Strong induction of PCSK9 gene expression through HNF1alpha and SREBP2: mechanism for the resistance to LDL-cholesterol lowering effect of statins in dyslipidemic hamsters

Short-term impact of low-dose atorvastatin on serum proprotein convertase subtilisin/kexin type 9

BACKGROUND AND OBJECTIVE

Several small studies have found that moderate- to high-dose statins (HMG-CoA reductase inhibitors) could increase the serum proprotein convertase subtilisin/kexin type 9 (PCSK9) level. However, little is known regarding the short-term, dose-dependent effects of low-dose atorvastatin and the rapid effects of a single dose of atorvastatin on PCSK9. The objective of this study was to investigate the short-term impact of low-dose atorvastatin on PCSK9 in humans.

METHODS

In this randomized study, data from 66 subjects were analyzed. In protocol I, 32 patients were randomized to atorvastatin 10 mg/day (n = 19) or 20 mg/day (n = 13) and eight healthy subjects without therapy were controls for 8 weeks. Serum PCSK9 and lipid profile were determined at day 0, week 4, and week 8. In protocol II, 26 patients were randomized to a single dose of atorvastatin 10 mg (n = 11) or 80 mg (n = 15), and serum levels of PCSK9 were measured at 24 h after treatment.

RESULTS

Atorvastatin 10 mg/day decreased low-density lipoprotein cholesterol (LDL-C) by 32 % at 4 weeks and by 33 % at 8 weeks, and atorvastatin 20 mg/day resulted in reduction of LDL-C by 41 % at 4 weeks and by 38 % at 8 weeks. Atorvastatin 10 mg/day slightly increased serum PCSK9 by 5-7 % but without a significant difference, while atorvastatin 20 mg/day significantly increased serum PCSK9 by 30 % at 4 weeks and by 35 % at 8 weeks (p = 0.009 and p = 0.002, respectively). In addition, 24 h after a single dose, atorvastatin 10 mg significantly increased serum PCSK9 by 13 % and atorvastatin 80 mg by 27 % (p = 0.042 and p = 0.001, respectively).

CONCLUSION

The short-term impact of low-dose atorvastatin on PCSK9 was time and dose dependent, with a rapid increase in PCSK9 levels being observed within 24 h of dosing.

CETP inhibitors downregulate hepatic LDL receptor and PCSK9 expression in vitro and in vivo through a SREBP2 dependent mechanism

BACKGROUND

CETP inhibitors block the transfer of cholesteryl ester from HDL-C to VLDL-C and LDL-C, thereby raising HDL-C and lowering LDL-C. In this study, we explored the effect of CETP inhibitors on hepatic LDL receptor (LDLR) and PCSK9 expression and further elucidated the underlying regulatory mechanism.

RESULTS

We first examined the effect of anacetrapib (ANA) and dalcetrapib (DAL) on LDLR and PCSK9 expression in hepatic cells in vitro. ANA exhibited a dose-dependent inhibition on both LDLR and PCSK9 expression in CETP-positive HepG2 cells and human primary hepatocytes as well as CETP-negative mouse primary hepatocytes (MPH). Moreover, the induction of LDLR protein expression by rosuvastatin in MPH was blunted by cotreatment with ANA. In both HepG2 and MPH ANA treatment reduced the amount of mature form of SREBP2 (SREBP2-M). In vivo, oral administration of ANA to dyslipidemic C57BL/6J mice at a daily dose of 50 mg/kg for 1 week elevated serum total cholesterol by approximately 24.5% (p < 0.05%) and VLDL-C by 70% (p < 0.05%) with concomitant reductions of serum PCSK9 and liver LDLR/SREBP2-M protein. Finally, we examined the in vitro effect of two other strong CETP inhibitors evacetrapib and torcetrapib on LDLR/PCSK9 expression and observed a similar inhibitory effect as ANA in a concentration range of 1-10 μM.

CONCLUSION

Our study revealed an unexpected off-target effect of CETP inhibitors that reduce the mature form of SREBP2, leading to attenuated transcription of hepatic LDLR and PCSK9. This negative regulation of SREBP pathway by ANA manifested in mice where CETP activity was absent and affected serum cholesterol metabolism.

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.

Enhanced circulating PCSK9 concentration by berberine through SREBP-2 pathway in high fat diet-fed rats

Background

Berberine (BBR), a natural plant extract, has been shown to improve lipid metabolism. However, its effects on PCSK9, a key factor involving in the lipid metabolism, have not yet been evaluated in vivo. The aim of the present study was to investigate the effect of BBR on PCSK9 expression in high fat diet-fed (HFD) rats.

Methods

Thirty-two male Sprague Dawley (SD) rats were randomized into the four groups (n = 8): normal diet (Control), HFD, HFD + simvastatin (Sim, 2 mg/kg/d) and HDF + BBR (400 mg/kg/d) for 6 weeks. The following parameters were determined: 1) body weight; 2) serum lipid profile; 3) serum PCSK9 measured by enzyme-linked immuno sorbent assay (ELISA) ; 4) hepatic expressions of low-density lipoprotein receptor (LDLR), sterol regulatory element binding protein-2 (SREBP-2) and hepatocyte nuclear factor 1 (HNF1) were examined by real time quantitative polymerase chain reaction (RT-PCR) and western blotting analysis.

Results

Compared with HFD rats, Sim and BBR significantly reduced body weight gain and improved lipid profile (P < 0.05 respectively). In addition, either of drug treatment for 6 weeks could increase serum concentration of PCSK9 in HFD rats (P < 0.05). This enhanced PCSK9 expression was demonstrated to be associated with the up-regulation of hepatic expression of LDLR and SREBP-2 and the down-regulation of hepatic expression of HNF1 (P < 0.05 respectively).

Conclusions

The data provided the first line of the evidence that BBR, similar to the Sim, could increase the expression of PCSK9 levels in HFD rats through SREBP-2 activation, suggesting that impacts of BBR on lipid profile may also be linked to SREBP-2 pathway.

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.

Plasma PCSK9 in nephrotic syndrome and in peritoneal dialysis: a cross-sectional study

BACKGROUND

Serum total and low-density lipoprotein (LDL) cholesterol levels are elevated in patients with nephrotic syndrome and those with kidney failure treated by peritoneal dialysis (PD), who are characterized by heavy losses of protein in urine and peritoneal dialysate, respectively. Hypercholesterolemia in nephrotic syndrome is associated with and largely due to acquired LDL receptor (LDLR) deficiency. Because PCSK9 (proprotein convertase subtilisin/kexin type 9) promotes degradation of LDLR, we tested the hypothesis that elevation of LDL cholesterol levels in patients with nephrotic syndrome and PD patients may be due to increased PCSK9 levels.

STUDY DESIGN

Cross-sectional study.

