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

Discovery of σ2R/TMEM97 as a Novel Biomarker for Atherosclerotic Plaques: A PET Imaging and Validation Study

BACKGROUND

The aims of this study were to evaluate σ2R (sigma-2 receptor)/TMEM97 (transmembrane protein 97) expression in atherosclerotic plaques, and assess the feasibility of in vivo atherosclerotic plaques imaging using the σ2R/TMEM97 targeting probe 1-(4-(5,6-dimethoxyisoindolin-2-yl)butyl)-3-(2-[18F]fluoroethyl)-1,3-dihydro-2H-benzo[d]imidazol-2-one ([18F]SYB-NF) developed in our laboratory.

METHODS

Hematoxylin and eosin and immunohistochemical staining were performed on both human coronary endarterectomy specimens and mouse samples. The expression of σ2R/TMEM97 in RAW264.7 cells incubated with ox-LDL (oxidized low-density lipoprotein) was analyzed using western blot analysis. Positron emission tomography imaging with [18F]SYB-NF, [18F]NaF, and [18F]fluoro-2-deoxy-d-glucose was conducted in wide-type C57BL/6 and ApoE-/- mice. Specific binding was evaluated by coinjecting [18F]SYB-NF with the σ2R/TMEM97 antagonist CM398. Autoradiography and Oil Red O staining were performed on harvested aortas and corresponding sections.

RESULTS

Staining results demonstrated significant upregulation of σ2R/TMEM97 expression during both early plaque formation and atherosclerosis progression. Western blot analysis indicated that incubation of macrophages with ox-LDL led to increased σ2R/TMEM97 expression. [18F]SYB-NF specifically accumulated in the aortic arch of ApoE-/- mice. Treatment with CM398 significantly reduced the standardized uptake value in the aortic arch of ApoE-/- mice. [18F]SYB-NF exhibited a higher standardized uptake value in the aortic arch (0.67±0.09 versus 0.51±0.07) and higher aortic arch-to-heart ratio (2.58 versus 0.56) in ApoE-/- mice compared with [18F]fluoro-2-deoxy-d-glucose, and a higher aortic arch-to-bone ratio (2.24 versus 0.44) compared with [18F]NaF. Autoradiography analysis revealed a strong correlation between the positive area in Oil Red O staining and autoradiography (Pearson correlation coefficient=0.993; P=0.001), further supporting the association between elevated σ2R/TMEM97 expression and plaque formation.

CONCLUSIONS

σ2R/TMEM97 may serve as a potential biomarker for atherosclerotic plaques, and σ2R/TMEM97 positron emission tomography imaging may be used to monitor plaque formation and progression, as well as the efficacy of emerging therapeutic strategies for atherosclerotic plaques.

Curcumin nicotinate increases LDL cholesterol uptake in hepatocytes through IDOL/LDL-R pathway regulation

BACKGROUND

Curcumin nicotinate (Curtn), derived from curcumin and niacin, reduces serum LDL-C levels, partly due to its influence on PCSK9. This study investigates IDOL's role in Curtn's lipid-lowering effects.

OBJECTIVE

To elucidate Curtn's regulation of the IDOL/LDLR pathway and potential molecular mechanisms in hepatocytes.

METHODS

Differential metabolites in Curtn-treated HepG2 cells were identified via LC-MS. Molecular docking assessed Curtn's affinity with IDOL. Cholesterol content and LDLR expression effects were studied in high-fat diet Wistar rats. In vitro evaluations determined Curtn's influence on IDOL overexpression's LDL-C uptake and LDLR expression in hepatocytes.

RESULTS

Lipids were the main differential metabolites in Curtn-treated HepG2 cells. Docking showed Curtn's higher affinity to IDOL's FERM domain compared to curcumin, suggesting potential competitive inhibition of IDOL's binding to LDLR. Curtn decreased liver cholesterol in Wistar rats and elevated LDLR expression. During in vitro experiments, Curtn significantly enhanced the effects of IDOL overexpression in HepG2 cells, leading to increased LDL-C uptake and elevated expression of LDL receptors.

CONCLUSION

Curtn modulates the IDOL/LDLR pathway, enhancing LDL cholesterol uptake in hepatocytes. Combined with its PCSK9 influence, Curtn emerges as a potential hyperlipidemia therapy.

