Emerging therapies for refractory hypercholesterolemia: a narrative review.

Refractory hypercholesterolemia (RH) is characterized by the failure of patients to achieve therapeutic targets for low-density lipoprotein-cholesterol (LDL-C) despite receiving maximal tolerable doses of standard lipid-lowering treatments. It predominantly impacts individuals with familial hypercholesterolemia (FH), thereby elevating the risk of cardiovascular complications. The prevalence of RH is now recognized to be substantially greater than previously thought. This review provides a comprehensive insight into current and emerging therapies for RH patients, including groundbreaking genetic-based therapeutic approaches. The review places emphasis on the dependency of therapies on low-density lipoprotein receptors (LDLRs) and highlights the critical role of considering LDLR activity in RH patients for individualization of the treatment.

Refractory hypercholesterolemia (RH) is a condition where patients are unable to get below target levels of 'bad' cholesterol despite receiving maximum doses of standard treatments. It is commonly present in those with a genetic disorder, called familial hypercholesterolemia (FH), known to increase the risk of heart complications. RH's prevalence is now understood to be higher than previously believed and this review offers insights into current and emerging therapies for RH, including genetic-based treatments. It stresses the importance of the mechanistic pathways behind cholesterol clearance, particularly low-density lipoprotein receptor (LDLR) activity, in RH treatment customization.

Comparison of the burden of familial hypercholesterolemia between two cohorts of French Canadians hospitalized 25 years apart for coronary heart disease.

BACKGROUND: Familial hypercholesterolemia (FH) is associated with lifelong elevated plasma concentrations of low-density lipoprotein cholesterol (LDL-C) and high risk of premature coronary heart disease (CHD). Clinical recommendations and treatments have emerged to facilitate the management of FH patients. Their impact on the burden of FH is however not well documented. OBJECTIVE: To compare the burden of FH between patients hospitalized for a CHD event 25 years apart in the French-Canadian founder population. METHODS: Lipid profiles, cardiovascular risk factors, treatments and FH status of 2,029 patients consecutively hospitalized for an acute CHD event between 2017 and 2022 (2022 Cohort) were compared to those of 2,506 patients with angiographically-confirmed CHD who were admitted between 1995 and 1998 (1998 Cohort). RESULTS: At the time of admission, 24.6 % of CHD patients had LDL-C levels >5.0 mmol/L in 1998 compared to 1.4 % in 2022, and FH was diagnosed in 9.6 % of patients in the 1998 cohort compared to 5.5 % in 2022 (p<0.001). FH patients hospitalized for a CHD event were older in 2022 than in 1998 (p <0.001). The prevalence of premature CHD requiring a hospitalization significantly decreased from 1998 to 2022 (64.3% vs. 44.1 %, p<0.001). At the moment of admission, 18.2 % of FH patients had LDL-C concentration <2.0 mmol/L in 2022 vs 0 % in 1998 (p <0.001). CONCLUSIONS: Over 25 years, FH patients tend to be older and contribute to a lower proportion of hospitalizations for CHD in the French-Canadian founder population. Despite significant improvement in diagnosis and treatment, FH management remains however sub-optimal.

Inhibition of Angiopoietin-Like Protein 3 or 3/8 Complex and ApoC-III in Severe Hypertriglyceridemia.