SETTING & PARTICIPANTS

Patients with nephrotic syndrome or treated by PD or hemodialysis and age- and sex-matched healthy Korean individuals (n=15 in each group).

PREDICTOR

Group and serum total and LDL cholesterol levels.

OUTCOMES

Plasma PCSK9 concentration.

MEASUREMENTS

Concentrations of fasting serum PCSK9, lipids, and albumin, and urine protein excretion.

RESULTS

Mean serum total and LDL cholesterol levels in patients with nephrotic syndrome (317.9±104.2 [SD] and 205.9±91.1mg/dL) and PD patients (200.0±27.6 and 126.7±18.5mg/dL) were significantly (P<0.05) higher than in hemodialysis patients (140.9±22.9 and 79.1±19.5mg/dL) and the control group (166.5±26.5 and 95.9±25.2mg/dL). This was associated with significantly (P<0.05) higher plasma PCSK9 levels in patients with nephrotic syndrome (15.13±4.99ng/mL) and PD patients (13.30±1.40ng/mL) than in the control (9.19±0.60ng/mL) and hemodialysis (7.30±0.50ng/mL) groups. Plasma PCSK9 level was directly related to total and LDL cholesterol concentrations in the study population (r=0.559 [P<0.001] and r=0.497 [P<0.001], respectively).

LIMITATIONS

Small number of participants may limit generalizability.

CONCLUSIONS

Nephrotic syndrome and PD are associated with higher plasma PCSK9 concentration, which can contribute to elevation of LDL levels by promoting LDLR deficiency.

Aldosterone aggravates glucose intolerance induced by high fructose

We previously reported that aldosterone impaired vascular insulin signaling in vivo and in vitro. Fructose-enriched diet induces metabolic syndrome including hypertension, insulin resistance, hyperlipidemia and diabetes in animal. In the current study, we hypothesized that aldosterone aggravated fructose feeding-induced glucose intolerance in vivo. Rats were divided into five groups for six-week treatment; uninephrectomy (Unx, n=8), Unx+aldosterone (aldo, 0.75 µg/h, s.c., n=8), Unx+fructose (fruc, 10% in drinking water, n=8), Unx+aldo+fruc, (aldo+fruc, n=8), and Unx+aldo+fruc+spironolactone, a mineralocorticoid receptor antagonist (aldo+fruc+spiro, 20mg/kg/day, p.o., n=8). Aldo+fruc rats manifested the hypertension, and induced glucose intolerance compared to fruc intake rats assessed by oral glucose tolerance test, homeostasis model assessment of insulin resistance and hyperinsulinemic-euglycemic clamp study. Spironolactone, significantly improved the aldosterone-accelerated glucose intolerance. Along with improvement in insulin resistance, spironolactone suppressed upregulated mineralocorticoid receptor (MR) target gene, serum and glucocorticoid-regulated kinases-1 mRNA expression in skeletal muscle in aldo+fruc rats. In conclusion, these data suggested that aldosterone aggravates fructose feeding-induced glucose intolerance through MR activation.

Plasma proprotein convertase subtilisin kexin type 9 levels are related to markers of cholesterol synthesis in familial combined hyperlipidemia

BACKGROUND AND AIMS

Two recent independent studies showed that patients with familial combined hyperlipidemia (FCHL) have elevated plasma levels of proprotein convertase subtilisin kexin type 9 (PCSK9) and markers of cholesterol synthesis. Both PCSK9 expression and cholesterol synthesis are downstream effects of hepatic activation of sterol regulatory element binding protein 2 (SREBP2). The present study was conducted to study the relationship between plasma PCSK9 and markers of cholesterol synthesis in FCHL.

METHODS AND RESULTS

Markers of cholesterol synthesis (squalene, desmosterol, lathosterol), cholesterol absorption (campesterol, sitosterol, cholestanol) and PCSK9 were measured in plasma of FCHL patients (n = 103) and their normolipidemic relatives (NLR; n = 240). Plasma PCSK9, lathosterol and desmosterol levels were higher in FCHL patients than their NLR (p < 0.001, age and sex adjusted). Heritability calculations demonstrated that 35% of the variance in PCSK9 levels could be explained by additive genetic effects (p < 0.001). Significant age- and sex-adjusted correlations were observed for the relationship between PCSK9 and lathosterol, both unadjusted and adjusted for cholesterol, in the overall FCHL population (both p < 0.001). Multivariate regression analyses, with PCSK9 as the dependent variable, showed that the regression coefficient for FCHL status decreased by 25% (from 0.8 to 0.6) when lathosterol was included. Nevertheless, FCHL status remained an independent contributor to plasma PCSK9 (p < 0.001).

CONCLUSIONS

The present study confirms the previously reported high and heritable PCSK9 levels in FCHL patients. Furthermore, we now show that high PCSK9 levels are, in part, explained by plasma lathosterol, suggesting that SREBP2 activation partly accounts for elevated PCSK9 levels in FCHL.

Role of PCSK9 and IDOL in the pathogenesis of acquired LDL receptor deficiency and hypercholesterolemia in nephrotic syndrome

BACKGROUND

Nephrotic syndrome (NS) leads to elevation of serum total and LDL cholesterol. This is largely due to impaired LDL clearance, which is caused by hepatic LDL receptor (LDLR) deficiency despite normal LDLR mRNA expression, pointing to a post-transcriptional process. The mechanism(s) by which NS causes LDLR deficiency is not known. By promoting degradation of LDLR, Proprotein Convertase Subtilisin/Kexin type 9 (PCSK9) and inducible degrader of the LDL receptor (IDOL) play a major role in post-translational regulation of LDLR. We, therefore, tested the hypothesis that LDLR deficiency despite its normal gene expression in NS may be due to upregulation of hepatic PCSK9 and IDOL.

METHODS

LDLR, IDOL and PCSK9 expressions and nuclear translocation of liver X receptor (LXR) that regulates IDOL expression were determined in the liver of rats with puromycin-induced NS and control (CTL) rats.

RESULTS

Compared with the CTLs, the NS rats showed marked elevation of serum total and LDL cholesterol and a significant reduction in hepatic LDLR protein expression. This was accompanied by marked upregulation of hepatic PCSK9 and IDOL expressions and heightened LXR activation.

CONCLUSIONS

LDLR deficiency, hypercholesterolemia and elevated plasma LDL in NS are associated with upregulation of PCSK9 and IDOL. Interventions targeting these pathways may be effective in the management of hypercholesterolemia and the associated cardiovascular and other complications of NS.

