Hepatic Glucagon Signaling Regulates PCSK9 and Low-Density Lipoprotein Cholesterol

Hepatic glucagon action: beyond glucose mobilization

Glucagon's ability to promote hepatic glucose production has been known for over a century, with initial observations touting this hormone as a diabetogenic agent. However, glucagon receptor agonism [when balanced with an incretin, including glucagon-like peptide 1 (GLP-1) to dampen glucose excursions] is now being developed as a promising therapeutic target in the treatment of metabolic diseases, like metabolic dysfunction-associated steatotic disease/metabolic dysfunction-associated steatohepatitis (MASLD/MASH), and may also have benefit for obesity and chronic kidney disease. Conventionally regarded as the opposing tag-team partner of the anabolic mediator insulin, glucagon is gradually emerging as more than just a "catabolic hormone." Glucagon action on glucose homeostasis within the liver has been well characterized. However, growing evidence, in part thanks to new and sensitive "omics" technologies, has implicated glucagon as more than just a "glucose liberator." Elucidation of glucagon's capacity to increase fatty acid oxidation while attenuating endogenous lipid synthesis speaks to the dichotomous nature of the hormone. Furthermore, glucagon action is not limited to just glucose homeostasis and lipid metabolism, as traditionally reported. Glucagon plays key regulatory roles in hepatic amino acid and ketone body metabolism, as well as mitochondrial turnover and function, indicating broader glucagon signaling consequences for metabolic homeostasis mediated by the liver. Here we examine the broadening role of glucagon signaling within the hepatocyte and question the current dogma, to appreciate glucagon as more than just that "catabolic hormone."

Glucagon-based therapy for people with diabetes and obesity: What is the sweet spot?

People with obesity and type 2 diabetes have a high prevalence of metabolic-associated steatotic liver disease, hyperlipidemia and cardiovascular disease. Glucagon increases hepatic glucose production; it also decreases hepatic fat accumulation, improves lipidemia and increases energy expenditure. Pharmaceutical strategies to antagonize the glucagon receptor improve glycemic outcomes in people with diabetes and obesity, but they increase hepatic steatosis and worsen dyslipidemia. Co-agonism of the glucagon and glucagon-like peptide-1 (GLP-1) receptors has emerged as a promising strategy to improve glycemia in people with diabetes and obesity. Addition of glucagon receptor agonism enhances weight loss, reduces liver fat and ameliorates dyslipidemia. Prior to clinical use, however, further studies are needed to investigate the safety and efficacy of glucagon and GLP-1 receptor co-agonists in people with diabetes and obesity and related conditions, with specific concerns regarding a higher prevalence of gastrointestinal side effects, loss of muscle mass and increases in heart rate. Furthermore, co-agonists with differing ratios of glucagon:GLP-1 receptor activity vary in their clinical effect; the optimum balance is yet to be identified.

Glucagon resistance and metabolic-associated steatotic liver disease: a review of the evidence

Metabolic-associated steatotic liver disease (MASLD) is closely associated with obesity. MASLD affects over 1 billion adults globally but there are few treatment options available. Glucagon is a key metabolic regulator, and its actions include the reduction of liver fat through direct and indirect means. Chronic glucagon signalling deficiency is associated with hyperaminoacidaemia, hyperglucagonaemia and increased circulating levels of glucagon-like peptide 1 (GLP-1) and fibroblast growth factor 21 (FGF-21). Reduction in glucagon activity decreases hepatic amino acid and triglyceride catabolism; metabolic effects include improved glucose tolerance, increased plasma cholesterol and increased liver fat. Conversely, glucagon infusion in healthy volunteers leads to increased hepatic glucose output, decreased levels of plasma amino acids and increased urea production, decreased plasma cholesterol and increased energy expenditure. Patients with MASLD share many hormonal and metabolic characteristics with models of glucagon signalling deficiency, suggesting that they could be resistant to glucagon. Although there are few studies of the effects of glucagon infusion in patients with obesity and/or MASLD, there is some evidence that the expected effect of glucagon on amino acid catabolism may be attenuated. Taken together, this evidence supports the notion that glucagon resistance exists in patients with MASLD and may contribute to the pathogenesis of MASLD. Further studies are warranted to investigate the direct effects of glucagon on metabolism in patients with MASLD.

Adaptive infusion of a glucagon-like peptide-1/glucagon receptor co-agonist G3215, in adults with overweight or obesity: Results from a phase 1 randomized clinical trial

AIMS

To determine whether a continuous infusion of a glucagon-like peptide receptor (GLP-1R)/glucagon receptor (GCGR) co-agonist, G3215 is safe and well tolerated in adults with overweight or obesity.

METHODS

A phase 1 randomized, double blind, placebo-controlled trial of G3215 in overweight or obese participants, with or without type 2 diabetes.

RESULTS

Twenty-six participants were recruited and randomized with 23 completing a 14-day subcutaneous infusion of G3215 or placebo. The most common adverse events were nausea or vomiting, which were mild in most cases and mitigated by real-time adjustment of drug infusion. There were no cardiovascular concerns with G3215 infusion. The pharmacokinetic characteristics were in keeping with a continuous infusion over 14 days. A least-squares mean body weight loss of 2.39 kg was achieved with a 14-day infusion of G3215, compared with 0.84 kg with placebo infusion (p < .05). A reduction in food consumption was also observed in participants receiving G3215 and there was no deterioration in glycaemia. An improved lipid profile was seen in G3215-treated participants and consistent with GCGR activation, a broad reduction in circulating amino acids was seen during the infusion period.