Regulation of cholesterol metabolism: New players for an old physiological process

Identified more than two centuries ago, cholesterol plays a pivotal role in human physiology. Since cholesterol metabolism is a physiologically significant process, it is not surprising that its alterations are associated with several pathologies. The discovery of new molecular targets or compounds able to modulate this sophisticated metabolism has been capturing the attention of research groups worldwide since many years. Endogenous and exogenous compounds are known to regulate cellular cholesterol synthesis and uptake, or reduce cholesterol absorption at the intestinal level, thereby regulating cholesterol homeostasis. However, there is a great need of new modulators and diverse new pathways have been uncovered. Here, after illustrating cholesterol metabolism and its well-known regulators, some new players of this important physiological process are also described.

A Review of Progress on Targeting LDL Receptor-Dependent and -Independent Pathways for the Treatment of Hypercholesterolemia, a Major Risk Factor of ASCVD

Since the discovery of the LDL receptor in 1973 by Brown and Goldstein as a causative protein in hypercholesterolemia, tremendous amounts of effort have gone into finding ways to manage high LDL cholesterol in familial hypercholesterolemic (HoFH and HeFH) individuals with loss-of-function mutations in the LDL receptor (LDLR) gene. Statins proved to be the first blockbuster drug, helping both HoFH and HeFH individuals by inhibiting the cholesterol synthesis pathway rate-limiting enzyme HMG-CoA reductase and inducing the LDL receptor. However, statins could not achieve the therapeutic goal of LDL. Other therapies targeting LDLR include PCSK9, which lowers LDLR by promoting LDLR degradation. Inducible degrader of LDLR (IDOL) also controls the LDLR protein, but an IDOL-based therapy is yet to be developed. Among the LDLR-independent pathways, such as angiopoietin-like 3 (ANGPTL3), apolipoprotein (apo) B, apoC-III and CETP, only ANGPTL3 offers the advantage of treating both HoFH and HeFH patients and showing relatively better preclinical and clinical efficacy in animal models and hypercholesterolemic individuals, respectively. While loss-of-LDLR-function mutations have been known for decades, gain-of-LDLR-function mutations have recently been identified in some individuals. The new information on gain of LDLR function, together with CRISPR-Cas9 genome/base editing technology to target LDLR and ANGPTL3, offers promise to HoFH and HeFH individuals who are at a higher risk of developing atherosclerotic cardiovascular disease (ASCVD).

The Cholesterol-Modulating Effect of the New Herbal Medicinal Recipe from Yellow Vine (Coscinium fenestratum (Goetgh.)), Ginger (Zingiber officinale Roscoe.), and Safflower (Carthamus tinctorius L.) on Suppressing PCSK9 Expression to Upregulate LDLR Expression in HepG2 Cells

PCSK9 is a promising target for developing novel cholesterol-lowering drugs. We developed a recipe that combined molecular docking, GC-MS/MS, and real-time PCR to identify potential PCSK9 inhibitors for herb ratio determination. Three herbs, Carthamus tinctorius, Coscinium fenestratum, and Zingiber officinale, were used in this study. This work aimed to evaluate cholesterol-lowering through a PCSK9 inhibitory mechanism of these three herbs for defining a suitable ratio. Chemical constituents were identified using GC-MS/MS. The PCSK9 inhibitory potential of the compounds was determined using molecular docking, real-time PCR, and Oil red O staining. It has been shown that most of the active compounds of C. fenestratum and Z. officinale inhibit PCSK9 when extracted with water, and C. fenestratum has been shown to yield tetraacetyl-d-xylonic nitrile (27.92%) and inositol, 1-deoxy-(24.89%). These compounds could inhibit PCSK9 through the binding of 6 and 5 hydrogen bonds, respectively, while the active compound in Z. officinale is 2-Formyl-9-[.beta.-d-ribofuranosyl] hypoxanthine (4.37%) inhibits PCSK9 by forming 8 hydrogen bonds. These results suggest that a recipe comprising three parts C. fenestratum, two parts Z. officinale, and one part C. tinctorius is a suitable herbal ratio for reducing lipid levels in the bloodstream through a PCSK9 inhibitory mechanism.