Targeting PCSK9 for hypercholesterolemia

Dyslipidemias are a predominant risk factor for cardiovascular disease. Biological and genetic research has led to the identification of several genes and proteins that may be pharmacologically targeted to improve lipoprotein profiles and possibly cardiovascular outcomes in patients with dyslipidemia. The observation that proprotein convertase subtilisin/kexin type 9 (PCSK9) regulates the levels of circulating low-density lipoprotein C (LDL-C) by enhancing the degradation of the hepatic low-density lipoprotein receptor (LDLR) prompted the search for drugs that inhibit PCSK9 activity. Several approaches to inhibiting PCSK9 activity have been proposed; these involve inhibitory antibodies, small molecules, and gene silencing. To date, the most promising and advanced approach relates to monoclonal antibodies, which can decrease LDL cholesterol by 65-70%, even as an add-on therapy to a maximal dose of a statin. Phase III studies and large, event-driven clinical trials are ongoing and will fully address the viability and role of these drugs in clinical practice.

Effects of elaidic acid on lipid metabolism in HepG2 cells, investigated by an integrated approach of lipidomics, transcriptomics and proteomics

Trans fatty acid consumption in the human diet can cause adverse health effects, such as cardiovascular disease, which is associated with higher total cholesterol, a higher low density lipoprotein-cholesterol level and a decreased high density lipoprotein-cholesterol level. The aim of the study was to elucidate the hepatic response to the most abundant trans fatty acid in the human diet, elaidic acid, to help explain clinical findings on the relationship between trans fatty acids and cardiovascular disease. The human HepG2 cell line was used as a model to investigate the hepatic response to elaidic acid in a combined proteomic, transcriptomic and lipidomic approach. We found many of the proteins responsible for cholesterol synthesis up-regulated together with several proteins involved in the esterification and hepatic import/export of cholesterol. Furthermore, a profound remodeling of the cellular membrane occurred at the phospholipid level. Our findings contribute to the explanation on how trans fatty acids from the diet can cause modifications in plasma cholesterol levels by inducing abundance changes in several hepatic proteins and the hepatic membrane composition.

FoxO3 transcription factor and Sirt6 deacetylase regulate low density lipoprotein (LDL)-cholesterol homeostasis via control of the proprotein convertase subtilisin/kexin type 9 (Pcsk9) gene expression

Elevated LDL-cholesterol is a risk factor for the development of cardiovascular disease. Thus, proper control of LDL-cholesterol homeostasis is critical for organismal health. Genetic analysis has identified PCSK9 (proprotein convertase subtilisin/kexin type 9) as a crucial gene in the regulation of LDL-cholesterol via control of LDL receptor degradation. Although biochemical characteristics and clinical implications of PCSK9 have been extensively investigated, epigenetic regulation of this gene is largely unknown. In this work we have discovered that Sirt6, an NAD(+)-dependent histone deacetylase, plays a critical role in the regulation of the Pcsk9 gene expression in mice. Hepatic Sirt6 deficiency leads to elevated Pcsk9 gene expression and LDL-cholesterol as well. Mechanistically, we have demonstrated that Sirt6 can be recruited by forkhead transcription factor FoxO3 to the proximal promoter region of the Pcsk9 gene and deacetylates histone H3 at lysines 9 and 56, thereby suppressing the gene expression. Also remarkably, overexpression of Sirt6 in high fat diet-fed mice lowers LDL-cholesterol. Overall, our data suggest that FoxO3 and Sirt6, two longevity genes, can reduce LDL-cholesterol levels through regulation of the Pcsk9 gene.

The promises of PCSK9 inhibition

PURPOSE OF REVIEW

In the past 10 years, the LDL receptor inhibitor proprotein convertase subtilisin kexin type 9 (PCSK9) has emerged as a validated target for lowering plasma LDL cholesterol levels. Here we review the most recent reports on PCSK9 out of a total of 500 publications published in print or online before March 2013 and indexed on PubMed.

RECENT FINDINGS

All published in 2012, phase I and II clinical trials demonstrate that fully human monoclonal antibodies targeting PCSK9 dramatically reduce LDL-C and enable patients to reach their target goals, without severe or serious safety issues.

SUMMARY

This review summarizes the discovery of PCSK9, its original mode of action as a secreted inhibitor of the LDL receptor, as well as its genetic regulation by statins. We then focus on the major results from the 2012 phase I and II PCSK9 inhibitor clinical trials. We also review the recent in-vivo studies demonstrating the potential cardiovascular benefits of long-term PCSK9 inhibition and discuss its potential side-effects.

Comparison of effects of pitavastatin versus pravastatin on serum proprotein convertase subtilisin/kexin type 9 levels in statin-naive patients with coronary artery disease

Proprotein convertase subtilisin/kexin type 9 (PCSK9) is a key regulator of serum low-density lipoprotein cholesterol levels. Although statins increase serum PCSK9 levels, the effects of different types of statins on the serum PCSK9 levels have not been examined in detail. The purpose of the present study was to compare the effects of pitavastatin versus pravastatin on the serum PCSK9 levels. A total of 164 patients with coronary artery disease who were not receiving lipid-lowering therapy were randomly assigned to receive either 4 mg/day of pitavastatin (intensive lipid-lowering therapy) or 20 mg/day of pravastatin (moderate lipid-lowering therapy). The serum PCSK9 levels were measured before statin treatment and 8 months after therapy. A significantly greater reduction in low-density lipoprotein cholesterol was observed in the pitavastatin group (-41% vs -28%, p = 0.0001). The serum levels of total PCSK9 and heterodimer PCSK9 significantly increased from 192 to 249 ng/ml (37%, p <0.0001) and 147 to 206 ng/ml (78%, p <0.0001) in the pitavastatin group and from 192 to 249 ng/ml (39%, p <0.0001) and 143 to 201 ng/ml (65%, p <0.0001) in the pravastatin group, respectively. The increase in total and heterodimer PSCK9 did not differ between the 2 groups. No significant correlations were found between the percentage of changes in heterodimer PCSK9 and changes in the various lipid parameters in either group. In conclusion, significant increases in the total and heterodimer PSCK9 levels were observed at 8 months after treatment with pitavastatin and pravastatin; however, these increases did not differ between the 2 statins.

