin resistance, and hepatic tissue repair. Given the potential number of humans exposed to this novel class of medications, careful evaluation of clinical trial results is warranted.

PCSK9 and LDLR degradation: regulatory mechanisms in circulation and in cells

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 FINDINGS

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.

SUMMARY

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.

PCSK9 inhibition fails to alter hepatic LDLR, circulating cholesterol, and atherosclerosis in the absence of ApoE

LDL cholesterol (LDL-C) contributes to coronary heart disease. Proprotein convertase subtilisin/kexin type 9 (PCSK9) increases LDL-C by inhibiting LDL-C clearance. The therapeutic potential for PCSK9 inhibitors is highlighted by the fact that PCSK9 loss-of-function carriers exhibit 15-30% lower circulating LDL-C and a disproportionately lower risk (47-88%) of experiencing a cardiovascular event. Here, we utilized pcsk9(-/-) mice and an anti-PCSK9 antibody to study the role of the LDL receptor (LDLR) and ApoE in PCSK9-mediated regulation of plasma cholesterol and atherosclerotic lesion development. We found that circulating cholesterol and atherosclerotic lesions were minimally modified in pcsk9(-/-) mice on either an LDLR- or ApoE-deficient background. Acute administration of an anti-PCSK9 antibody did not reduce circulating cholesterol in an ApoE-deficient background, but did reduce circulating cholesterol (-45%) and TGs (-36%) in APOE*3Leiden.cholesteryl ester transfer protein (CETP) mice, which contain mouse ApoE, human mutant APOE3*Leiden, and a functional LDLR. Chronic anti-PCSK9 antibody treatment in APOE*3Leiden.CETP mice resulted in a significant reduction in atherosclerotic lesion area (-91%) and reduced lesion complexity. Taken together, these results indicate that both LDLR and ApoE are required for PCSK9 inhibitor-mediated reductions in atherosclerosis, as both are needed to increase hepatic LDLR expression.

Macrophage-derived apoESendai suppresses atherosclerosis while causing lipoprotein glomerulopathy in hyperlipidemic mice

Lipoprotein glomerulopathy (LPG) is a renal disease often accompanied by dyslipidemia and increased serum apoE levels. apoESendai (Arg145Pro), a rare mutant based on the apoE3 sequence carrying an apoE2 charge, causes LPG in humans and transgenic mice, but its effects on the artery wall are unknown. Macrophage expression of apoESendai may also directly influence renal and arterial homeostasis. We investigated the effects of macrophage-expressed apoESendai in apoE(-/-) mice with or without LDL receptor (LDLR). Murine bone marrow transduced to express apoE2, apoE3, or apoESendai was transplanted into lethally irradiated mice. Macrophage apoESendai expression reduced aortic lesion size and inflammation by 32 and 28%, respectively, compared with apoE2 in apoE(-/-) recipients. No differences in lesion size or inflammation were found between apoESendai and apoE3 in apoE(-/-) recipients. Macrophage apoESendai expression also reduced aortic lesion size by 18% and inflammation by 29% compared with apoE2 in apoE(-/-)/LDLR(-/-) recipients. Glomerular lesions compatible with LPG with increased mesangial matrix, extracellular lipid accumulation, and focal mesangiolysis were only observed in apoE(-/-)/LDLR(-/-) mice expressing apoESendai. Thus, macrophage expression of apoESendai protects against atherosclerosis while causing lipoprotein glomerulopathy. This is the first demonstration of an apoprotein variant having opposing effects on vascular and renal homeostasis.

Thyroid hormone reduces PCSK9 and stimulates bile acid synthesis in humans

Reduced plasma LDL-cholesterol is a hallmark of hyperthyroidism and is caused by transcriptional stimulation of LDL receptors in the liver. Here, we investigated whether thyroid hormone (TH) actions involve other mechanisms that may also account for the reduction in LDL-cholesterol, including effects on proprotein convertase subtilisin/kexin type 9 (PCSK9) and bile acid synthesis. Twenty hyperthyroid patients were studied before and after clinical normalization, and the responses to hyperthyroidism were compared with those in 14 healthy individuals after 14 days of treatment with the liver-selective TH analog eprotirome. Both hyperthyroidism and eprotirome treatment reduced circulating PCSK9, lipoprotein cholesterol, apoB and AI, and lipoprotein(a), while cholesterol synthesis was stable. Hyperthyroidism, but not eprotirome treatment, markedly increased bile acid synthesis and reduced fibroblast growth factor (FGF) 19 and dietary cholesterol absorption. Eprotirome treatment, but not hyperthyroidism, reduced plasma triglycerides. Neither hyperthyroidism nor eprotirome treatment altered insulin, glucose, or FGF21 levels. TH reduces circulating PSCK9, thereby likely contributing to lower plasma LDL-cholesterol in hyperthyroidism. TH also stimulates bile acid synthesis, although this response is not critical for its LDL-lowering effect.