PURPOSE OF REVIEW: The role of the inhibition of ANGPTL3 in severe or refractory hypercholesterolemia is well documented, less in severe hyperTG. This review focuses on the preclinical and clinical development of ApoC-III inhibitors and ANGPTL3, 4, and 3/8 complex inhibitors for the treatment of severe or refractory forms of hypertriglyceridemia to prevent cardiovascular disease or other morbidities. RECENT FINDINGS: APOC3 and ANGPTL3 became targets for drug development following the identification of naturally occurring loss of function variants in families with a favorable lipid profile and low cardiovascular risk. The inhibition of ANGPTL3 covers a broad spectrum of lipid disorders from severe hypercholesterolemia to severe hypertriglyceridemia, while the inhibition of ApoC-III can treat hypertriglyceridemia regardless of the severity. Preclinical and clinical data suggest that ApoC-III inhibitors, ANGPTL3 inhibitors, and inhibitors of the ANGPTL3/8 complex that is formed postprandially are highly effective for the treatment of severe or refractory hypertriglyceridemia. Inhibition of ANGPTL3 or the ANGPTL3/8 complex upregulates LPL and facilitates the hydrolysis and clearance of triglyceride-rich lipoproteins (TRL) (LPL-dependent mechanisms), whereas ApoC-III inhibitors contribute to the management and clearance of TRL through both LPL-dependent and LPL-independent mechanisms making it possible to successfully lower TG in subjects completely lacking LPL (familial chylomicronemia syndrome). Most of these agents are biologicals including monoclonal antibodies (mAb), antisense nucleotides (ASO), small interfering RNA (siRNA), or CRISPR-cas gene editing strategies.

Atherosclerotic plaque regression in homozygous familial hypercholesterolaemia: a case report of a long-term lipid-lowering therapy involving LDL-receptor-independent mechanisms.

Background: Homozygous familial hypercholesterolaemia (HoFH) is a rare and life-threatening genetic disease characterized by extremely elevated low-density lipoprotein cholesterol (LDL-C) levels, important xanthomatosis and increased risk of premature atherosclerotic cardiovascular disease. Management of HoFH at an early stage is recommended but conventional lipid-lowering therapies (LLTs) dependent on the LDL-receptor for clearance of LDL particles, are usually not sufficient. However, agents acting independently of the LDL-receptor, such as inhibitors of microsomal triglyceride transfer protein (MTP) or angiopoietin-like protein 3 (ANGPTL3), administered in combination, on top of standard-of-care LLT constitute a promising therapy for HoFH. Case summary: The present case describes a long-term (>10 years) follow-up of a 52-year-old woman with severe HoFH, who was treated with conventional lipid-lowering medications (i.e. statins and ezetimibe) for several years before experiencing the risks and benefits that were encountered with the use of LDL-receptor-independent agents (MTP and ANGPTL3 inhibitors). This combination therapy demonstrated a good long-term safety and efficacy profile, while continuous monitoring of hepatic enzymes (sometimes requiring dose adjustments) and fat accumulation is recommended when using lomitapide. Discussion: Treating this HoFH patient with an LLT involving the combination of MTP and ANGPTL3 LDL-receptor-independent inhibitors (lomitapide and evinacumab, respectively) showed remarkable improvement in LDL-C levels, disappearance of xanthomatosis and regression in atherosclerotic plaques. In addition to safety and efficacy, one should question the affordability and access hurdle that emerging combination of expensive therapies might constitute in the future for the payers. These challenges could eventually limit the clinical use of those innovative treatments despite their clinical benefit.

Correlation between chylomicronemia diagnosis scores and post-heparin lipoprotein lipase activity.

INTRODUCTION: Sustained chylomicronemia is a defect in post-prandial triglyceride management characterized by severe hypertriglyceridemia (triglyceride > 10 mmol/L) due to functional or genetic defects in lipoprotein lipase (LPL)-mediated triglyceride-rich lipoprotein lipolysis. Familial chylomicronemia syndrome (FCS) is a rare mendelian form of chylomicronemia caused by loss-of-function variants in LPL or LPL-related genes. Most individuals with chylomicronemia however present multifactorial chylomicronemia (MCS), in which LPL bio-availability and activity are variable. FCS and MCS differ in terms of clinical characteristics and risk of disease, and diagnosis scoring systems have been proposed to accurately distinguish FCS from MCS. OBJECTIVE: The aim of this study was to assess the strength of the relationship between plasma post-heparin LPL activity and two published chylomicronemia diagnosis scoring systems. DESIGN AND METHODS: Post-heparin plasma LPL activity was measured using colorimetric assays in a sample of 29 subjects with sustained chylomicronemia (20 FCS and 9 MCS). Chylomicronemia diagnosis scores were obtained for all subjects using the scoring system A (model A), which integrates apolipoprotein B and free glycerol, a surrogate marker of triglyceride hydrolysis, and the scoring system B (model B). Correlation analyses were conducted to estimate the linear relationship between LPL activity and the two diagnosis scoring systems. RESULTS: There was a significant (p < 0.001) difference in post-heparin LPL activity between FCS and MCS. Both scoring systems significantly correlated with post-heparin LPL activity (model A: rs = -0.64, p < 0.001; model B: rs = -0.54, p = 0.002). CONCLUSIONS: These result suggest that chylomicronemia diagnosis scoring systems correlate with LPL activity and adequately contribute to distinguish FCS from MCS.