Activation of 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase during high fat diet feeding

The liver plays a central role in regulating cholesterol homeostasis. High fat diets have been shown to induce obesity and hyperlipidemia. Despite considerable advances in our understanding of cholesterol metabolism, the regulation of liver cholesterol biosynthesis in response to high fat diet feeding has not been fully addressed. The aim of the present study was to investigate mechanisms by which a high fat diet caused activation of liver 3-hydroxy-3-methylglutaryl coenzyme A reductase (HMG-CoA reductase) leading to increased cholesterol biosynthesis. Mice were fed a high fat diet (60% kcal fat) for 5weeks. High fat diet feeding induced weight gain and elevated lipid levels (total cholesterol and triglyceride) in both the liver and serum. Despite cholesterol accumulation in the liver, there was a significant increase in hepatic HMG-CoA reductase mRNA and protein expression as well as enzyme activity. The DNA binding activity of sterol regulatory element binding protein (SREBP)-2 and specific protein 1 (Sp1) were also increased in the liver of mice fed a high fat diet. To validate the in vivo findings, HepG2 cells were treated with palmitic acid. Such a treatment activated SREBP-2 as well as increased the mRNA and enzyme activity of HMG-CoA reductase leading to intracellular cholesterol accumulation. Inhibition of Sp1 by siRNA transfection abolished palmitic acid-induced SREBP-2 and HMG-CoA reductase mRNA expression. These results suggest that Sp1-mediated SREBP-2 activation contributes to high fat diet induced HMG-CoA reductase activation and increased cholesterol biosynthesis. This may play a role in liver cholesterol accumulation and hypercholesterolemia.

The next generation of novel low-density lipoprotein cholesterol-lowering agents: proprotein convertase subtilisin/kexin 9 inhibitors

Proprotein convertase subtilisin/kexin 9 (PCSK9) has been shown to degrade hepatic low-density lipoprotein receptors (LDLR). Gain-of-function mutations promote the development of familial hypercholesterolemia, whereas loss-of-function mutations are associated with lower levels of circulating low-density lipoprotein cholesterol (LDL-C) and significant protection against coronary heart disease. The major classes of commonly prescribed lipid-lowering medications, such as statins, increase serum PCSK9 levels, thus PCSK9 inhibition would increase the efficacy of statins on LDL-C lowering. Therefore, PCSK9 is an attractive therapeutic target for the new generation of cholesterol-lowering drugs. Here, we present a brief overview of the development of PCSK9 inhibitors and highlight the effect of currently prescribed LDL-C-lowering drugs on PCSK9, and the strategies that are being explored for its therapeutic inhibition. Current research and clinical trial results indicate that a PCSK9 inhibitor may be an exciting new therapeutic drug for the treatment of dyslipidemia and relevant cardiovascular diseases.

High-fructose diet downregulates long-chain acyl-CoA synthetase 3 expression in liver of hamsters via impairing LXR/RXR signaling pathway

Long-chain acyl-CoA synthetases (ACSL) play key roles in fatty acid metabolism in liver and other metabolic tissues in an isozyme-specific manner. In this study, we examined the effects of a fructose-enriched diet on expressions of ACSL isoforms in the liver of hamsters. We showed that the fructose diet markedly reduced the mRNA and protein expressions of ACSL3 in hamster liver without significant effects on other ACSLs. The decrease in ACSL3 abundance was accompanied by a reduction in ACSL-catalyzed synthesis of arachidonyl-CoA and oleoyl-CoA in liver homogenates of hamsters fed the fructose diet as opposed to normal diet. We further showed that fructose diet specifically reduced expressions of three key components of the LXR signaling pathway, namely, liver X receptor (LXR)α, LXRβ, and retinoid X receptor (RXR)β. Exogenous expression and activation of LXRα/β increased hamster ACSL3 promoter activities in a LXR-responsive element (LXRE)-dependent fashion. Finally, we showed that treating hamsters with LXR agonist GW3965 increased hepatic ACSL3 expression without affecting other ACSL isoforms. Furthermore, the ligand-induced increases of ACSL3 expression were accompanied with the reduction of hepatic triglyceride levels in GW3965-treated hamster liver. Altogether, our studies demonstrate that fructose diet has a negative impact on LXR signaling pathway in liver tissue and reduction of ACSL3 expression/activity could be a causal factor for fructose-induced hepatic steatosis.

Upregulating Reverse Cholesterol Transport With Cholesteryl Ester Transfer Protein Inhibition Requires Combination With the LDL-Lowering Drug Berberine in Dyslipidemic Hamsters

Objective—

This study aimed to investigate whether cholesteryl ester transfer protein inhibition promotes in vivo reverse cholesterol transport in dyslipidemic hamsters.

Methods and Results—

In vivo reverse cholesterol transport was measured after an intravenous injection of 3 H-cholesteryl-oleate–labeled/oxidized low density lipoprotein particles ( 3 H-oxLDL), which are rapidly cleared from plasma by liver-resident macrophages for further 3 H-tracer egress in plasma, high density lipoprotein (HDL), liver, and feces. A first set of hamsters made dyslipidemic with a high-fat and high-fructose diet was treated with vehicle or torcetrapib 30 mg/kg (TOR) over 2 weeks. Compared with vehicle, TOR increased apolipoprotein E–rich HDL levels and significantly increased 3 H-tracer appearance in HDL by 30% over 72 hours after 3 H-oxLDL injection. However, TOR did not change 3 H-tracer recovery in liver and feces, suggesting that uptake and excretion of cholesterol deriving from apolipoprotein E-rich HDL is not stimulated. As apoE is a potent ligand for the LDL receptor, we next evaluated the effects of TOR in combination with the LDL-lowering drug berberine, which upregulates LDL receptor expression in dyslipidemic hamsters. Compared with TOR alone, treatment with TOR+berberine 150 mg/kg resulted in lower apolipoprotein E–rich HDL levels. After 3 H-oxLDL injection, TOR+berberine significantly increased 3 H-tracer appearance in fecal cholesterol by 109%.

Conclusion—

Our data suggest that cholesteryl ester transfer protein inhibition alone does not stimulate reverse cholesterol transport in dyslipidemic hamsters and that additional effects mediated by the LDL-lowering drug berberine are required to upregulate this process.

Proprotein convertase subtilisin/kexin type 9 inhibition

PURPOSE OF REVIEW

There are now ample data that demonstrate that inhibition of PCSK9 (proprotein convertase subtilisin/kexin type 9) can safely lower LDL cholesterol synergistically with statins. Considering that PCSK9 was first identified less than a decade ago, the last few years have shown rapid and remarkable advancements in our understanding and knowledge of the structure and function of PCSK9.

RECENT FINDINGS

Therapeutic developments have not lagged far behind with some monoclonal antibodies currently entering phase III trials. Of the many approaches to PCSK9 inhibition, these compounds are the furthest advanced in their clinical development while small molecule oral inhibitors seem a distant prospect.

SUMMARY

This review summarizes the discovery and history of PCSK9 and in particular its mode of action as an inhibitor of the LDL receptor. It also recapitulates key studies that have demonstrated the potential of inhibiting PCSK9 to further decrease LDL-cholesterol levels safely and synergistically with statins. Finally, we review the strategies that are currently in development to inhibit PCSK9, with a special emphasis on the spectacular results from recent phase-I and phase-II clinical trials.