CONCLUSION

An adaptive continuous infusion of the GLP-1/GCGR co-agonist, G3215, is safe and well tolerated offering a unique strategy to control drug exposure. By allowing rapid, response-directed titration, this strategy may allow for mitigation of adverse effects and afford significant weight loss within shorter time horizons than is presently possible with weekly GLP-1R and multi-agonists. These results support ongoing development of G3215 for the treatment of obesity and metabolic disease.

Downregulation of the kidney glucagon receptor, essential for renal function and systemic homeostasis, contributes to chronic kidney disease

The glucagon receptor (GCGR) in the kidney is expressed in nephron tubules. In humans and animal models with chronic kidney disease, renal GCGR expression is reduced. However, the role of kidney GCGR in normal renal function and in disease development has not been addressed. Here, we examined its role by analyzing mice with constitutive or conditional kidney-specific loss of the Gcgr. Adult renal Gcgr knockout mice exhibit metabolic dysregulation and a functional impairment of the kidneys. These mice exhibit hyperaminoacidemia associated with reduced kidney glucose output, oxidative stress, enhanced inflammasome activity, and excess lipid accumulation in the kidney. Upon a lipid challenge, they display maladaptive responses with acute hypertriglyceridemia and chronic proinflammatory and profibrotic activation. In aged mice, kidney Gcgr ablation elicits widespread renal deposition of collagen and fibronectin, indicative of fibrosis. Taken together, our findings demonstrate an essential role of the renal GCGR in normal kidney metabolic and homeostatic functions. Importantly, mice deficient for kidney Gcgr recapitulate some of the key pathophysiological features of chronic kidney disease.

Geranylgeranyl isoprenoids and hepatic Rap1a regulate basal and statin-induced expression of PCSK9

LDL-C lowering is the main goal of atherosclerotic cardiovascular disease prevention, and proprotein convertase subtilisin/kexin type 9 (PCSK9) inhibition is now a validated therapeutic strategy that lowers serum LDL-C and reduces coronary events. Ironically, the most widely used medicine to lower cholesterol, statins, has been shown to increase circulating PCSK9 levels, which limits their efficacy. Here, we show that geranylgeranyl isoprenoids and hepatic Rap1a regulate both basal and statin-induced expression of PCSK9 and contribute to LDL-C homeostasis. Rap1a prenylation and activity is inhibited upon statin treatment, and statin-mediated PCSK9 induction is dependent on geranylgeranyl synthesis and hepatic Rap1a. Accordingly, treatment of mice with a small-molecule activator of Rap1a lowered PCSK9 protein and plasma cholesterol and inhibited statin-mediated PCSK9 induction in hepatocytes. The mechanism involves inhibition of the downstream RhoA-ROCK pathway and regulation of PCSK9 at the post-transcriptional level. These data further identify Rap1a as a novel regulator of PCSK9 protein and show that blocking Rap1a prenylation through lowering geranylgeranyl levels contributes to statin-mediated induction of PCSK9.

Rap1 in the Context of PCSK9, Atherosclerosis, and Diabetes

PURPOSE OF REVIEW

The focus of this article is to highlight the importance of the small GTPase, Ras-associated protein 1 (Rap1), in proprotein convertase subtilisin/kexin type 9 (PCSK9) regulation and atherosclerosis and type 2 diabetes etiology and discuss the potential therapeutic implications of targeting Rap1 in these disease areas.

REVIEW FINDINGS

Cardiometabolic disease characterized by obesity, glucose intolerance, dyslipidemia, and atherosclerotic cardiovascular disease remain an important cause of mortality. Evidence using mouse models of obesity and insulin resistance indicates that Rap1 deficiency increases proatherogenic PCSK9 and low-density lipoprotein cholesterol levels and predisposes these mice to develop obesity- and statin-induced hyperglycemia, which highlights Rap1's role in cardiometabolic dysfunction. Rap1 may also contribute to cardiovascular disease through its effects on vascular wall cells involved in the atherosclerosis progression. Rap1 activation, specifically in the liver, could be beneficial in the prevention of cardiometabolic perturbations, including type 2 diabetes, hypercholesterolemia, and atherosclerosis.

Rap1 Activation Protects Against Fatty Liver and Non-Alcoholic Steatohepatitis Development

We previously demonstrated that hepatic activation of a small G protein of the Ras family, Rap1a, is suppressed in obesity, which results in increased hepatic glucose production and glucose intolerance in obese mice. Here, we show that Rap1a inhibition in obese mice liver also results in fatty liver formation, which is characteristic of the diabetic liver. Specifically, we report that Rap1a activity is decreased in the livers of patients with non-alcoholic steatohepatitis (NASH) and mouse models of non-alcoholic fatty liver disease (NAFLD) and NASH. Restoring hepatic Rap1a activity by overexpressing a constitutively active mutant form of Rap1a lowered the mature, processed form of lipogenic transcription factor, Srebp1, without an effect on the unprocessed Srebp1 and suppressed hepatic TG accumulation, whereas liver Rap1a deficiency increased Srebp1 processing and exacerbated steatosis. Mechanistically, we show that mTORC1, which promotes Srebp1 cleavage, is hyperactivated upon Rap1a deficiency despite disturbed insulin signaling. In proof-of-principle studies, we found that treatment of obese mice with a small molecule activator of Rap1a (8-pCPT) or inhibiting Rap1a's endogenous inhibitor, Rap1Gap, recapitulated our hepatic gain-of-function model and resulted in improved hepatic steatosis and lowered lipogenic genes. Thus, hepatic Rap1a serves as a signaling molecule that suppresses both hepatic gluconeogenesis and steatosis, and inhibition of its activity in the liver contributes to the pathogenesis of glucose intolerance and NAFLD/NASH development.