Lessons learned from the evinacumab trials in the treatment of homozygous familial hypercholesterolemia.

Homozygous familial hypercholesterolemia (HoFH) is a life-threatening disease characterized by extremely elevated LDL cholesterol (LDL-C) levels which result in premature atherosclerotic cardiovascular disease. As conventional lipid-lowering therapies, which mainly depend on LDL receptors for LDL particle clearance, remain insufficient for reaching the recommended LDL-C levels in HoFH, agents acting independently of LDL receptors, such as ANGPTL3 inhibitors, constitute a promising target. Evinacumab, a monoclonal antibody directed against ANGPTL3, was approved in the USA in 2021 for treating patients with HoFH. Evinacumab has shown an adequate safety profile with strong LDL-lowering efficacy. This review highlights the development path of evinacumab and provides insight on the lessons learned from trials as well as the hurdles facing accessibility.

Homozygous familial hypercholesterolemia (HoFH) is a life-threatening rare genetic disorder characterized by extremely elevated 'bad' cholesterol and resulting in heart disease at a young age. As the current treatments that tend to lower the levels of 'bad' cholesterol remain insufficient for treating patients with this disease, emerging agents, such as inhibitors of the protein ANGPTL3, follow different biological pathways than the ones targeted by the traditional therapies and constitute a promising target for HoFH. Evinacumab, which is highly selective for ANGPTL3 inhibition, was approved in the USA in 2021 for treating patients with HoFH. Evinacumab has been shown to be safe and well tolerated, with a strong effect on decreasing 'bad' cholesterol levels. This review highlights the development path of evinacumab and provides insight into the lessons learned from trials as well as the hurdles facing accessibility.

Influence of the LDL-Receptor Genotype on Statin Response in Heterozygous Familial Hypercholesterolemia: Insights From the Canadian FH Registry.

BACKGROUND: Whether low-density lipoprotein (LDL) receptor (LDLR) residual activity influences the LDL-lowering effect of statins in heterozygous familial hypercholesterolemia (HeFH) remains unclear. The objective of this study was to investigate the relationship between the LDLR genotype and statin-induced LDL cholesterol (LDL-C) reductions in HeFH. METHODS: A total of 615 individuals with HeFH (receptor-defective [RD] genotype: n = 226; receptor-negative [RN] genotype: n = 389) from 7 lipid clinics across Canada who initiated statin monotherapy were included in this retrospective longitudinal study. Statin-induced reductions in LDL-C among individuals with RD and RN genotypes were compared with the use of linear models. RESULTS: There were 334 women and 281 men with a mean untreated LDL-C concentrations of 6.97 ± 1.65 mmol/L. Untreated and on-statin LDL-C levels where higher among patients with an RN genotype: untreated: RN 7.24 (95% confidence interval [CI] 6.98-7.50) mmol/L vs RD 6.70 (95% CI 6.41-6.98) mmol/L (P = 0.0002); on-statin: RN 4.50 (95% CI 4.31-4.70) vs RD 4.05 (95% CI 3.84-4.26) mmol/L (P = 0.0004). After adjustments for age, sex, smoking status, untreated LDL-C concentrations, statin type and dose, as well as the clinic where the patients were treated, the LDL-C-lowering effect of statins was significantly weaker for individuals with an RN mutation than for individuals with an RD mutation: RN: -31.1% (95% CI -34.7% to -27.4) vs RD -36.5% (95% CI -40.4% to -32.6%); P < 0.0001. The LDLR genotype was the strongest nonmodifiable independent correlate of statin-induced LDL-C reductions (R2 = 2.3%; P = 0.0001). CONCLUSION: The LDLR genotype is significantly associated with statin-induced reductions in LDL-C concentrations in HeFH.