Potentially important considerations in choosing specific statin treatments to reduce overall morbidity and mortality

Hypercholesterolemia and dyslipidemia are independent risk factors for cardiovascular disease and death. Statins are the drugs of choice to decrease plasma cholesterol and have other beneficial actions beyond lipid-lowering leading to substantial improvements in cardiovascular morbidity and mortality. However, evaluation of the effects of statins to reduce overall morbidity and mortality must integrate metabolic consequences of statin therapy with its lipid-lowering effect. Indeed, reduction in LDL-cholesterol to target level achieved by statins does not completely eliminate risk of cardiovascular disease and may elevate metabolic risk factors that contribute to dysregulation of metabolic homeostasis. This may lead to increased incidence of diabetes and its cardiovascular complications that are explained, in part, by reciprocal relationships between insulin resistance and endothelial dysfunction. Genetic factors may determine 40-60% of total cholesterol levels and 70% of the efficacy of statin treatments. Metabolic and cardiovascular phenotypes that are either genetically determined or environmentally acquired are also important determinants of responses to specific statins. Moreover, differences between biological outcomes of specific statins or increasing dosages of statins result in differential metabolic actions due to off-target or unknown mechanism that have important implications for the use of statins to reduce overall morbidity and mortality. In this review, we discuss differential cardiovascular and metabolic pleiotropic actions of specific statins that interact in a context-dependent manner with patient phenotypes and genotypes. These important considerations may influence progression of atherosclerosis, risk of diabetes, and modulation of insulin resistance that help determine overall morbidity and mortality in patients undergoing statin therapy.

Nutraceutical pill containing berberine versus ezetimibe on plasma lipid pattern in hypercholesterolemic subjects and its additive effect in patients with familial hypercholesterolemia on stable cholesterol-lowering treatment

Background

Although statins (STs) are drugs of first choice in hypercholesterolemic patients, especially in those at high cardiovascular risk, some of them are intolerant to STs or refuse treatment with these drugs. In view of this, we have evaluated the lipid-lowering effect of a nutraceutical pill containing berberine (BBR) and of ezetimibe, as alternative treatments, in monotherapy or in combination, in 228 subjects with primary hypercholesterolemia (HCH), with history of STs intolerance or refusing STs treatment. In addition, since PCSK9 was found up-regulated by STs dampening their effect through an LDL receptors (LDLRs) degradation, and BBR suppressed PCSK9 expression in cellular studies, we supplemented the stable lipid-lowering therapy of 30 genotype-confirmed Familial Hypercholesterolemia heterozygotes (HeFH) with BBR, searching for a further plasma cholesterol reduction. Plasma lipid pattern was evaluated at baseline and during treatments.

Results

In HCH subjects the nutraceutical pill resulted more effective than EZE in lowering LDL cholesterol (−31.7% vs −25.4%, P < 0.001) and better tolerated. On treatment, LDL-C level below 3.36 mmol/L (≤130 mg/dl) was observed in 28.9% of subjects treated with the nutraceutical pill and 11.8% of those treated with EZE (P <0.007). In the group treated with EZE the subjects carrying the G allele of the g.1679 C > G silent polymorphism of NPC1L1 gene showed a higher response to EZE than homozygous for the common allele (GG + CG: LDL-C −29.4±5.0%, CC −23.6±6.5%, P <0.001). Combined treatment with these drugs was as effective as STs in moderate doses (LDL cholesterol −37%, triglycerides −23%). In HeFH patients the addition of BBR resulted in LDL cholesterol reductions inversely related to those induced by the stable therapy (r = −0.617, P <0.0001), with mean 10.5% further decrease.

Conclusions

The alternative treatments tested in our HCH subjects were rather effective and safe. The findings in HeFH patients suggest that BBR might act in vivo increasing expression and stability of LDLRs and/or suppressing PCSK9 expression.

Delineation of molecular pathways that regulate hepatic PCSK9 and LDL receptor expression during fasting in normolipidemic hamsters

BACKGROUND

PCSK9 has emerged as a key regulator of serum LDL-C metabolism by promoting the degradation of hepatic LDL receptor (LDLR). In this study, we investigated the effect of fasting on serum PCSK9, LDL-C, and hepatic LDLR expression in hamsters and further delineated the molecular pathways involved in fasting-induced repression of PCSK9 transcription.

RESULTS

Fasting had insignificant effects on serum total cholesterol and HDL-C levels, but reduced LDL-C, triglyceride and insulin levels. The decrease in serum LDL-C was accompanied by marked reductions of hepatic PCSK9 mRNA and serum PCSK9 protein levels with concomitant increases of hepatic LDLR protein amounts. Fasting produced a profound impact on SREBP1 expression and its transactivating activity, while having modest effects on mRNA expressions of SREBP2 target genes in hamster liver. Although PPARα mRNA levels in hamster liver were elevated by fasting, ligand-induced activation of PPARα with WY14643 compound in hamster primary hepatocytes did not affect PCSK9 mRNA or protein expressions. Further investigation on HNF1α, a critical transactivator of PCSK9, revealed that fasting did not alter its mRNA expression, however, the protein abundance of HNF1α in nuclear extracts of hamster liver was markedly reduced by prolonged fasting.

CONCLUSION

Fasting lowered serum LDL-C in hamsters by increasing hepatic LDLR protein amounts via reductions of serum PCSK9 levels. Importantly, our results suggest that attenuation of SREBP1 transactivating activity owing to decreased insulin levels during fasting is primarily responsible for compromised PCSK9 gene transcription, which was further suppressed after prolonged fasting by a reduction of nuclear HNF1α protein abundance.

The PCSK9 decade

PCSK9 proprotein convertase subtilisin/kexin type (PCSK9) is a crucial protein in LDL cholesterol (LDL-C) metabolism by virtue of its pivotal role in the degradation of the LDL receptor. In recent years, both in vitro and in vivo studies have greatly supplemented our understanding of the (patho)physiological role of PCSK9 in human biology. In the current review, we summarize studies published or in print before May 2012 concerning the physiological role of PCSK9 in cholesterol metabolism. Moreover, we briefly describe the clinical phenotypes encountered in carriers of mutations in the gene encoding PCSK9. As PCSK9 has emerged as a novel target for LDL-C lowering therapy, methods to inhibit PCSK9 will also be reviewed. Initial data from investigations of PCSK9 inhibition in humans are promising and indicate that PCSK9 inhibition may be a viable new therapeutic option for the treatment of dyslipidemia and associated cardiovascular diseases.