Geranylgeranyl Isoprenoids and Hepatic Rap1a Regulate Basal and Statin-Induced Expression of PCSK9

Low-density lipoprotein cholesterol (LDL-C) lowering is the main goal of atherosclerotic cardiovascular disease prevention, and proprotein convertase subtilisin/kexin type 9 (PCSK9) inhibition is now a validated therapeutic strategy that lowers serum LDL-C and reduces coronary events. Ironically, the most widely used medicine to lower cholesterol, statins, has been shown to increase circulating PCSK9 levels, which limits their efficacy. Here, we show that geranylgeranyl isoprenoids and hepatic Rap1a regulate both basal and statin induced expression of PCSK9 and contribute to LDL-C homeostasis. Rap1a prenylation and activity is inhibited upon statin treatment, and statin mediated PCSK9 induction is dependent on geranylgeranyl synthesis and hepatic Rap1a. Accordingly, treatment of mice with a small molecule activator of Rap1a lowered PCSK9 protein and plasma cholesterol and inhibited statin mediated PCSK9 induction in hepatocytes. The mechanism involves inhibition of the downstream RhoA-ROCK pathway and regulation of PCSK9 at the post transcriptional level. These data further identify Rap1a as a novel regulator of PCSK9 protein and show that blocking Rap1a prenylation through lowering geranylgeranyl levels contributes to statin-mediated induction of PCSK9.

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.

The Regulation and Secretion of Glucagon in Response to Nutrient Composition: Unraveling Their Intricate Mechanisms

Glucagon was initially regarded as a hyperglycemic substance; however, recent research has revealed its broader role in metabolism, encompassing effects on glucose, amino acids (AAs), and lipid metabolism. Notably, the interplay of glucagon with nutrient intake, particularly of AAs, and non-nutrient components is central to its secretion. Fasting and postprandial hyperglucagonemia have long been linked to the development and progression of type 2 diabetes (T2DM). However, recent studies have brought to light the positive impact of glucagon agonists on lipid metabolism and energy homeostasis. This review explores the multifaceted actions of glucagon, focusing on its regulation, signaling pathways, and effects on glucose, AAs, and lipid metabolism. The interplay between glucagon and other hormones, including insulin and incretins, is examined to provide a mechanistic understanding of its functions. Notably, the liver-α-cell axis, which involves glucagon and amino acids, emerges as a critical aspect of metabolic regulation. The dysregulation of glucagon secretion and its impact on conditions such as T2DM are discussed. The review highlights the potential therapeutic applications of targeting the glucagon pathway in the treatment of metabolic disorders.

Glucagon and Its Receptors in the Mammalian Heart

Glucagon exerts effects on the mammalian heart. These effects include alterations in the force of contraction, beating rate, and changes in the cardiac conduction system axis. The cardiac effects of glucagon vary according to species, region, age, and concomitant disease. Depending on the species and region studied, the contractile effects of glucagon can be robust, modest, or even absent. Glucagon is detected in the mammalian heart and might act with an autocrine or paracrine effect on the cardiac glucagon receptors. The glucagon levels in the blood and glucagon receptor levels in the heart can change with disease or simultaneous drug application. Glucagon might signal via the glucagon receptors but, albeit less potently, glucagon might also signal via glucagon-like-peptide-1-receptors (GLP1-receptors). Glucagon receptors signal in a species- and region-dependent fashion. Small molecules or antibodies act as antagonists to glucagon receptors, which may become an additional treatment option for diabetes mellitus. Hence, a novel review of the role of glucagon and the glucagon receptors in the mammalian heart, with an eye on the mouse and human heart, appears relevant. Mouse hearts are addressed here because they can be easily genetically modified to generate mice that may serve as models for better studying the human glucagon receptor.

Triple-Hormone-Receptor Agonist Retatrutide for Obesity - A Phase 2 Trial

BACKGROUND

Retatrutide (LY3437943) is an agonist of the glucose-dependent insulinotropic polypeptide, glucagon-like peptide 1, and glucagon receptors. Its dose-response relationships with respect to side effects, safety, and efficacy for the treatment of obesity are not known.

METHODS

We conducted a phase 2, double-blind, randomized, placebo-controlled trial involving adults who had a body-mass index (BMI, the weight in kilograms divided by the square of the height in meters) of 30 or higher or who had a BMI of 27 to less than 30 plus at least one weight-related condition. Participants were randomly assigned in a 2:1:1:1:1:2:2 ratio to receive subcutaneous retatrutide (1 mg, 4 mg [initial dose, 2 mg], 4 mg [initial dose, 4 mg], 8 mg [initial dose, 2 mg], 8 mg [initial dose, 4 mg], or 12 mg [initial dose, 2 mg]) or placebo once weekly for 48 weeks. The primary end point was the percentage change in body weight from baseline to 24 weeks. Secondary end points included the percentage change in body weight from baseline to 48 weeks and a weight reduction of 5% or more, 10% or more, or 15% or more. Safety was also assessed.