Discovery of a dual Ras and ARF6 inhibitor from a GPCR endocytosis screen.

Internalization and intracellular trafficking of G protein-coupled receptors (GPCRs) play pivotal roles in cell responsiveness. Dysregulation in receptor trafficking can lead to aberrant signaling and cell behavior. Here, using an endosomal BRET-based assay in a high-throughput screen with the prototypical GPCR angiotensin II type 1 receptor (AT1R), we sought to identify receptor trafficking inhibitors from a library of ~115,000 small molecules. We identified a novel dual Ras and ARF6 inhibitor, which we named Rasarfin, that blocks agonist-mediated internalization of AT1R and other GPCRs. Rasarfin also potently inhibits agonist-induced ERK1/2 signaling by GPCRs, and MAPK and Akt signaling by EGFR, as well as prevents cancer cell proliferation. In silico modeling and in vitro studies reveal a unique binding modality of Rasarfin within the SOS-binding domain of Ras. Our findings unveil a class of dual small G protein inhibitors for receptor trafficking and signaling, useful for the inhibition of oncogenic cellular responses.

Identifying Markers of Cardiovascular Event-Free Survival in Familial Hypercholesterolemia.

Familial hypercholesterolemia (FH) is an autosomal dominant trait characterized by elevated low-density lipoprotein-cholesterol (LDL-C) concentrations appearing at birth and is associated with increased risk of premature atherosclerotic cardiovascular disease (CVD). However, in some cases, FH subjects over 70 years of age have surprisingly never experienced any CVD symptoms throughout their entire lives. The objective of this study consists of identifying biological and environmental markers acting as cardioprotective factors and associated with unexpected survival in FH. Upon age and reported cardiovascular events (CVE) stratification, we identified a total of 458 French-Canadian FH subjects with premature reported CVE, and 1297 young adults as well as 24 elderly subjects (≥70 years) who have never reported CVE requiring hospitalization. Logistic regression models were used to depict cardioprotective markers among FH survivors (≥70 years). Regression analyses of the FH cohort showed that female sex (odds ratio (OR) = 12.92 (4.23-39.46); p < 0.0001), high levels of high-density lipoprotein (HDL)-C (OR = 6.76 (2.43-18.79); p = 0.0002) and elevated concentrations of adiponectin (OR = 71.40 (5.20-980.47); p = 0.001) were significant contributory factors in reducing FH-related CVD risk. Notably, female (OR = 11.45 (1.25-105.98); p = 0.031) and high HDL-C (OR = 9.78 (1.75-54.67); p = 0.009) were shown to be significant covariates associated with survival in FH. Non-smoking (OR = 11.73 (4.36-31.56); p < 0.0001) was also identified as an environmental factor associated with CVE-free survival. Based on this configured model of premature CVE occurrence, these results demonstrated that, beyond LDL-C levels, female sex, high HDL-C, elevated adiponectin and non-smoking are important markers that contribute to a reduced risk of CVD and CVE-free survival in FH.

Lomitapide for treatment of homozygous familial hypercholesterolemia: The Québec experience.