Pharmacologic profile of the Adnectin BMS-962476, a small protein biologic alternative to PCSK9 antibodies for low-density lipoprotein lowering

Proprotein convertase subtilisin kexin-9 (PCSK9) is an important pharmacological target for decreasing low-density lipoprotein (LDL) in cardiovascular disease, although seemingly inaccessible to small molecule approaches. Compared with therapeutic IgG antibodies currently in development, targeting circulating PCSK9 with smaller molecular scaffolds could offer different profiles and reduced dose burdens. This inspired genesis of PCSK9-binding Adnectins, a protein family derived from human fibronectin-10th-type III-domain and engineered for high-affinity target binding. BMS-962476, an ∼11-kDa polypeptide conjugated to polyethylene glycol to enhance pharmacokinetics, binds with subnanomolar affinity to human. The X-ray cocrystal structure of PCSK9 with a progenitor Adnectin shows ∼910 Å(2) of PCSK9 surface covered next to the LDL receptor binding site, largely by residues of a single loop of the Adnectin. In hypercholesterolemic, overexpressing human PCSK9 transgenic mice, BMS-962476 rapidly lowered cholesterol and free PCSK9 levels. In genomic transgenic mice, BMS-962476 potently reduced free human PCSK9 (ED50 ∼0.01 mg/kg) followed by ∼2-fold increases in total PCSK9 before return to baseline. Treatment of cynomolgus monkeys with BMS-962476 rapidly suppressed free PCSK9 >99% and LDL-cholesterol ∼55% with subsequent 6-fold increase in total PCSK9, suggesting reduced clearance of circulating complex. Liver sterol response genes were consequently downregulated, following which LDL and total PCSK9 returned to baseline. These studies highlight the rapid dynamics of PCSK9 control over LDL and liver cholesterol metabolism and characterize BMS-962476 as a potent and efficacious PCSK9 inhibitor.

Proprotein convertase subtilisin kexin type 9 promotes intestinal overproduction of triglyceride-rich apolipoprotein B lipoproteins through both low-density lipoprotein receptor-dependent and -independent mechanisms

BACKGROUND

Proprotein convertase subtilisin kexin type 9 (PCSK9) promotes the degradation of the low-density lipoprotein (LDL) receptor (LDLR), and its deficiency in humans results in low plasma LDL cholesterol and protection against coronary heart disease. Recent evidence indicates that PCSK9 also modulates the metabolism of triglyceride-rich apolipoprotein B (apoB) lipoproteins, another important coronary heart disease risk factor. Here, we studied the effects of physiological levels of PCSK9 on intestinal triglyceride-rich apoB lipoprotein production and elucidated for the first time the cellular and molecular mechanisms involved.

METHODS AND RESULTS

Treatment of human enterocytes (CaCo-2 cells) with recombinant human PCSK9 (10 μg/mL for 24 hours) increased cellular and secreted apoB48 and apoB100 by 40% to 55% each (P<0.01 versus untreated cells), whereas short-term deletion of PCSK9 expression reversed this effect. PCSK9 stimulation of apoB was due to a 1.5-fold increase in apoB mRNA (P<0.01) and to enhanced apoB protein stability through both LDLR-dependent and LDLR-independent mechanisms. PCSK9 decreased LDLR protein (P<0.01) and increased cellular apoB stability via activation of microsomal triglyceride transfer protein. PCSK9 also increased levels of the lipid-generating enzymes FAS, SCD, and DGAT2 (P<0.05). In mice, human PCSK9 at physiological levels increased intestinal microsomal triglyceride transfer protein levels and activity regardless of LDLR expression.

CONCLUSIONS

PCSK9 markedly increases intestinal triglyceride-rich apoB production through mechanisms mediated in part by transcriptional effects on apoB, microsomal triglyceride transfer protein, and lipogenic genes and in part by posttranscriptional effects on the LDLR and microsomal triglyceride transfer protein. These findings indicate that targeted PCSK9-based therapies may also be effective in the management of postprandial hypertriglyceridemia.