Peroxisome Proliferator-activated receptor γ activation by ligands and dephosphorylation induces proprotein convertase subtilisin kexin type 9 and low density lipoprotein receptor expression

Proprotein convertase subtilisin kexin type 9 (PCSK9) plays an important role in cholesterol homeostasis by enhancing the degradation of LDL receptor (LDLR) protein. Peroxisome proliferator-activated receptor γ (PPARγ) has been shown to be atheroprotective. PPARγ can be activated by ligands and/or dephosphorylation with ERK1/2 inhibitors. The effect of PPARγ on PCSK9 and LDLR expression remains unknown. In this study, we investigated the effects of PPARγ on PCSK9 and LDLR expression. At the cellular levels, PPARγ ligands induced PCSK9 mRNA and protein expression in HepG2 cells. PCSK9 expression was induced by inhibition of ERK1/2 activity but inhibited by ERK1/2 activation. The mutagenic study and promoter activity assay suggested that the induction of PCSK9 expression by ERK1/2 inhibitors was tightly linked to PPARγ dephosphorylation. However, PPARγ activation by ligands or ERK1/2 inhibitors induced hepatic LDLR expression. The promoter assay indicated that the induction of LDLR expression by PPARγ was sterol regulatory element-dependent because PPARγ enhanced sterol regulatory element-binding protein 2 (SREBP2) processing. In vivo, administration of pioglitazone or U0126 alone increased PCSK9 expression in mouse liver but had little effect on PCSK9 secretion. However, the co-treatment of pioglitazone and U0126 enhanced both PCSK9 expression and secretion. Similar to in vitro, the increased PCSK9 expression by pioglitazone and/or U0126 did not result in decreased LDLR expression and function. In contrast, pioglitazone and/or U0126 increased LDLR protein expression and membrane translocation, SREBP2 processing, and CYP7A1 expression in the liver, which led to decreased total and LDL cholesterol levels in serum. Our results indicate that although PPARγ activation increased PCSK9 expression, PPARγ activation induced LDLR and CYP7A1 expression that enhanced LDL cholesterol metabolism.

Regulation of hepatic LDL receptors by mTORC1 and PCSK9 in mice

Individuals with type 2 diabetes have an increased risk of atherosclerosis. One factor underlying this is dyslipidemia, which in hyperinsulinemic subjects with early type 2 diabetes is typically characterized by increased VLDL secretion but normal LDL cholesterol levels, possibly reflecting enhanced catabolism of LDL via hepatic LDLRs. Recent studies have also suggested that hepatic insulin signaling sustains LDLR levels. We therefore sought to elucidate the mechanisms linking hepatic insulin signaling to regulation of LDLR levels. In WT mice, insulin receptor knockdown by shRNA resulted in decreased hepatic mTORC1 signaling and LDLR protein levels. It also led to increased expression of PCSK9, a known post-transcriptional regulator of LDLR expression. Administration of the mTORC1 inhibitor rapamycin caused increased expression of PCSK9, decreased levels of hepatic LDLR protein, and increased levels of VLDL/LDL cholesterol in WT but not Pcsk9-/- mice. Conversely, mice with increased hepatic mTORC1 activity exhibited decreased expression of PCSK9 and increased levels of hepatic LDLR protein levels. Pcsk9 is regulated by the transcription factor HNF1α, and our further detailed analyses suggest that increased mTORC1 activity leads to activation of PKCδ, reduced activity of HNF4α and HNF1α, decreased PCSK9 expression, and ultimately increased hepatic LDLR protein levels, which result in decreased circulating LDL levels. We therefore suggest that PCSK9 inhibition could be an effective way to reduce the adverse side effect of increased LDL levels that is observed in transplant patients taking rapamycin as immunosuppressive therapy.

An anti-PCSK9 antibody reduces LDL-cholesterol on top of a statin and suppresses hepatocyte SREBP-regulated genes

Proprotein convertase subtilisin/kexin type 9 (PCSK9) is a promising therapeutic target for treating coronary heart disease. We report a novel antibody 1B20 that binds to PCSK9 with sub-nanomolar affinity and antagonizes PCSK9 function in-vitro. In CETP/LDLR-hemi mice two successive doses of 1B20, administered 14 days apart at 3 or 10 mpk, induced dose dependent reductions in LDL-cholesterol (≥ 25% for 7-14 days) that correlated well with the extent of PCSK9 occupancy by the antibody. In addition, 1B20 induces increases in total plasma antibody-bound PCSK9 levels and decreases in liver mRNA levels of SREBP-regulated genes PCSK9 and LDLR, with a time course that parallels decreases in plasma LDL-cholesterol (LDL-C). Consistent with this observation in mice, in statin-responsive human primary hepatocytes, 1B20 lowers PCSK9 and LDLR mRNA levels and raises serum steady-state levels of antibody-bound PCSK9. In addition, mRNA levels of several SREBP regulated genes involved in cholesterol and fatty-acid synthesis including ACSS2, FDPS, IDI1, MVD, HMGCR, and CYP51A1 were decreased significantly with antibody treatment of primary human hepatocytes. In rhesus monkeys, subcutaneous (SC) dosing of 1B20 dose-dependently induces robust LDL-C lowering (maximal ~70%), which is correlated with increases in target engagement and total antibody-bound PCSK9 levels. Importantly, a combination of 1B20 and Simvastatin in dyslipidemic rhesus monkeys reduced LDL-C more than either agent alone, consistent with a mechanism of action that predicts additive effects of anti-PCSK9 agents with statins. Our results suggest that antibodies targeting PCSK9 could provide patients powerful LDL lowering efficacy on top of statins, and lower cardiovascular risk.

Rosuvastatin, proprotein convertase subtilisin/kexin type 9 concentrations, and LDL cholesterol response: the JUPITER trial

BACKGROUND

Although statin therapy is known to increase concentrations of PCSK9, whether this effect is related to the magnitude of LDL reduction is uncertain. This study was undertaken to understand the extent of this effect and examine the relationship between PCSK9 and LDL cholesterol (LDL-C) reduction.

METHODS

We measured plasma PCSK9 concentrations by ELISA at baseline and at 1 year in 500 men and 500 women participating in the Justification for Use of Statins in Prevention: an Intervention Trial Evaluating Rosuvastatin (JUPITER) trial that randomly allocated participants to rosuvastatin 20 mg daily or placebo. We also evaluated rs11591147, a single nucleotide polymorphism known to have an impact on plasma PCSK9 concentrations.

RESULTS

At baseline, median (interquartile range) PCSK9 concentrations were higher in women [73 (62-90)] ng/mL than in men [69 (57-81) ng/mL] (P<0.005). During 1 year, there was no change in PCSK9 concentrations in the placebo arm, suggesting stability in time. In contrast, the rosuvastatin increased PCSK9 by 35% in women [101 (82-117) ng/mL] and 28% in men [89 (71-109) ng/mL] (P<0.0001). Among those allocated to rosuvastatin, greater reductions in LDL-C were associated with greater increases in PCSK9 on both absolute and relative scales (r=-0.15, P<0.0005). Furthermore PCSK9 (rs11591147) did not alter the magnitude of LDL-C reduction associated with rosuvastatin use.