BACKGROUND AND AIMS: Homozygous familial hypercholesterolemia (HoFH) is an orphan disease, most often caused by bi-allelic mutations of the LDLR gene. Patients with HoFH have elevated LDL-C levels >13 mmol/L, tendinous xanthomata and severe, premature atherosclerotic cardiovascular disease (ASCVD). Untreated, most HoFH patients die of ASCVD in youth. New therapeutic modalities include lomitapide, an inhibitor of microsomal triglyceride transfer protein that lowers hepatic LDL-C production. We have recently identified 79 Canadian patients with HoFH. Here, we describe our experience with lomitapide in the province of Quebec, a geographic area known to have a high prevalence of HoFH. METHODS: This is a retrospective case series of 12 HoFH patients followed at three lipidology centers in the province of Quebec. RESULTS: Mean age of the patients was 44 ± 18 years; age at time of HoFH diagnosis ranged from 2 to 59 years. All patients were on a statin and ezetimibe 10 mg/day and five patients were treated with LDL apheresis. Treatment with lomitapide reduced LDL-C levels by 38% (intention-to-treat). Intolerable gastrointestinal side effects were observed in 3/12 patients and were the main reason for treatment discontinuation. Three patients tolerated lomitapide at doses ranging between 5 and 30 mg/day without major side effects. Downwards drug titration was necessary in the 6 remaining patients because of gastrointestinal side effects (n = 5) and elevated liver enzymes (n = 1), and 2 of them finally discontinued treatment. CONCLUSIONS: Lomitapide may be used to further decrease LDL-C in HoFH patients; gastrointestinal side effects and hepatic toxicity may limit adherence.

Dissection of Clinical and Gene Expression Signatures of Familial versus Multifactorial Chylomicronemia.

Familial chylomicronemia syndrome (FCS) is a rare disorder associated with chylomicronemia (CM) and an increased risk of pancreatitis. Most individuals with CM do not have FCS but exhibit multifactorial CM (MCM), which differs from FCS in terms of risk and disease management. This study aimed to investigate clinical and gene expression profiles of FCS and MCM patients. Anthropometrics, clinical, and biochemical variables were analyzed in 57 FCS and 353 MCM patients. Gene expression analyses were performed in a subsample of 19 FCS, 28 MCM, and 15 normolipidemic controls. Receiver operating characteristic (ROC) curve analyses were performed to analyze the capacity of variables to discriminate FCS from MCM. Sustained fasting triglycerides ≥20 mmol/L (>15 mmol/L with eruptive xanthomas), history of pancreatitis, poor response to fibrates, diagnosis of CM at childhood, body mass index <22 kg/m2, and delipidated apolipoprotein B or glycerol levels <0.9 g/L and <0.05 mmol/L, respectively, had an area under the ROC curve ≥0.7. Gene expression analyses identified 142 probes differentially expressed in FCS and 32 in MCM compared with controls. Among them, 13 probes are shared between FCS and MCM; 63 are specific to FCS and 2 to MCM. Most FCS-specific or shared biomarkers are involved in inflammatory, immune, circadian, postprandial metabolism, signaling, docking systems, or receptor-mediated clearance mechanisms. This study reveals differential signatures of FCS and MCM. It opens the door to the identification of key mechanisms of CM expression and potential targets for the development of new treatments.

Preclinical discovery and development of evolocumab for the treatment of hypercholesterolemia.

Introduction: Proprotein convertase subtilisin/kexin type 9 (PCSK9) is a serine protease that binds and promotes the lysosomal degradation of the low-density lipoprotein receptors (LDLR). Upon its discovery in 2002, PCSK9 inhibition has subsequently emerged as a novel target for lowering LDL-cholesterol (LDL-C) and reducing coronary heart disease. Evolocumab, a monoclonal antibody directed against human PCSK9, was approved in 2015 as an adjunct to lipid-lowering therapy for treating patients with familial hypercholesterolemia (FH) or patients with high cardiovascular risk, who are treated with maximally tolerated lipid-lowering agents and have not reached the recommended LDL-C levels.Areas covered: The authors illustrate the rapid pace of the drug development process that monoclonal antibodies, including evolocumab, have demonstrated during the last decade. In less than 15 years from its discovery, this lipid-lowering target has successfully progressed from bench-side to clinical practice and has been recently approved to reduce cardiovascular events in patients with established atherosclerotic cardiovascular disease (ASCVD).Expert opinion: Evolocumab has demonstrated a good safety profile and robust efficacy in terms of its lipid-lowering effect and ASCVD risk reduction, yet affordability, accessibility, and cost-effectiveness of PCSK9 monoclonal antibodies remain a hurdle in the 'real-world' setting. These challenges facing the upcoming generation of precision medicine therapies must be addressed upfront.