Effects of statins on the inducible degrader of low-density lipoprotein receptor in familial hypercholesterolemia

The inducible degrader of low-density lipoprotein receptor (IDOL) is an E3 ubiquitin ligase involved in the post-transcriptional regulation of LDL receptor (LDLR). Statins lower plasma LDL by activating transcription of hepatic LDLR expression, and we have determined whether statins modulate IDOL expression and influence LDLR protein abundance. IDOL expression in monocytes and serum IDOL level was determined in statin-treated familial hypercholesterolemia (FH) patients and compared with control subjects. Serum IDOL level was also evaluated in a group of untreated FH patients before and after the initiation of statin. The mechanism underlying the inhibitory effect of statin on IDOL expression was investigated in vitro. In statin-treated FH patients, serum IDOL level and its expression in monocytes was reduced compared with control (P < 0.05). In contrast, untreated FH patients had higher serum levels of IDOL and proprotein convertase subtilisin/kexintype 9 (PCSK9) than control (P < 0.05), and serum IDOL level decreased after statin therapy (P < 0.05) whereas an increase was observed in PCSK9 level (P < 0.01). In vitro, atorvastatin significantly decreased IDOL abundance in a dose-dependent manner in cultured macrophages and hepatocytes with a concomitant increase in LDLR expression. The transcription of IDOL was restored by adding either an LXR agonist T0901317 or oxysterol 22(R)-hydroxycholesterol, indicating that statin inhibited IDOL expression by reducing LXR activation. The LXR-IDOL-LDLR axis can be modulated by statins in vitro and in vivo. Statins inhibit IDOL expression by reducing LXR activation and upregulate LDLR, and statins exert the opposite effect on IDOL and PCSK9.

Inclisiran in lipid management: A Literature overview and future perspectives

Primary and secondary prevention protocols aim at reducing the plasma levels of lipids - with particular reference to low-density lipoprotein cholesterol (LDL-C) plasma concentrations - in order to improve the overall survival and reduce the occurrence of major adverse cardiovascular events. The use of statins has been widely considered as the first-line approach in lipids management as they can dramatically impact on the cardiovascular risk profile of individuals. The introduction of ezetimibe and proprotein convertase subtilisin-kexin type 9 (PCSK9) inhibitors overcame the adverse effects of statins and ameliorate the achievement of the target lipids levels. Indeed, advances in therapies promote the use of specific molecules - i.e. short strands of RNA named small-interfering RNAs (siRNAs) - to suppress the transcription of genes related to lipids metabolism. Recently, the inclisiran has been developed: this is a siRNA able to block the mRNA of the PCSK9 gene. About 50% reduction in low-density lipoprotein cholesterol levels have been observed in randomized controlled trials with inclisiran. The aim of this review was to summarize the literature regarding inclisiran and its possible role in the general management of patients with lipid disorders and/or in primary/secondary prevention protocols.

Hepatic cholesterol transport and its role in non-alcoholic fatty liver disease and atherosclerosis

Non-alcoholic fatty liver disease (NAFLD) is a quickly emerging global health problem representing the most common chronic liver disease in the world. Atherosclerotic cardiovascular disease represents the leading cause of mortality in NAFLD patients. Cholesterol metabolism has a crucial role in the pathogenesis of both NAFLD and atherosclerosis. The liver is the major organ for cholesterol metabolism. Abnormal hepatic cholesterol metabolism not only leads to NAFLD but also drives the development of atherosclerotic dyslipidemia. The cholesterol level in hepatocytes reflects the dynamic balance between endogenous synthesis, uptake, esterification, and export, a process in which cholesterol is converted to neutral cholesteryl esters either for storage in cytosolic lipid droplets or for secretion as a major constituent of plasma lipoproteins, including very-low-density lipoproteins, chylomicrons, high-density lipoproteins, and low-density lipoproteins. In this review, we describe decades of research aimed at identifying key molecules and cellular players involved in each main aspect of hepatic cholesterol metabolism. Furthermore, we summarize the recent advances regarding the biological processes of hepatic cholesterol transport and its role in NAFLD and atherosclerosis.