RESULTS

We enrolled 338 adults, 51.8% of whom were men. The least-squares mean percentage change in body weight at 24 weeks in the retatrutide groups was -7.2% in the 1-mg group, -12.9% in the combined 4-mg group, -17.3% in the combined 8-mg group, and -17.5% in the 12-mg group, as compared with -1.6% in the placebo group. At 48 weeks, the least-squares mean percentage change in the retatrutide groups was -8.7% in the 1-mg group, -17.1% in the combined 4-mg group, -22.8% in the combined 8-mg group, and -24.2% in the 12-mg group, as compared with -2.1% in the placebo group. At 48 weeks, a weight reduction of 5% or more, 10% or more, and 15% or more had occurred in 92%, 75%, and 60%, respectively, of the participants who received 4 mg of retatrutide; 100%, 91%, and 75% of those who received 8 mg; 100%, 93%, and 83% of those who received 12 mg; and 27%, 9%, and 2% of those who received placebo. The most common adverse events in the retatrutide groups were gastrointestinal; these events were dose-related, were mostly mild to moderate in severity, and were partially mitigated with a lower starting dose (2 mg vs. 4 mg). Dose-dependent increases in heart rate peaked at 24 weeks and declined thereafter.

CONCLUSIONS

In adults with obesity, retatrutide treatment for 48 weeks resulted in substantial reductions in body weight. (Funded by Eli Lilly; ClinicalTrials.gov number, NCT04881760.).

Glucagon receptor antagonism impairs and glucagon receptor agonism enhances triglycerides metabolism in mice

OBJECTIVE

Treatment with glucagon receptor antagonists (GRAs) reduces blood glucose but causes dyslipidemia and accumulation of fat in the liver. We investigated the acute and chronic effects of glucagon on lipid metabolism in mice.

METHODS

Chronic effects of glucagon receptor signaling on lipid metabolism were studied using oral lipid tolerance tests (OLTTs) in overnight fasted glucagon receptor knockout (Gcgr^-/-) mice, and in C57Bl/6JRj mice treated with a glucagon receptor antibody (GCGR Ab) or a long-acting glucagon analogue (GCGA) for eight weeks. Following treatment, liver tissue was harvested for RNA-sequencing and triglyceride measurements. Acute effects were studied in C57Bl/6JRj mice treated with a GRA or GCGA 1 h or immediately before OLTTs, respectively. Direct effects of glucagon on hepatic lipolysis were studied using isolated perfused mouse liver preparations. To investigate potential effects of GCGA and GRA on gastric emptying, paracetamol was, in separate experiments, administered immediately before OLTTs.

RESULTS

Plasma triglyceride concentrations increased 2-fold in Gcgr^-/- mice compared to their wild-type littermates during the OLTT (P = 0.001). Chronic treatment with GCGR Ab increased, whereas GCGA treatment decreased, plasma triglyceride concentrations during OLTTs (P < 0.05). Genes involved in lipid metabolism were upregulated upon GCGR Ab treatment while GCGA treatment had opposite effects. Acute GRA and GCGA treatment, respectively, increased (P = 0.02) and decreased (P = 0.003) plasma triglyceride concentrations during OLTTs. Glucagon stimulated hepatic lipolysis, evident by an increase in free fatty acid concentrations in the effluent from perfused mouse livers. In line with this, GCGR Ab treatment increased, while GCGA treatment decreased, liver triglyceride concentrations. The effects of glucagon appeared independent of changes in gastric emptying of paracetamol.

CONCLUSIONS

Glucagon receptor signaling regulates triglyceride metabolism, both chronically and acutely, in mice. These data expand glucagon´s biological role and implicate that intact glucagon signaling is important for lipid metabolism. Glucagon agonism may have beneficial effects on hepatic and peripheral triglyceride metabolism.

Pancreatic PCSK9 controls the organization of the β-cell secretory pathway via LDLR-cholesterol axis

BACKGROUND

Cholesterol is central to pancreatic β-cell physiology and alterations of its homeostasis contribute to β-cell dysfunction and diabetes. Proper intracellular cholesterol levels are maintained by different mechanisms including uptake via the low-density lipoprotein receptor (LDLR). In the liver, the proprotein convertase subtilisin/kexin type 9 (PCSK9) routes the LDLR to lysosomes for degradation, thus limiting its recycling to the membrane. PCSK9 is also expressed in the pancreas and loss of function mutations of PCSK9 result in higher plasma glucose levels and increased risk of Type 2 diabetes mellitus. Aim of this study was to investigate whether PCSK9 also impacts β-cells function.

METHODS

Pancreas-specific Pcsk9 null mice (Pdx1Cre/Pcsk9 fl/fl) were generated and characterized for glucose tolerance, insulin release and islet morphology. Isolated Pcsk9-deficient islets and clonal β-cells (INS1E) were employed to characterize the molecular mechanisms of PCSK9 action.

RESULTS

Pdx1Cre/Pcsk9 fl/fl mice exhibited normal blood PCSK9 and cholesterol levels but were glucose intolerant and had defective insulin secretion in vivo. Analysis of PCSK9-deficient islets revealed comparable β-cell mass and insulin content but impaired stimulated secretion. Increased proinsulin/insulin ratio, modifications of SNARE proteins expression and decreased stimulated‑calcium dynamics were detected in PCSK9-deficient β-cells. Mechanistically, pancreatic PCSK9 silencing impacts β-cell LDLR expression and cholesterol content, both in vivo and in vitro. The key role of LDLR is confirmed by the demonstration that LDLR downregulation rescued the phenotype.

CONCLUSIONS

These findings establish pancreatic PCSK9 as a novel critical regulator of the functional maturation of the β-cell secretory pathway, via modulation of cholesterol homeostasis.