Pharmacogenomics of statin-related myopathy: Meta-analysis of rare variants from whole-exome sequencing.

AIMS: Statin-related myopathy (SRM), which includes rhabdomyolysis, is an uncommon but important adverse drug reaction because the number of people prescribed statins world-wide is large. Previous association studies of common genetic variants have had limited success in identifying a genetic basis for this adverse drug reaction. We conducted a multi-site whole-exome sequencing study to investigate whether rare coding variants confer an increased risk of SRM. METHODS AND RESULTS: SRM 3-5 cases (N = 505) and statin treatment-tolerant controls (N = 2047) were recruited from multiple sites in North America and Europe. SRM 3-5 was defined as symptoms consistent with muscle injury and an elevated creatine phosphokinase level >4 times upper limit of normal without another likely cause of muscle injury. Whole-exome sequencing and variant calling was coordinated from two analysis centres, and results of single-variant and gene-based burden tests were meta-analysed. No genome-wide significant associations were identified. Given the large number of cases, we had 80% power to identify a variant with minor allele frequency of 0.01 that increases the risk of SRM 6-fold at genome-wide significance. CONCLUSIONS: In this large whole-exome sequencing study of severe statin-related muscle injury conducted to date, we did not find evidence that rare coding variants are responsible for this adverse drug reaction. Larger sample sizes would be required to identify rare variants with small effects, but it is unclear whether such findings would be clinically actionable.

Review of the long-term safety of lomitapide: a microsomal triglycerides transfer protein inhibitor for treating homozygous familial hypercholesterolemia.

INTRODUCTION: Homozygous familial hypercholesterolemia (HoFH) is a rare and life-threatening lipid disorder characterized by extremely elevated low-density lipoprotein-cholesterol (LDL-C) concentrations and premature atherosclerotic cardiovascular disease (CVD). Conventional lipid-lowering agents remain insufficient in managing this disease, which emphasize the unmet medical need for potential therapies capable of lowering LDL-C and decreasing CVD risk in this patient population. AREAS COVERED: Novel LDL receptor (LDLR) independent drugs have been recently approved or are in development for the treatment of HoFH, including lomitapide (Juxtapid®). This oral microsomal triglyceride transfer protein (MTP) inhibitor was approved in 2012 in several countries as an adjunct to a low-fat diet and other lipid-lowering drugs with or without LDL apheresis to treat patients with HoFH. This review summarizes key safety and efficacy data of lomitapide from clinical trials and 'real-life' experience. EXPERT OPINION: While lomitapide is an interesting therapy for treating HoFH, long-term safety, as well as cardiovascular outcome data, are yet to be provided. Precision medicine has recently contributed to the development of several agents designed to address the unmet medical need of HoFH. However, combining safety, efficacy, accessibility, and affordability in a single therapy constitutes very challenging individual and societal paradigms in HoFH treatment.

Familial hypercholesterolemia in Canada: Initial results from the FH Canada national registry.

BACKGROUND AND AIMS: Familial hypercholesterolemia (FH) is under-diagnosed and under-treated in most of the world, including Canada. National registries play a key role in identifying patients with FH, understanding gaps in care, and advancing the science of FH to better treat these patients. METHODS: FH Canada has established a national registry across 19 academic sites acting as "hubs" in Canada to increase awareness and access to standard-of-care therapies. RESULTS: To-date, more than 3000 patients with FH have been entered into a secure, web-based database. Early outcomes of this initiative include a greater understanding of treatment gaps for patients with FH in Canada, the development of a new, simplified Canadian definition of FH, and tools to aid in the diagnosis of FH, including imputation of baseline levels of LDL cholesterol. CONCLUSIONS: As the national registry expands in size and scope, further learning will emerge with ultimate benefit for the diagnosis and treatment of FH in Canada.

Cardiovascular disease in familial hypercholesterolemia: Validation and refinement of the Montreal-FH-SCORE.