LDL Receptor Pathway Regulation by miR-224 and miR-520d

MicroRNAs (miRNA) have emerged as important post-transcriptional regulators of metabolic pathways that contribute to cellular and systemic lipoprotein homeostasis. Here, we identify two conserved miRNAs, miR-224, and miR-520d, which target gene networks regulating hepatic expression of the low-density lipoprotein (LDL) receptor (LDLR) and LDL clearance. In silico prediction of miR-224 and miR-520d target gene networks showed that they each repress multiple genes impacting the expression of the LDLR, including the chaperone molecules PCSK9 and IDOL that limit LDLR expression at the cell surface and the rate-limiting enzyme for cholesterol synthesis HMGCR, which is the target of LDL-lowering statin drugs. Using gain- and loss-of-function studies, we tested the role of miR-224 and miR-520d in the regulation of those predicted targets and their impact on LDLR expression. We show that overexpression of miR-224 or miR-520d dose-dependently reduced the activity of PCSK9, IDOL, and HMGCR 3'-untranslated region (3'-UTR)-luciferase reporter constructs and that this repression was abrogated by mutation of the putative miR-224 or miR-520d response elements in the PCSK9, IDOL, and HMGCR 3'-UTRs. Compared to a control miRNA, overexpression of miR-224 or miR-520d in hepatocytes inhibited PCSK9, IDOL, and HMGCR mRNA and protein levels and decreased PCSK9 secretion. Furthermore, miR-224 and miR-520d repression of PCSK9, IDOL, and HMGCR was associated with an increase in LDLR protein levels and cell surface expression, as well as enhanced LDL binding. Notably, the effects of miR-224 and miR-520d were additive to the effects of statins in upregulating LDLR expression. Finally, we show that overexpression of miR-224 in the livers of Ldlr ^+/- mice using lipid nanoparticle-mediated delivery resulted in a 15% decrease in plasma levels of LDL cholesterol, compared to a control miRNA. Together, these findings identify roles for miR-224 and miR-520d in the posttranscriptional control of LDLR expression and function.

Inhibition of Bromodomain and Extraterminal Domain (BET) Proteins by JQ1 Unravels a Novel Epigenetic Modulation to Control Lipid Homeostasis

The homeostatic control of lipid metabolism is essential for many fundamental physiological processes. A deep understanding of its regulatory mechanisms is pivotal to unravel prospective physiopathological factors and to identify novel molecular targets that could be employed to design promising therapies in the management of lipid disorders. Here, we investigated the role of bromodomain and extraterminal domain (BET) proteins in the regulation of lipid metabolism. To reach this aim, we used a loss-of-function approach by treating HepG2 cells with JQ1, a powerful and selective BET inhibitor. The main results demonstrated that BET inhibition by JQ1 efficiently decreases intracellular lipid content, determining a significant modulation of proteins involved in lipid biosynthesis, uptake and intracellular trafficking. Importantly, the capability of BET inhibition to slow down cell proliferation is dependent on the modulation of cholesterol metabolism. Taken together, these data highlight a novel epigenetic mechanism involved in the regulation of lipid homeostasis.

The E3 ubiquitin ligase inducible degrader of the LDL receptor/myosin light chain interacting protein in health and disease

PURPOSE OF REVIEW

The RING E3 ubiquitin ligase inducible degrader of the LDL receptor (IDOL, also known as MYLIP) promotes ubiquitylation and subsequent lysosomal degradation of the LDL receptor (LDLR), thus acting to limit uptake of lipoprotein-derived cholesterol into cells. Next to the LDLR, IDOL also promotes degradation of two related receptors, the very LDL receptor (VLDLR) and apolipoprotein E receptor 2 (APOER2), which have important signaling functions in the brain. We review here the emerging role of IDOL in lipoprotein and energy metabolism, neurodegenerative diseases, and the potential for therapeutic targeting of IDOL.

RECENT FINDINGS

Genetic studies suggest an association between IDOL and lipoprotein metabolism in humans. Studies in rodents and nonhuman primates support an in-vivo role for IDOL in lipoprotein metabolism, and also uncovered an unexpected role in whole-body energy metabolism. Recent evaluation of IDOL function in the brain revealed a role in memory formation and progression of Alzheimer's disease. The report of the first IDOL inhibitor may facilitate further investigations on therapeutic strategies to target IDOL.

SUMMARY

IDOL is emerging as an important determinant of lipid and energy metabolism in metabolic disease as well as in Alzheimer's disease. IDOL targeting may be beneficial in treating these conditions.

Experimental models of hyperlipoproteinemia and atherosclerosis

The first experimental model of atherosclerosis (in rabbits) is more than hundred years old. Several animal species have been used to produce hyperlipoproteinemia and possible atherosclerosis. The gene manipulation produced the most used models recently. This review acknowledges the extensive study of atherosclerotic changes in experimental models of hyperlipoproteinemia and atherosclerosis to come to light thus far and the purpose here is not only to summarize the published data but also to try to add some details of our experience in using these models. In addition to rabbit (the old but also improved model by reno-vascular hypertension) dog, birds, pig, hamster, mice, rat and non-human primate's animal models are described. The gene manipulation produced the most used models two decades ago. Germline genetically engineered (without apoE or LDL receptor genes) animals have become the most used models producing atherosclerotic changes in the aorta. Recent new models also producing atherosclerotic changes but without germline genetic manipulation are also described.