CONCLUSIONS

In this randomized trial, rosuvastatin increased plasma concentration of PCSK9 in proportion to the magnitude of LDL-C reduction; the LDL-C response to statin could not be inferred by PCSK9 concentrations.

Anacetrapib promotes reverse cholesterol transport and bulk cholesterol excretion in Syrian golden hamsters

Cholesteryl ester transfer protein (CETP) transfers cholesteryl ester (CE) and triglyceride between HDL and apoB-containing lipoproteins. Anacetrapib (ANA), a reversible inhibitor of CETP, raises HDL cholesterol (HDL-C) and lowers LDL cholesterol in dyslipidemic patients; however, the effects of ANA on cholesterol/lipoprotein metabolism in a dyslipidemic hamster model have not been demonstrated. To test whether ANA (60 mg/kg/day, 2 weeks) promoted reverse cholesterol transport (RCT), ³H-cholesterol-loaded macrophages were injected and (3)H-tracer levels were measured in HDL, liver, and feces. Compared to controls, ANA inhibited CETP (94%) and increased HDL-C (47%). ³H-tracer in HDL increased by 69% in hamsters treated with ANA, suggesting increased cholesterol efflux from macrophages to HDL. ³H-tracer in fecal cholesterol and bile acids increased by 90% and 57%, respectively, indicating increased macrophage-to-feces RCT. Mass spectrometry analysis of HDL from ANA-treated hamsters revealed an increase in free unlabeled cholesterol and CE. Furthermore, bulk cholesterol and cholic acid were increased in feces from ANA-treated hamsters. Using two independent approaches to assess cholesterol metabolism, the current study demonstrates that CETP inhibition with ANA promotes macrophage-to-feces RCT and results in increased fecal cholesterol/bile acid excretion, further supporting its development as a novel lipid therapy for the treatment of dyslipidemia and atherosclerotic vascular disease.

Proprotein convertase substilisin/kexin type 9 antagonism reduces low-density lipoprotein cholesterol in statin-treated hypercholesterolemic nonhuman primates

Proprotein convertase substilisin/kexin type 9 (PCSK9) promotes the degradation of low-density lipoprotein (LDL) receptor (LDLR) and thereby increases serum LDL-cholesterol (LDL-C). We have developed a humanized monoclonal antibody that recognizes the LDLR binding domain of PCSK9. This antibody, J16, and its precursor mouse antibody, J10, potently inhibit PCSK9 binding to the LDLR extracellular domain and PCSK9-mediated down-regulation of LDLR in vitro. In vivo, J10 effectively reduces serum cholesterol in C57BL/6 mice fed normal chow. J16 reduces LDL-C in healthy and diet-induced hypercholesterolemic cynomologous monkeys, but does not significantly affect high-density lipoprotein-cholesterol. Furthermore, J16 greatly lowered LDL-C in hypercholesterolemic monkeys treated with the HMG-CoA reductase inhibitor simvastatin. Our data demonstrate that anti-PCSK9 antibody is a promising LDL-C-lowering agent that is both efficacious and potentially additive to current therapies.

Mechanisms and genetic determinants regulating sterol absorption, circulating LDL levels, and sterol elimination: implications for classification and disease risk

This review integrates historical biochemical and modern genetic findings that underpin our understanding of the low-density lipoprotein (LDL) dyslipidemias that bear on human disease. These range from life-threatening conditions of infancy through severe coronary heart disease of young adulthood, to indolent disorders of middle- and old-age. We particularly focus on the biological aspects of those gene mutations and variants that impact on sterol absorption and hepatobiliary excretion via specific membrane transporter systems (NPC1L1, ABCG5/8); the incorporation of dietary sterols (MTP) and of de novo synthesized lipids (HMGCR, TRIB1) into apoB-containing lipoproteins (APOB) and their release into the circulation (ANGPTL3, SARA2, SORT1); and receptor-mediated uptake of LDL and of intestinal and hepatic-derived lipoprotein remnants (LDLR, APOB, APOE, LDLRAP1, PCSK9, IDOL). The insights gained from integrating the wealth of genetic data with biological processes have important implications for the classification of clinical and presymptomatic diagnoses of traditional LDL dyslipidemias, sitosterolemia, and newly emerging phenotypes, as well as their management through both nutritional and pharmaceutical means.

Plasma proprotein convertase subtilisin kexin type 9 is a heritable trait of familial combined hyperlipidaemia

The aim of the present study was to investigate the relationship between circulating PCSK9 (proprotein convertase subtilisin kexin type 9) and FCHL (familial combined hyperlipidaemia) and, when positive, to determine the strength of its heritability. Plasma PCSK9 levels were measured in FCHL patients (n=45), NL (normolipidaemic) relatives (n=139) and their spouses (n=72). In addition, 11 FCHL patients were treated with atorvastatin to study the response in PCSK9 levels. PCSK9 levels were higher in FCHL patients compared with NL relatives and spouses: 96.1 compared with 78.7 and 82.0 ng/ml (P=0.004 and P=0.002 respectively). PCSK9 was significantly associated with both TAG (triacylglycerol) and apolipoprotein B levels (P<0.001). The latter relationship was accounted for by LDL (low-density lipoprotein)–apolipoprotein B (r=0.31, P=0.02), not by VLDL (very-low-density lipoprotein)–apolipoprotein B (r=0.09, P=0.49) in a subgroup of subjects (n=59). Heritability calculations for PCSK9 using SOLAR and FCOR software yielded estimates of 67–84% respectively (P<0.0001). PCSK9 increased from 122 to 150 ng/ml in 11 FCHL patients treated with atorvastatin (40 mg) once daily for 8 weeks (P=0.018). In conclusion, plasma PCSK9 is a heritable trait associated with both FCHL diagnostic hallmarks. These results, combined with the significant rise in PCSK9 levels after statin therapy, warrant further studies in order to unravel the exact role of PCSK9 in the pathogenesis and treatment of this highly prevalent genetic dyslipidaemia.