PCSK9: a key modulator of cardiovascular health

Since the discovery of proprotein convertase subtilisin kexin 9 (PCSK9) in 2003, this PC has attracted a lot of attention from the scientific community and pharmaceutical companies. Secreted into the plasma by the liver, the proteinase K-like serine protease PCSK9 binds the low-density lipoprotein (LDL) receptor at the surface of hepatocytes, thereby preventing its recycling and enhancing its degradation in endosomes/lysosomes, resulting in reduced LDL-cholesterol clearance. Surprisingly, in a nonenzymatic fashion, PCSK9 enhances the intracellular degradation of all its target proteins. Rare gain-of-function PCSK9 variants lead to higher levels of LDL-cholesterol and increased risk of cardiovascular disease; more common loss-of-function PCSK9 variants are associated with reductions in both LDL-cholesterol and risk of cardiovascular disease. It took 9 years to elaborate powerful new PCSK9-based therapeutic approaches to reduce circulating levels of LDL-cholesterol. Presently, PCSK9 monoclonal antibodies that inhibit its function on the LDL receptor are evaluated in phase III clinical trials. This review will address the biochemical, genetic, and clinical aspects associated with PCSK9's biology and pathophysiology in cells, rodent and human, with emphasis on the clinical benefits of silencing the expression/activity of PCSK9 as a new modality in the treatment of hypercholesterolemia and associated pathologies.

Hepatic overexpression of idol increases circulating protein convertase subtilisin/kexin type 9 in mice and hamsters via dual mechanisms: sterol regulatory element-binding protein 2 and low-density lipoprotein receptor-dependent pathways

OBJECTIVE

Low-density lipoprotein receptor (LDLR) is degraded by inducible degrader of LDLR (Idol) and protein convertase subtilisin/kexin type 9 (PCSK9), thereby regulating circulating LDL levels. However, it remains unclear whether, and if so how, these LDLR degraders affect each other. We therefore investigated effects of liver-specific expression of Idol on LDL/PCSK9 metabolism in mice and hamsters.

APPROACH AND RESULTS

Injection of adenoviral vector expressing Idol (Ad-Idol) induced a liver-specific reduction in LDLR expression which, in turn, increased very-low-density lipoprotein/LDL cholesterol levels in wild-type mice because of delayed LDL catabolism. Interestingly, hepatic Idol overexpression markedly increased plasma PCSK9 levels. In LDLR-deficient mice, plasma PCSK9 levels were already elevated at baseline and unchanged by Idol overexpression, which was comparable with the observation for Ad-Idol-injected wild-type mice, indicating that Idol-induced PCSK9 elevation depended on LDLR. In wild-type mice, but not in LDLR-deficient mice, Ad-Idol enhanced hepatic PCSK9 expression, with activation of sterol regulatory element-binding protein 2 and subsequently increased expression of its target genes. Supporting in vivo findings, Idol transactivated PCSK9/LDLR in sterol regulatory element-binding protein 2/LDLR-dependent manners in vitro. Furthermore, an in vivo kinetic study using (125)I-labeled PCSK9 revealed delayed clearance of circulating PCSK9, which could be another mechanism. Finally, to extend these findings into cholesteryl ester transfer protein-expressing animals, we repeated the above in vivo experiments in hamsters and obtained similar results.

CONCLUSIONS

A vicious cycle in LDLR degradation might be generated by PCSK9 induced by hepatic Idol overexpression via dual mechanisms: sterol regulatory element-binding protein 2/LDLR. Furthermore, these effects would be independent of cholesteryl ester transfer protein expression.

Elevated plasma PCSK9 level is equally detrimental for patients with nonfamilial hypercholesterolemia and heterozygous familial hypercholesterolemia, irrespective of low-density lipoprotein receptor defects

OBJECTIVES

Do elevated proprotein convertase subtilisin/kexin type 9 (PCSK9) levels constitute an even greater risk for patients who already have reduced low-density lipoprotein receptor (LDLR) levels, such as those with heterozygous familial hypercholesterolemia (HeFH)?

BACKGROUND

As a circulating inhibitor of LDLR, PCSK9 is an attractive target for lowering LDL-cholesterol (LDL-C) levels.