Hepatocyte Rap1a contributes to obesity- and statin-associated hyperglycemia

Excessive hepatic glucose production contributes to the development of hyperglycemia and is a key feature of type 2 diabetes. Here, we report that activation of hepatocyte Rap1a suppresses gluconeogenic gene expression and glucose production, whereas Rap1a silencing stimulates them. Rap1a activation is suppressed in obese mouse liver, and restoring its activity improves glucose intolerance. As Rap1a′s membrane localization and activation depends on its geranylgeranylation, which is inhibited by statins, we show that statin-treated hepatocytes and the human liver have lower active-Rap1a levels. Similar to Rap1a inhibition, statins stimulate hepatic gluconeogenesis and increase fasting blood glucose in obese mice. Geranylgeraniol treatment, which acts as the precursor for geranylgeranyl isoprenoids, restores Rap1a activity and improves statin-mediated glucose intolerance. Mechanistically, Rap1a activation induces actin polymerization, which suppresses gluconeogenesis by Akt-mediated FoxO1 inhibition. Thus, Rap1a regulates hepatic glucose homeostasis, and blocking its activity, via lowering geranylgeranyl isoprenoids, contributes to statin-induced glucose intolerance.

Wang et al. show that activation of hepatic Rap1a suppresses gluconeogenic gene expression and improves glucose intolerance via Akt-mediated FoxO1 inhibition. Statins lower intracellular isoprenoid levels and inhibit Rap1a activation, which contributes to their hyperglycemic effect. These findings identify a role of hepatic Rap1a in obesity- and statin-associated glucose homeostasis.

Insights into the Role of Glucagon Receptor Signaling in Metabolic Regulation from Pharmacological Inhibition and Tissue-Specific Knockout Models

While glucagon has long been recognized as the primary counter hormone to insulin’s actions, it has recently gained recognition as a metabolic regulator with its effects extending beyond control of glycemia. Recently developed models of tissue-specific glucagon receptor knockouts have advanced our understanding of this hormone, providing novel insight into the role it plays within organs as well as its systemic effects. Studies where the pharmacological blockade of the glucagon receptor has been employed have proved similarly valuable in the study of organ-specific and systemic roles of glucagon signaling. Studies carried out employing these tools demonstrate that glucagon indeed plays a role in regulating glycemia, but also in amino acid and lipid metabolism, systemic endocrine, and paracrine function, and in the response to cardiovascular injury. Here, we briefly review recent progress in our understanding of glucagon’s role made through inhibition of glucagon receptor signaling utilizing glucagon receptor antagonists and tissue specific genetic knockout models.

Molecular and cellular biology of PCSK9: impact on glucose homeostasis

Proprotein convertase substilisin/kexin 9 (PCSK9) inhibitors (PCSK9i) revolutionised the lipid-lowering therapy. However, a risk of type 2 diabetes mellitus (T2DM) is evoked under PCSK9i therapy. In this review, we summarise the current knowledge on the link of PCSK9 with T2DM. A significant correlation was found between PCSK9 and insulin, homeostasis model assessment (HOMA) of insulin resistance and glycated haemoglobin. PCSK9 is also involved in inflammation. PCSK9 loss-of-function variants increased T2DM risk by altering insulin secretion. Local pancreatic low PCSK9 regulates β-cell LDLR expression which in turn promotes intracellular cholesterol accumulation and hampers insulin secretion. Nevertheless, the association of PCSK9 loss-of-function variants and T2DM is inconsistent. InsLeu and R46L polymorphisms were associated with T2DM, low HOMA for β-cell function and impaired fasting glucose, while the C679X polymorphism was associated with low fasting glucose in Black South African people. Hence, we assume that the impact of these variants on glucose homeostasis may vary depending on the genetic background of the studied populations and the type of effect caused by those genetic variants on the PCSK9 protein. Accordingly, these factors should be considered when choosing a genetic variant of PCSK9 to assess the impact of long-term use of PCSK9i on glucose homeostasis.

Emerging roles of Sirtuins in alleviating alcoholic liver Disease: A comprehensive review

Sirtuins (SIRTs), a NAD⁺ family of dependent deacetylases, are involved in the regulation of various human diseases. Recently, accumulating evidence has uncovered number of substrates and crucial roles of SIRTs in the pathogenesis of alcoholic liver disease (ALD). However, systematic reports are still lacking, so this review provides a comprehensive profile of the crucial physiological functions of SIRTs and its role in attenuating ALD, including alcoholic liver steatosis, steatohepatitis, and fibrosis. SIRTs play beneficial roles in energy/lipid metabolism, oxidative stress, inflammatory response, mitochondrial homeostasis, autophagy and necroptosis of ALD via regulating multiple signaling transduction pathways such as AMPK, LKB1, SREBP1, Lipin1, PGC-1α, PPARα/γ, FoxO1/3a, Nrf2/p62, mTOR, TFEB, RIPK1/3, HMGB1, NFATc4, NF-κB, TLR4, NLRP3, P2X7R, MAPK, TGF1β/Smads and Wnt/β-catenin. In addition, the mechanism and clinical application of natural/ synthetic SIRTs agonists in ALD are summarized, which provide a new idea for the treatment of ALD and basic foundation for further studies into target drugs.

Multi-organ Coordination of Lipoprotein Secretion by Hormones, Nutrients and Neural Networks

Plasma triglyceride-rich lipoproteins (TRL), particularly atherogenic remnant lipoproteins, contribute to atherosclerotic cardiovascular disease. Hypertriglyceridemia may arise in part from hypersecretion of TRLs by the liver and intestine. Here we focus on the complex network of hormonal, nutritional, and neuronal interorgan communication that regulates secretion of TRLs and provide our perspective on the relative importance of these factors. Hormones and peptides originating from the pancreas (insulin, glucagon), gut [glucagon-like peptide 1 (GLP-1) and 2 (GLP-2), ghrelin, cholecystokinin (CCK), peptide YY], adipose tissue (leptin, adiponectin) and brain (GLP-1) modulate TRL secretion by receptor-mediated responses and indirectly via neural networks. In addition, the gut microbiome and bile acids influence lipoprotein secretion in humans and animal models. Several nutritional factors modulate hepatic lipoprotein secretion through effects on the central nervous system. Vagal afferent signaling from the gut to the brain and efferent signals from the brain to the liver and gut are modulated by hormonal and nutritional factors to influence TRL secretion. Some of these factors have been extensively studied and shown to have robust regulatory effects whereas others are "emerging" regulators, whose significance remains to be determined. The quantitative importance of these factors relative to one another and relative to the key regulatory role of lipid availability remains largely unknown. Our understanding of the complex interorgan regulation of TRL secretion is rapidly evolving to appreciate the extensive hormonal, nutritional, and neural signals emanating not only from gut and liver but also from the brain, pancreas, and adipose tissue.