PCSK9 reduces the protein levels of the LDL receptor in mouse brain during development and after ischemic stroke

Proprotein convertase subtilisin/kexin type 9 (PCSK9) plays a major role in cholesterol homeostasis through enhanced degradation of the LDL receptor (LDLR) in liver. As novel inhibitors/silencers of PCSK9 are now being tested in clinical trials to treat hypercholesterolemia, it is crucial to define the physiological consequences of the lack of PCSK9 in various organs. LDLR regulation by PCSK9 has not been extensively described during mouse brain development and injury. Herein, we show that PCSK9 and LDLR are co-expressed in mouse brain during development and at adulthood. Although the protein levels of LDLR and apolipoprotein E (apoE) in the adult brain of Pcsk9(-/-) mice are similar to those of wild-type (WT) mice, LDLR levels increased and were accompanied by a reduction of apoE levels during development. This suggests that the upregulation of LDLR protein levels in Pcsk9(-/-) mice enhances apoE degradation. Upon ischemic stroke, PCSK9 was expressed in the dentate gyrus between 24 h and 72 h following brain reperfusion. Although mouse behavior and lesion volume were similar, LDLR protein levels dropped ∼2-fold less in the Pcsk9(-/-)-lesioned hippocampus, without affecting apoE levels and neurogenesis. Thus, PCSK9 downregulates LDLR levels during brain development and following transient ischemic stroke in adult mice.

Effects of currently prescribed LDL-C-lowering drugs on PCSK9 and implications for the next generation of LDL-C-lowering agents

Background

During the past decade, proprotein convertase subtilisin kexin type 9 (PCSK9) has been identified as a key regulator of serum LDL-cholesterol (LDL-C) levels. PCSK9 is secreted by the liver into the plasma and binds the hepatic LDL receptor, causing its subsequent degradation. In humans, gain-of-function mutations in PCSK9 cause a form of familial hypercholesterolemia that manifests with dramatically increased serum levels of LDL-C, while loss-of-function mutations in PCSK9 are associated with significantly decreased LDL-C and cardiovascular risk.

Results

Initial studies in animals and cultured cells demonstrated that statins increased PCSK9 mRNA expression, resulting in many research groups exploring the effect of statins on PCSK9 levels in humans. We first reported that statins increased human PCSK9 circulating protein levels. Additional researchers subsequently confirmed these observations, further prompting many laboratories including our own to examine the effect of other lipid lowering medications on PCSK9 levels. Our observation that fenofibrate (200 mg/day) significantly increased PCSK9 levels was confirmed by another laboratory, and an additional group demonstrated that ezetimibe also increased PCSK9 levels.

Conclusions

It has become clear that the major classes of commonly prescribed lipid-lowering medications increase serum PCSK9 levels. These observations almost certainly explain why these agents are not more effective in lowering LDL-C and suggest that efforts should be made toward the development of new LDL-C lowering medications that either do not increase circulating PCSK9 levels or work through decreasing or inhibiting PCSK9.

Improved efficacy for ezetimibe and rosuvastatin by attenuating the induction of PCSK9

Reducing circulating LDL-cholesterol (LDL-c) reduces the risk of cardiovascular disease in people with hypercholesterolemia. Current approaches to reduce circulating LDL-c include statins, which inhibit cholesterol synthesis, and ezetimibe, which blocks cholesterol absorption. Both elevate serum PCSK9 protein levels in patients, which could attenuate their efficacy by reducing the amount of cholesterol cleared from circulation. To determine whether PCSK9 inhibition could enhance LDL-c lowering of both statins and ezetimibe, we utilized small interfering RNAs (siRNAs) to knock down Pcsk9, together with ezetimibe, rosuvastatin, and an ezetimibe/rosuvastatin combination in a mouse model with a human-like lipid profile. We found that ezetimibe, rosuvastatin, and ezetimibe/rosuvastatin combined lower serum cholesterol but induce the expression of Pcsk9 as well as the Srebp-2 hepatic cholesterol biosynthesis pathway. Pcsk9 knockdown in combination with either treatment led to greater reductions in serum non-HDL with a near-uniform reduction of all LDL-c subfractions. In addition to reducing serum cholesterol, the combined rosuvastatin/ezetimibe/Pcsk9 siRNA treatment exhibited a significant reduction in serum APOB protein and triglyceride levels. Taken together, these data provide evidence that PCSK9 inhibitors, in combination with current therapies, have the potential to achieve greater reductions in both serum cholesterol and triglycerides.

Analyses of PCSK9 post-translational modifications using time-of-flight mass spectrometry

Post-translational modification(s) can affect a protein's function - changing its half-life/stability, its protein-protein interactions, biological activity and/or sub-cellular localization. Following translation, a protein can be modified in several ways, including (i) disulfide bridge formation, (ii) chemical conversion of its constituent amino acids (for instance, glutamine can undergo deamidation to glutamic acid), (iii) sulfation, phosphorylation, de/acetylation, and glycosylation (to name a few), (iv) addition of other proteins as occurs during sumoylation and ubiquitination, and (v) proteolytic cleavage(s). There are several techniques available to identify and monitor post-translational modifications of proteins and peptides including mass spectrometry, two-dimensional sodium dodecyl sulfate polyacrylamide electrophoresis (2D-SDS-PAGE), radiolabeling, and immunoblotting. Ciphergen's surface-enhanced laser desorption/ionization time-of-flight mass spectrometer (SELDI-TOF-MS) has been used successfully for protein/peptide profiling in disease states and for the detection of protein/peptide biomarkers (1-4). In this chapter, the secreted proprotein convertase subtilisin/kexin 9 (PCSK9), which we study in our lab, is used to demonstrate coupling of immunoprecipitation with Ciphergen's time-of-flight mass spectrometer and its ProteinChip software to detect and analyze the common post-translational modifications of phosphorylation and glycosylation. The following topics are covered (1): preparation of cell extracts/samples/spent media (2), processing of samples by immunoprecipitation including optimization of conditions and (3) data acquisition by mass spectrometry and its subsequent analyses.

High-dose atorvastatin causes a rapid sustained increase in human serum PCSK9 and disrupts its correlation with LDL cholesterol

Proprotein convertase subtilisin kexin type 9 (PCSK9) is a key regulator of serum LDL-cholesterol (LDL-C) levels. PCSK9 is secreted by the liver into the plasma and binds the hepatic LDL receptor (LDLR), causing its subsequent degradation. We first demonstrated that a moderate dose of atorvastatin (40 mg) increases PCSK9 serum levels, suggesting why increasing statin doses may have diminished efficacy with regard to further LDL-C lowering. Since that initial observation, at least two other groups have reported statin-induced PCSK9 increases. To date, no analysis of the effect of high-dose atorvastatin (80 mg) on PCSK9 over time has been conducted. Therefore, we studied the time course of atorvastatin (80 mg) in human subjects. We measured PCSK9 and lipid levels during a 2-week lead-in baseline period and every 4 weeks thereafter for 16 weeks. We observed that atorvastatin (80 mg) caused a rapid 47% increase in serum PCSK9 at 4 weeks that was sustained throughout 16 weeks of dosing. Importantly, while PCSK9 levels were highly correlated with total cholesterol (TC), LDL-C, and triglyceride (TG) levels at baseline, atorvastatin (80 mg) completely abolished all of these correlations. Together, these results further suggest an explanation for why increasing doses of statins fail to achieve proportional LDL-C lowering.