METHODS

Circulating PCSK9 levels were measured by enzyme-linked immunosorbent assay in nontreated patients with HeFH carrying a D206E (n = 237), V408M (n = 117), or D154N (n = 38) LDLR missense mutation and in normolipidemic controls (n = 152). Skin fibroblasts and lymphocytes were isolated from a subset of patients and grown in 0.5% serum and mevastatin with increasing amounts of recombinant PCSK9. LDLR abundance at the cell surface was determined by flow cytometry.

RESULTS

PCSK9 reduced LDLR expression in a dose-dependent manner in control and FH fibroblasts to similar extents, by up to 77 ± 8% and 82 ± 7%, respectively. Likewise, PCSK9 reduced LDLR abundance by 39 ± 8% in nonfamilial hypercholesterolemia (non-FH) and by 45 ± 10% in HeFH lymphocytes, irrespective of their LDLR mutation status. We found positive correlations of the same magnitude between PCSK9 and LDL-C levels in controls (beta = 0.22; p = 0.0003), D206E (beta = 0.20; p = 0.0002), V408M (beta = 0.24; p = 0.0002), and D154N (beta = 0.25; p = 0.048) patients with HeFH. The strengths of these associations were all similar.

CONCLUSIONS

Elevated PCSK9 levels are equally detrimental for patients with HeFH or non-FH: a 100-ng/ml increase in PCSK9 will lead to an increase in LDL-C of 0.20 to 0.25 mmol/l in controls and HeFH alike, irrespective of their LDLR mutation. This explains why patients with non-FH or HeFH respond equally well to monoclonal antibodies targeting PCSK9.

The Sirt1 activator SRT3025 provides atheroprotection in Apoe-/- mice by reducing hepatic Pcsk9 secretion and enhancing Ldlr expression

AIMS

The deacetylase sirtuin 1 (Sirt1) exerts beneficial effects on lipid metabolism, but its roles in plasma LDL-cholesterol regulation and atherosclerosis are controversial. Thus, we applied the pharmacological Sirt1 activator SRT3025 in a mouse model of atherosclerosis and in hepatocyte culture.

METHODS AND RESULTS

Apolipoprotein E-deficient (Apoe(-/-)) mice were fed a high-cholesterol diet (1.25% w/w) supplemented with SRT3025 (3.18 g kg(-1) diet) for 12 weeks. In vitro, the drug activated wild-type Sirt1 protein, but not the activation-resistant Sirt1 mutant; in vivo, it increased deacetylation of hepatic p65 and skeletal muscle Foxo1. SRT3025 treatment decreased plasma levels of LDL-cholesterol and total cholesterol and reduced atherosclerosis. Drug treatment did not change mRNA expression of hepatic LDL receptor (Ldlr) and proprotein convertase subtilisin/kexin type 9 (Pcsk9), but increased their protein expression indicating post-translational effects. Consistent with hepatocyte Ldlr and Pcsk9 accumulation, we found reduced plasma levels of Pcsk9 after pharmacological Sirt1 activation. In vitro administration of SRT3025 to cultured AML12 hepatocytes attenuated Pcsk9 secretion and its binding to Ldlr, thereby reducing Pcsk9-mediated Ldlr degradation and increasing Ldlr expression and LDL uptake. Co-administration of exogenous Pcsk9 with SRT3025 blunted these effects. Sirt1 activation with SRT3025 in Ldlr(-/-) mice reduced neither plasma Pcsk9, nor LDL-cholesterol levels, nor atherosclerosis.

CONCLUSION

We identify reduction in Pcsk9 secretion as a novel effect of Sirt1 activity and uncover Ldlr as a prerequisite for Sirt1-mediated atheroprotection in mice. Pharmacological activation of Sirt1 appears promising to be tested in patients for its effects on plasma Pcsk9, LDL-cholesterol, and atherosclerosis.

The hypercholesterolemia-risk gene SORT1 facilitates PCSK9 secretion

Circulating PCSK9 destines low-density lipoprotein receptor for degradation in lysosomes, resulting in increased LDL cholesterol. Accordingly, it is an attractive drug target for hypercholesterolemia, and results from clinical trials are promising. While the physiological role of PCSK9 in cholesterol metabolism is well described, its complex mechanism of action remains poorly understood, although it is known to depend on intracellular trafficking. We here identify sortilin, encoded by the hypercholesterolemia-risk gene SORT1, as a high-affinity sorting receptor for PCSK9. Sortilin colocalizes with PCSK9 in the trans-Golgi network and facilitates its secretion from primary hepatocytes. Accordingly, sortilin-deficient mice display decreased levels of circulating PCSK9, while sortilin overexpression in the liver confers increased plasma PCSK9. Furthermore, circulating PCSK9 and sortilin were positively correlated in a human cohort of healthy individuals, suggesting that sortilin is involved in PCSK9 secretion in humans. Taken together, our findings establish sortilin as a critical regulator of PCSK9 activity.