Hepatic Sensing Loop Regulates PCSK9 Secretion in Response to Inhibitory Antibodies

BACKGROUND

Monoclonal antibodies against proprotein convertase subtilisin/kexin type 9 (PCSK9i) lower LDL-C by up to 60% and increase plasma proprotein convertase subtilisin/kexin type 9 (PCSK9) levels by 10-fold.

OBJECTIVES

The authors studied the reasons behind the robust increase in plasma PCSK9 levels by testing the hypothesis that mechanisms beyond clearance via the low-density lipoprotein receptor (LDLR) contribute to the regulation of cholesterol homeostasis.

METHODS

In clinical cohorts, animal models, and cell-based studies, we measured kinetic changes in PCSK9 production and clearance in response to PCSK9i.

RESULTS

In a patient cohort receiving PCSK9i therapy, plasma PCSK9 levels rose 11-fold during the first 3 months and then plateaued for 15 months. In a cohort of healthy volunteers, a single injection of PCSK9i increased plasma PCSK9 levels within 12 hours; the rise continued for 9 days until it plateaued at 10-fold above baseline. We recapitulated the rapid rise in PCSK9 levels in a mouse model, but only in the presence of LDLR. In vivo turnover and in vitro pulse-chase studies identified 2 mechanisms contributing to the rapid increase in plasma PCSK9 levels in response to PCSK9i: 1) the expected delayed clearance of the antibody-bound PCSK9; and 2) the unexpected post-translational increase in PCSK9 secretion.

CONCLUSIONS

PCSK9 re-entry to the liver via LDLR triggers a sensing loop regulating PCSK9 secretion. PCSK9i therapy enhances the secretion of PCSK9, an effect that contributes to the increased plasma PCSK9 levels in treated subjects.

Allosteric MAPKAPK2 inhibitors improve plaque stability in advanced atherosclerosis

Atherosclerotic vascular disease resulting from unstable plaques is the leading cause of morbidity and mortality in subjects with type 2 diabetes (T2D), and thus a major therapeutic goal is to discover T2D drugs that can also promote atherosclerotic plaque stability. Genetic or pharmacologic inhibition of mitogen-activated protein kinase-activated protein kinase-2 (MAPKAPK2 or MK2) in obese mice improves glucose homeostasis and enhances insulin sensitivity. We developed two novel orally active small-molecule inhibitors of MK2, TBX-1 and TBX-2, and tested their effects on metabolism and atherosclerosis in high-fat Western diet (WD)-fed Ldlr-/- mice. Ldlr-/- mice were first fed the WD to allow atherosclerotic lesions to become established, and the mice were then treated with TBX-1 or TBX-2. Both compounds improved glucose metabolism and lowered plasma cholesterol and triglyceride, without an effect on body weight. Most importantly, the compounds decreased lesion area, lessened plaque necrosis, and increased fibrous cap thickness in the aortic root lesions of the mice. Thus, in a preclinical model of high-fat feeding and established atherosclerosis, MK2 inhibitors improved metabolism and also enhanced atherosclerotic plaque stability, suggesting potential for further clinical development to address the epidemic of T2D associated with atherosclerotic vascular disease.

New Insight Into Metformin-Induced Cholesterol-Lowering Effect Crosstalk Between Glucose and Cholesterol Homeostasis via ChREBP (Carbohydrate-Responsive Element-Binding Protein)-Mediated PCSK9 (Proprotein Convertase Subtilisin/Kexin Type 9) Regulation

[Figure: see text].

Sirt6 Deacetylase: A Potential Key Regulator in the Prevention of Obesity, Diabetes and Neurodegenerative Disease

Sirtuins, NAD + dependent proteins belonging to class III histone deacetylases, are involved in regulating numerous cellular processes including cellular stress, insulin resistance, inflammation, mitochondrial biogenesis, chromatin silencing, cell cycle regulation, transcription, and apoptosis. Of the seven mammalian sirtuins present in humans, Sirt6 is an essential nuclear sirtuin. Until recently, Sirt6 was thought to regulate chromatin silencing, but new research indicates its role in aging, diabetes, cardiovascular disease, lipid metabolism, neurodegenerative diseases, and cancer. Various murine models demonstrate that Sirt6 activation is beneficial in alleviating many disease conditions and increasing lifespan, showing that Sirt6 is a critical therapeutic target in the treatment of various disease conditions in humans. Sirt6 also regulates the pathogenesis of multiple diseases by acting on histone proteins and non-histone proteins. Endogenous and non-endogenous modulators regulate both activation and inhibition of Sirt6. Few Sirt6 specific non-endogenous modulators have been identified. Hence the identification of Sirt6 specific modulators may have potential therapeutic roles in the diseases described above. In this review, we describe the development of Sirt6, the role it plays in the human condition, the functional role and therapeutic importance in disease processes, and specific modulators and molecular mechanism of Sirt6 in the regulation of metabolic homeostasis, cardiovascular disease, aging, and neurodegenerative disease.