Drugs for hypercholesterolaemia - from statins to pro-protein convertase subtilisin kexin 9 (PCSK9) inhibition

Cardiovascular disease (CVD) remains one of the commonest sources of morbidity and mortality in the world. Lipids and especially low density lipoprotein cholesterol (LDL-C) contribute to the risk of CVD events. Statins are the primary therapy for hypercholesterolaemia and recent evidence supports the use of ezetimibe as a second-line agent. Pro-protein convertase subtilisin kexin 9 (PCSK9) is a regulator of LDL receptor expression. Activating mutations in PCSK9 give rise to a form of familial hypercholesterolaemia, while inactivating mutations lead to lower LDL-C levels and fewer CVD events. Therapies to inhibit PCSK9 are in development and two antibody-based therapies - alirocumab and evolocumab - have recently been licensed. This article reviews the actions of PCSK9, the novel therapeutics targeted on this molecule and how they are likely to be used in clinical practice until large scale CVD outcome studies with PCSK9 inhibitors are published.

Human PCSK9 promotes hepatic lipogenesis and atherosclerosis development via apoE- and LDLR-mediated mechanisms

AIMS

Proprotein convertase subtilisin/kexin type 9 (PCSK9) promotes the degradation of hepatic low-density lipoprotein (LDL) receptors (LDLR), thereby, decreasing hepatocyte LDL-cholesterol (LDL-C) uptake. However, it is unknown whether PCSK9 has effects on atherogenesis that are independent of lipid changes. The present study investigated the effect of human (h) PCSK9 on plasma lipids, hepatic lipogenesis, and atherosclerotic lesion size and composition in transgenic mice expressing hPCSK9 (hPCSK9tg) on wild-type (WT), LDLR⁻/⁻, or apoE⁻/⁻ background.

METHODS AND RESULTS

hPCSK9 expression significantly increased plasma cholesterol (+91%), triglycerides (+18%), and apoB (+57%) levels only in WT mice. The increase in plasma lipids was a consequence of both decreased hepatic LDLR and increased hepatic lipid production, mediated transcriptionally and post-transcriptionally by PCSK9 and dependent on both LDLR and apoE. Despite the lack of changes in plasma lipids in mice expressing hPCSK9 and lacking LDLR (the main target for PCSK9) or apoE (a canonical ligand for the LDLR), hPCSK9 expression increased aortic lesion size in the absence of apoE (268 655 ± 97 972 µm² in hPCSK9tg/apoE⁻/⁻ vs. 189 423 ± 65 700 µm(2) in apoE⁻/⁻) but not in the absence of LDLR. Additionally, hPCSK9 accumulated in the atheroma and increased lesion Ly6C(hi) monocytes (by 21%) in apoE⁻/⁻ mice, but not in LDLR⁻/⁻ mice.

CONCLUSIONS

PCSK9 increases hepatic lipid and lipoprotein production via apoE- and LDLR-dependent mechanisms. However, hPCSK9 also accumulate in the artery wall and directly affects atherosclerosis lesion size and composition independently of such plasma lipid and lipoprotein changes. These effects of hPCSK9 are dependent on LDLR but are independent of apoE.

IDOL, inducible degrader of low-density lipoprotein receptor, serves as a potential therapeutic target for dyslipidemia

Low-density lipoprotein cholesterol (LDL-C) is the hall marker for the atherosclerotic cardiovascular disease (ASCVD). It has been shown that over 70% of circulating LDL-C is metabolized through binding and activation of hepatic LDL receptor (LDLR). Genetic LDLR mutations cause hypercholesterolemia in the patients. Therefore, elevation of LDLR levels is beneficial for the treatment of dyslipidemia. LDLR expression is regulated by the SREBP2/PCSK9 pathways. Targeting SREBP2/PCSK9 pathways by statins and human monoclonal PCSK9 antibody has been shown to reduce the progression of ASVCD. Recent studies identified that inducible degrader of LDLR (IDOL) is a novel regulator of LDLR. IDOL is an E3-ubiquitin ligase regulated via liver X receptors (LXRs) binding to the upstream of translation start site of IDOL. IDOL modulates LDLR distribution through ubiquitination and degradation of LDLR in lysosomes. Genome-wide association studies (GWAS) have revealed that the nonsynonymous substitution rs9370867 of IDOL probably contributes to the variability of circulating LDL levels. Recently studies also demonstrated that IDOL influences PCSK9 expression in a LDLR/SREBP2-dependent manner. Based upon these novel findings, we hypothesize that IDOL and PCSK9 would have a synergistic effect on LDLR distribution. Specifically, loss of IDOL increases LDLR distribution in the hepatic cell, and subsequently reduces serum LDL-C levels in dyslipidemic patients. IDOL might be a potential therapeutic target for the treatment of ASCVD.