Linton M, Kon V, Fazio S, Sampson UK. Angiotensin-induced abdominal aortic aneurysms in hypercholesterolemic mice: role of serum cholesterol and temporal effects of exposure

Objective

Understanding variations in size and pattern of development of angiotensin II (Ang II)-induced abdominal aortic aneurysms (AAA) may inform translational research strategies. Thus, we sought insight into the temporal evolution of AAA in apolipoprotein (apo)E^−/− mice.

Approach

A cohort of mice underwent a 4-week pump-mediated infusion of saline (n = 23) or 1500 ng/kg/min of Ang II (n = 85) and AAA development was tracked via in vivo ultrasound imaging. We adjusted for hemodynamic covariates in the regression models for AAA occurrence in relation to time.

Results

The overall effect of time was statistically significant (p<0.001). Compared to day 7 of AngII infusion, there was no decrease in the log odds of AAA occurrence by day 14 (−0.234, p = 0.65), but compared to day 21 and 28, the log odds decreased by 9.07 (p<0.001) and 2.35 (p = 0.04), respectively. Hemodynamic parameters were not predictive of change in aortic diameter (Δ) (SBP, p = 0.66; DBP, p = 0.66). Mean total cholesterol (TC) was higher among mice with large versus small AAA (601 vs. 422 mg/ml, p<0.0001), and the difference was due to LDL. AngII exposure was associated with 0.43 mm (95% CI, 0.27 to 0.61, p<0.0001) increase in aortic diameter; and a 100 mg/dl increase in mean final cholesterol level was associated with a 12% (95% CI, 5.68 to 18.23, p<0.0001) increase in aortic diameter. Baseline cholesterol was not associated with change in aortic diameter (p = 0.86).

Conclusions

These are the first formal estimates of a consistent pattern of Ang II-induced AAA development. The odds of AAA occurrence diminish after the second week of Ang II infusion, and TC is independently associated with AAA size.

PCSK9: From discovery to therapeutic applications

The proprotein convertase subtilisin/kexin type 9 (PCSK9) regulates cholesterol metabolism mainly by targeting the low-density lipoprotein receptor (LDLR) for degradation in the liver. Gain-of-function mutations in PCSK9 are one of the genetic causes of autosomal dominant hypercholesterolaemia. Conversely, loss-of-function mutations are associated with lower concentrations of LDL cholesterol (LDL-C) and reduced coronary heart disease. As these loss-of-function mutations are not associated with apparent deleterious effects, PCSK9 inhibition is an attractive new strategy for lowering LDL-C concentration. Among the various approaches to PCSK9 inhibition, human data are only available for inhibition of PCSK9 binding to LDLR by monoclonal antibodies. In phase II studies, the two most advanced monoclonal antibodies in development (alirocumab and evolocumab) decreased atherogenic lipoproteins very effectively and were well tolerated. A dramatic decrease in LDL-C up to 70% can be obtained with the most efficacious doses. Efficacy has been evaluated so far in addition to statins in hypercholesterolaemic patients with or without familial hypercholesterolaemia, in patients with intolerance to statin therapy and in monotherapy. Large phase III programmes are ongoing to evaluate the long-term efficacy and safety of these very promising new agents.

Internalized PCSK9 dissociates from recycling LDL receptors in PCSK9-resistant SV-589 fibroblasts