β-Cell-Specific Deletion of HMG-CoA (3-hydroxy-3-methylglutaryl-coenzyme A) Reductase Causes Overt Diabetes due to Reduction of β-Cell Mass and Impaired Insulin Secretion

Inhibitors of 3-hydroxy-3-methylglutaryl-CoA reductase (HMGCR), statins, which are used to prevent cardiovascular diseases, are associated with a modest increase in the risk of new-onset diabetes. To investigate the role of HMGCR in the development of β-cells and glucose homeostasis, we deleted Hmgcr in a β-cell-specific manner by using the Cre-loxP technique. Mice lacking Hmgcr in β-cells (β-KO) exhibited hypoinsulinemic hyperglycemia as early as postnatal day 9 (P9) due to decreases in both β-cell mass and insulin secretion. Ki67-positive cells were reduced in β-KO mice at P9; thus, β-cell mass reduction was caused by proliferation disorder immediately after birth. The mRNA expression of neurogenin3 (Ngn3), which is transiently expressed in endocrine progenitors of the embryonic pancreas, was maintained despite a striking reduction in the expression of β-cell-associated genes, such as insulin, pancreatic and duodenal homeobox 1 (Pdx1), and MAF BZIP transcription factor A (Mafa) in the islets from β-KO mice. Histological analyses revealed dysmorphic islets with markedly reduced numbers of β-cells, some of which were also positive for glucagon. In conclusion, HMGCR plays critical roles not only in insulin secretion but also in the development of β-cells in mice.

Inclusion of endophenotypes in a standard GWAS facilitate a detailed mechanistic understanding of genetic elements that control blood lipid levels

Dyslipidemia is the primary cause of cardiovascular disease, which is a serious human health problem in large parts of the world. Therefore, it is important to understand the genetic and molecular mechanisms that regulate blood levels of cholesterol and other lipids. Discovery of genetic elements in the regulatory machinery is often based on genome wide associations studies (GWAS) focused on end-point phenotypes such as total cholesterol level or a disease diagnosis. In the present study, we add endophenotypes, such as serum levels of intermediate metabolites in the cholesterol synthesis pathways, to a GWAS analysis and use the pig as an animal model. We do this to increase statistical power and to facilitate biological interpretation of results. Although the study population was limited to ~ 300 individuals, we identify two genome-wide significant associations and ten suggestive associations. Furthermore, we identify 28 tentative associations to loci previously associated with blood lipids or dyslipidemia associated diseases. The associations with endophenotypes may inspire future studies that can dissect the biological mechanisms underlying these previously identified associations and add a new level of understanding to previously identified associations.

Optimization of S. aureus dCas9 and CRISPRi Elements for a Single Adeno-Associated Virus that Targets an Endogenous Gene

The power of CRISPRi to decrease targeted gene expression for clinical applications has been inhibited by delivery challenges. Existing constructs are too large to fit within the ∼4.7 kb packaging size limitation of adeno-associated virus (AAV), the only FDA approved viral vector for clinical use. Therefore, we optimized CRISPRi components to generate a single AAV vector that contains all functional elements and effectively knocks down expression of an endogenous gene in vivo. First, we increased nuclear targeting of Staphylococcus aureus deactivated Cas9 (SadCas9) 4-fold by using a helical linker and the c-Myc nuclear localization signal. Second, we identified an amino-terminal Krüppel associated box (KRAB) construct as the most effective in decreasing expression of target genes in vitro. Third, we optimized promoters for guide RNA and evaluated mini-promoters for expression of KRAB-SadCas9 in liver cells. Our final construct decreased protein convertase subtilisin/kexin type 9 (Pcsk9) mRNA and secreted protein 5-fold in vitro. The corresponding AAV2/8 vector was localized in nuclei of liver cells and decreased Pcsk9 mRNA and serum protein levels by 30% in vivo. This single AAV approach provides a potential clinically translatable method for decreasing targeted gene transcription by CRISPRi in vivo.

Impact of Sex and Age on the Mevalonate Pathway in the Brain: A Focus on Effects Induced by Maternal Exposure to Exogenous Compounds

The mevalonate pathway produces cholesterol and other compounds crucial for numerous cellular processes. It is well known that age and sex modulate this pathway in the liver. Recently, similar effects were also noted in different brain areas, suggesting that alterations of the mevalonate pathway are at the root of marked sex-specific disparities in some neurodevelopmental disorders related to disturbed cholesterol homeostasis. Here, we show how the mevalonate pathway is modulated in a sex-, age- and region-specific manner, and how maternal exposure to exogenous compounds can disturb the regulation of this pathway in the brain, possibly inducing functional alterations.

An Update on the Role of PCSK9 in Atherosclerosis

Atherosclerosis is initiated by functional changes in the endothelium accompanied by accumulation, oxidation, and glycation of LDL-cholesterol in the inner layer of the arterial wall and continues with the expression of adhesion molecules and release of chemoattractants. PCSK9 is a proprotein convertase that increases circulating LDL levels by directing hepatic LDL receptors into lysosomes for degradation. The effects of PCSK9 on hepatic LDL receptors and contribution to atherosclerosis via the induction of hyperlipidemia are well defined. Monoclonal PCSK9 antibodies that block the effects of PCSK9 on LDL receptors demonstrated beneficial results in cardiovascular outcome trials. In recent years, extrahepatic functions of PCSK9, particularly its direct effects on atherosclerotic plaques have received increasing attention. Experimental trials have revealed that PCSK9 plays a significant role in every step of atherosclerotic plaque formation. It contributes to foam cell formation by increasing the uptake of LDL by macrophages via scavenger receptors and inhibiting cholesterol efflux from macrophages. It induces the expression of inflammatory cytokines, adhesion molecules, and chemoattractants, thereby increasing monocyte recruitment, inflammatory cell adhesion, and inflammation at the atherosclerotic vascular wall. Moreover, low shear stress is associated with increased PCSK9 expression. PCSK9 may induce endothelial cell apoptosis and autophagy and stimulate the differentiation of smooth muscle cells from the contractile phenotype to synthetic phenotype. Increasing evidence indicates that PCSK9 is a molecular target in the development of novel approaches toward the prevention and treatment of atherosclerosis. This review focuses on the molecular roles of PCSK9 in atherosclerotic plaque formation.