BACKGROUND: Familial hypercholesterolemia (FH) is a disease characterized by increased low-density lipoprotein cholesterol and premature cardiovascular disease (CVD) but there is marked individuality in the occurrence of CVD events. Recently, the Montreal-FH-SCORE (MFHS) has been shown to stratify CVD frequency in FH subjects, but this score has not yet been validated. OBJECTIVE: The aims of the present study were to conduct an independent external validation of the MFHS in a retrospective cohort of heterozygous FH and to identify additional variables that could significantly improve the prediction of prevalent CVD. METHODS: The MFHS calculation is based on 5 variables: age, high-density lipoprotein cholesterol, gender, hypertension, and smoking status. This score was validated in a cohort of 718 adult FH using receiver operating characteristic (ROC) curves analysis to determine the discriminatory ability of the MFHS. The performance of the MFHS was compared to a novel Combined-FH-SCORE in 1388 FH. RESULTS: The MFHS had an excellent discrimination for prevalent CVD events in the validation cohort, with an area under the receiver operating characteristic curve of 0.799 (0.766-0.832, P < .0001). Patients with a high MFHS score presented a significant 8.8-fold increased odd of CVD events compared with patients with a low score (95% confidence interval 5.8-13.3, P < .0001). The addition of lipoprotein(a) to the score did not improve the prediction of CVD events (area under the receiver operating characteristic curve 0.823 vs 0.817, P = .11). CONCLUSION: This study confirmed that the MFHS is a strong predictor of prevalent CVD in FH and that the addition of lipoprotein(a) offers a minor improvement in the discrimination of the score.

A new inhibitor of the ß-arrestin/AP2 endocytic complex reveals interplay between GPCR internalization and signalling.

In addition to G protein-coupled receptor (GPCR) desensitization and endocytosis, ß-arrestin recruitment to ligand-stimulated GPCRs promotes non-canonical signalling cascades. Distinguishing the respective contributions of ß-arrestin recruitment to the receptor and ß-arrestin-promoted endocytosis in propagating receptor signalling has been limited by the lack of selective analytical tools. Here, using a combination of virtual screening and cell-based assays, we have identified a small molecule that selectively inhibits the interaction between ß-arrestin and the ß2-adaptin subunit of the clathrin adaptor protein AP2 without interfering with the formation of receptor/ß-arrestin complexes. This selective ß-arrestin/ß2-adaptin inhibitor (Barbadin) blocks agonist-promoted endocytosis of the prototypical ß2-adrenergic (ß2AR), V2-vasopressin (V2R) and angiotensin-II type-1 (AT1R) receptors, but does not affect ß-arrestin-independent (transferrin) or AP2-independent (endothelin-A) receptor internalization. Interestingly, Barbadin fully blocks V2R-stimulated ERK1/2 activation and blunts cAMP accumulation promoted by both V2R and ß2AR, supporting the concept of ß-arrestin/AP2-dependent signalling for both G protein-dependent and -independent pathways.

Monitoring G protein-coupled receptor and ß-arrestin trafficking in live cells using enhanced bystander BRET.

Endocytosis and intracellular trafficking of receptors are pivotal to maintain physiological functions and drug action; however, robust quantitative approaches are lacking to study such processes in live cells. Here we present new bioluminescence resonance energy transfer (BRET) sensors to quantitatively monitor G protein-coupled receptors (GPCRs) and ß-arrestin trafficking. These sensors are based on bystander BRET and use the naturally interacting chromophores luciferase (RLuc) and green fluorescent protein (rGFP) from Renilla. The versatility and robustness of this approach are exemplified by anchoring rGFP at the plasma membrane or in endosomes to generate high dynamic spectrometric BRET signals on ligand-promoted recruitment or sequestration of RLuc-tagged proteins to, or from, specific cell compartments, as well as sensitive subcellular BRET imaging for protein translocation visualization. These sensors are scalable to high-throughput formats and allow quantitative pharmacological studies of GPCR trafficking in real time, in live cells, revealing ligand-dependent biased trafficking of receptor/ß-arrestin complexes.