Secreted PCSK9 binds to cell surface LDL receptor (LDLR) and directs the receptor for lysosomal degradation. PCSK9 is potent at inducing LDLR degradation in cultured liver-derived cells, but it is considerably less active in immortalized fibroblasts. We examined PCSK9 trafficking in SV-589 human skin fibroblasts incubated with purified recombinant wild-type PCSK9 or gain-of-function mutant PCSK9-D374Y with increased LDLR binding affinity. Despite LDLR-dependent PCSK9 uptake, cell surface LDLR levels in SV-589 fibroblasts were only modestly reduced by wild-type PCSK9, even at high nonphysiological concentrations (20 µg/ml). Internalized ¹²⁵I-labeled wild-type PCSK9 underwent lysosomal degradation at high levels, indicating its dissociation from recycling LDLRs. PCSK9-D374Y (2 µg/ml) reduced cell surface LDLRs by approximately 50%, but this effect was still blunted compared with HepG2 hepatoma cells. Radioiodinated PCSK9-D374Y was degraded less efficiently in SV-589 fibroblasts, and Alexa488-labeled PCSK9-D374Y trafficked to both lysosomes and endocytic recycling compartments. Endocytic recycling assays showed that more than 50% of internalized PCSK9-D374Y recycled to the cell surface compared with less than 10% for wild-type PCSK9. These data support that wild-type PCSK9 readily dissociates from the LDLR within early endosomes of SV-589 fibroblasts, contributing to PCSK9-resistance. Although a large proportion of gain-of-function PCSK9-D374Y remains bound to LDLR in these cells, degradative activity is still diminished.

Loss of plasma proprotein convertase subtilisin/kexin 9 (PCSK9) after lipoprotein apheresis

RATIONALE

Lipoprotein apheresis (LA) reduces low-density lipoprotein (LDL) levels in patients with severe familial hypercholesterolemia (FH). We have recently reported that >30% of plasma proprotein convertase subtilisin/kexin 9 (PCSK9) is bound to LDL, thus we predicted that LA would also reduce plasma PCSK9 levels by removing LDL.

OBJECTIVE

Pre- and post-apheresis plasma from 6 patients with familial hypercholesterolemia on 3 consecutive treatment cycles was used to determine changes in PCSK9 levels.

METHODS AND RESULTS

LA drastically reduced plasma LDL (by 77 ± 4%). Concomitantly, PCSK9 levels fell by 52 ± 5%, strongly correlating with the LDL drop (P=0.0322; r(2)=0.26), but not with decreases in triglyceride (49 ± 13%) or high-density lipoprotein levels (18 ± 2%). Levels of albumin, creatinine, and CK-MB did not show significant changes after LA. Similar to LDL, PCSK9 levels returned to pretreatment values between cycles (2-week intervals). Fractionation of pre- and post-apheresis plasma showed that 81 ± 11% of LDL-bound PCSK9 and 48 ± 14% of apolipoprotein B-free PCSK9 were removed. Separation of whole plasma, purified LDL, or the apolipoprotein B-free fraction through a scaled-down, experimental dextran sulfate cellulose beads column produced similar results.

CONCLUSIONS

Our results show, for the first time, that modulation of LDL levels by LA directly affects plasma PCSK9 levels, and suggest that PCSK9 reduction is an additional benefit of LA. Because the loss of PCSK9 could contribute to the LDL-lowering effect of LA, then (1) anti-PCSK9 therapies may reduce frequency of LA in patients currently approved for therapy, and (2) LA and anti-PCSK9 therapies may be used synergistically to reduce treatment burden.

Hepatic cholesterol homeostasis: is the low-density lipoprotein pathway a regulatory or a shunt pathway?

OBJECTIVE

The hypothesis that cholesterol that enters the cell within low-density lipoprotein (LDL) particles rapidly equilibrates with the regulatory pool of intracellular cholesterol and maintains cholesterol homeostasis by reducing cholesterol and LDL receptor synthesis was validated in the fibroblast but not in the hepatocyte. Accordingly, the present studies were designed to compare the effects of cholesterol that enters the hepatocyte within an LDL particle with those of cholesterol that enters via other lipoprotein particles.

APPROACH AND RESULTS

We measured cholesterol synthesis and esterification in hamster hepatocytes treated with LDL and other lipoprotein particles, including chylomicron remnants and VLDL. Endogenous cholesterol synthesis was not significantly reduced by uptake of LDL, but cholesterol esterification (280%) and acyl CoA:cholesterol acyltransferase 2 expression (870%) were increased. In contrast, cholesterol synthesis was significantly reduced (70% decrease) with other lipoprotein particles. Furthermore, more cholesterol that entered the hepatocyte within LDL particles was secreted within VLDL particles (480%) compared with cholesterol from other sources.

CONCLUSIONS

Much of the cholesterol that enters the hepatocyte within LDL particles is shunted through the cell and resecreted within VLDL particles without reaching equilibrium with the regulatory pool.