A MARCH6 and IDOL E3 Ubiquitin Ligase Circuit Uncouples Cholesterol Synthesis from Lipoprotein Uptake in Hepatocytes

Cholesterol synthesis and lipoprotein uptake are tightly coordinated to ensure that the cellular level of cholesterol is adequately maintained. Hepatic dysregulation of these processes is associated with pathological conditions, most notably cardiovascular disease. Using a genetic approach, we have recently identified the E3 ubiquitin ligase MARCH6 as a regulator of cholesterol biosynthesis, owing to its ability to promote degradation of the rate-limiting enzymes 3-hydroxy-3-methyl-glutaryl coenzyme A reductase (HMGCR) and squalene epoxidase (SQLE). Here, we present evidence for MARCH6 playing a multifaceted role in the control of cholesterol homeostasis in hepatocytes. We identify MARCH6 as an endogenous inhibitor of the sterol regulatory element binding protein (SREBP) transcriptional program. Accordingly, loss of MARCH6 increases expression of SREBP-regulated genes involved in cholesterol biosynthesis and lipoprotein uptake. Unexpectedly, this is associated with a decrease in cellular lipoprotein uptake, induced by enhanced lysosomal degradation of the low-density lipoprotein receptor (LDLR). Finally, we provide evidence that induction of the E3 ubiquitin ligase IDOL represents the molecular mechanism underlying this MARCH6-induced phenotype. Our study thus highlights a MARCH6-dependent mechanism to direct cellular cholesterol accretion that relies on uncoupling of cholesterol synthesis from lipoprotein uptake.

β-Estradiol results in a proprotein convertase subtilisin/kexin type 9-dependent increase in low-density lipoprotein receptor levels in human hepatic HuH7 cells

The lower risk of coronary artery disease in premenopausal women than in men and postmenopausal women implicates sex steroids in cardioprotective processes. β-Estradiol upregulates liver low-density lipoprotein receptor (LDLR), which, in turn, decreases circulating levels of low-density lipoprotein, which is a risk factor for coronary artery disease. Conversely, LDLR protein is negatively regulated by proprotein convertase subtilisin/kexin type 9 (PCSK9). Herein, we investigated PCSK9 regulation by β-estradiol and its impact on LDLR in human hepatocarcinoma HuH7 cells grown in the presence or absence of β-estradiol. Immunoblot analysis showed upregulation of LDLR at 3 μm β-estradiol (140%), and the upregulation reached 220% at 10 μm β-estradiol; only at the latter dose was an increase in LDLR mRNA detected by qPCR, suggesting post-translational regulation of LDLR. No changes in PCSK9 mRNA or secreted protein levels were detected by qPCR or ELISA, respectively. β-estradiol-conditioned medium devoid of PCSK9 failed to upregulate LDLR. Similarly, PCSK9 knockdown cells showed no upregulation of LDLR by β-estradiol. Together, these results indicate a requirement for PCSK9 in the β-estradiol-induced upregulation of LDLR. A radiolabeling assay showed a significant, dose-dependent decrease in the ratio of secreted phosphoPCSK9 to total secreted PCSK9 with increasing β-estradiol levels, suggesting a change in the functional state of PCSK9 in the presence of β-estradiol. Our results indicate that the protein upregulation of LDLR at subtranscriptionally effective doses of β-estradiol, and its supratranscriptional upregulation at 10 μm β-estradiol, occur through an extracellular PCSK9-dependent mechanism.

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.

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.

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.

The UPS and downs of cholesterol homeostasis

An emerging theme in the regulation of cholesterol homeostasis is the role of the ubiquitin proteasome system (UPS), through which proteins are ubiquitylated and then degraded in response to specific signals. The UPS controls all aspects of cholesterol metabolism including its synthesis, uptake, and efflux. We review here recent work uncovering the ubiquitylation and degradation of key players in cholesterol homeostasis. This includes the low-density lipoprotein (LDL) receptor, transcription factors (sterol regulatory element binding proteins and liver X receptors), flux-controlling enzymes in cholesterol synthesis (3-hydroxy-3-methylglutaryl-CoA reductase and squalene monooxygenase), and cholesterol exporters (ATP-binding cassette transporters ABCA1 and ABCG1). We explore which E3 ligases are involved, and identify areas deserving of further research.