Circulating PCSK9 and cardiovascular events in FH patients with standard lipid-lowering therapy

BACKGROUND

Proprotein convertase subtilisin/kexin 9 (PCSK9) has been proposed as a novel target for coronary artery disease (CAD). Familial hypercholesterolemia (FH) is characterized by high prevalence of CAD and major cardiovascular events (MACEs). However, no data is available on the association between PCSK9 levels and MACEs in FH patients with standard lipid lowering therapy.

METHODS

A total of 338 consecutive heterozygous FH (Dutch Lipid Clinic Network score ≥ 6) was enrolled and followed up for the occurrence of MACEs. Multidetector CT and coronary angiography were performed to determine coronary artery calcification score (CACS) and Gensini score (GS). Multivariable Cox regression analyses were used to calculate hazard ratios (HRs) with 95% confidence intervals (CIs). Plasma PCSK9 concentrations were determined by enzyme immunoassay.

RESULTS

PCSK9 was independently and positively associated CACS and GS at baseline. During a mean follow-up of 3 years, 33 (9.8%) events occurred. Patients with MACEs had higher median PCSK9 compared with those without (332.47 vs. 311.89 ng/mL, p = 0.038). Kaplan-Meier analysis revealed that patients with higher PCSK9 presented lower event-free survival (p = 0.0017). PCSK9 was statistically correlated with MACEs after adjusting for confounding factors, with the HR per SD being 1.86 (1.31-2.65) and 3.70 (1.16-11.82) for the highest tertile compared with the lowest tertile. Adding PCSK9 to Cox prediction model led to a statistical improvement in net reclassification and integrated discrimination.

CONCLUSION

Elevated levels of PCSK9 were positively associated with the development of CAD and future cardiovascular events, suggesting that measurement of PCSK9 concentration might be useful for cardiovascular risk stratification. Further studies are needed to confirm our results.

Proprotein convertase subtilisin/kexin type 9 and lipid metabolism

PURPOSE OF REVIEW

The purpose of this review is to highlight the recent findings of one of the most promising therapeutic targets in LDL cholesterol (LDL-C) management, proprotein convertase subtilisin/kexin type 9 (PCSK9).

RECENT FINDINGS

Endoplasmic reticulum cargo receptor, surfeit locus protein 4 interacts with PCSK9 and regulates its exit from endoplasmic reticulum and its secretion. Once secreted, PCSK9 binds to heparin sulfate proteoglycans on the hepatocyte surface and this binding is required for PCSK9-LDL receptor (LDLR) complex formation and LDLR degradation. Posttranscriptionally, recent work has shown that PCSK9 gets degraded in lysosomes by activation of the glucagon receptor signaling, providing more data on the hormonal regulation of PCSK9. Finally, human studies with PCSK9 inhibitors offered more evidence on their benefits and safe use.

SUMMARY

Recent work on the regulation of PCSK9 has enhanced our understanding of its biology, which may provide important information for future PCSK9-based therapies.

Changes in circulating pro-protein convertase subtilisin/kexin type 9 levels - experimental and clinical approaches with lipid-lowering agents

Regulation of pro-protein convertase subtilisin/kexin type 9 (PCSK9) by drugs has led to the development of a still small number of agents with powerful activity on low-density lipoprotein cholesterol levels, associated with a significant reduction of cardiovascular events in patients in secondary prevention. The Further Cardiovascular Outcomes Research with PCSK9 Inhibition in Subjects with Elevated Risk (FOURIER) and Evaluation of Cardiovascular Outcomes After an Acute Coronary Syndrome During Treatment With Alirocumab (ODYSSEY OUTCOMES) studies, with the two available PCSK9 antagonists, i.e. evolocumab and alirocumab, both reported a 15% reduction in major adverse cardiovascular events. Regulation of PCSK9 expression is dependent upon a number of factors, partly genetic and partly associated to a complex transcriptional system, mainly controlled by sterol regulatory element binding proteins. PCSK9 is further regulated by concomitant drug treatments, particularly by statins, enhancing PCSK9 secretion but decreasing its stimulatory phosphorylated form (S688). These complex transcriptional mechanisms lead to variable circulating levels making clinical measurements of plasma PCSK9 for cardiovascular risk assessment a debated matter. Determination of total PCSK9 levels may provide a diagnostic tool for explaining an apparent resistance to PCSK9 inhibitors, thus indicating the need for other approaches. Newer agents targeting PCSK9 are in clinical development with a major interest in those with a longer duration of action, e.g. RNA silencing, allowing optimal patient compliance. Interest has been expanded to areas not only limited to low-density lipoprotein cholesterol reduction but also investigating other non-lipid pathways raising cardiovascular risk, in particular inflammation associated to raised high-sensitivity C-reactive protein levels, not significantly affected by the present PCSK9 antagonists.