Amy C. Burke

Naringenin prevents obesity, hepatic steatosis, and glucose intolerance in male mice independent of fibroblast growth factor 21.

The molecular mechanisms and metabolic pathways whereby the citrus flavonoid, naringenin, reduces dyslipidemia and improves glucose tolerance were investigated in C57BL6/J wild-type mice and fibroblast growth factor 21 (FGF21) null (Fgf21(-/-)) mice. FGF21 regulates energy homeostasis and the metabolic adaptation to fasting. One avenue of this regulation is through induction of peroxisome proliferator-activated receptor-γ coactivator-1α (Pgc1a), a regulator of hepatic fatty acid oxidation and ketogenesis. Because naringenin is a potent activator of hepatic FA oxidation, we hypothesized that induction of FGF21 might be an integral part of naringenins mechanism of action. Furthermore, we predicted that FGF21 deficiency would potentiate high-fat diet (HFD)-induced metabolic dysregulation and compromise metabolic protection by naringenin. The absence of FGF21 exacerbated the response to a HFD. Interestingly, naringenin supplementation to the HFD robustly prevented obesity in both genotypes. Gene expression analysis suggested that naringenin was not primarily targeting fatty acid metabolism in white adipose tissue. Naringenin corrected hepatic triglyceride concentrations and normalized hepatic expression of Pgc1a, Cpt1a, and Srebf1c in both wild-type and Fgf21(-/-) mice. HFD-fed Fgf21(-/-) mice displayed greater muscle triglyceride deposition, hyperinsulinemia, and impaired glucose tolerance as compared with wild-type mice, confirming the role of FGF21 in insulin sensitivity; however, naringenin supplementation improved these metabolic parameters in both genotypes. We conclude that FGF21 deficiency exacerbates HFD-induced obesity, hepatic steatosis, and insulin resistance. Furthermore, FGF21 is not required for naringenin to protect mice from HFD-induced metabolic dysregulation. Collectively these studies support the concept that naringenin has potent lipid-lowering effects and may act as an insulin sensitizer in vivo.

Peroxisome Proliferator–Activated Receptor δ Agonist GW1516 Attenuates Diet-Induced Aortic Inflammation, Insulin Resistance, and Atherosclerosis in Low-Density Lipoprotein Receptor Knockout Mice

Objective— The peroxisome proliferator–activated receptor (PPAR) &dgr; regulates systemic lipid homeostasis and inflammation. However, the ability of PPAR&dgr; agonists to improve the pathology of pre-established lesions and whether PPAR&dgr; activation is atheroprotective in the setting of insulin resistance have not been reported. Here, we examine whether intervention with a selective PPAR&dgr; agonist corrects metabolic dysregulation and attenuates aortic inflammation and atherosclerosis. Approach and Results— Low-density lipoprotein receptor knockout mice were fed a chow or a high-fat, high-cholesterol (HFHC) diet (42% fat, 0.2% cholesterol) for 4 weeks. For a further 8 weeks, the HFHC group was fed either HFHC or HFHC plus GW1516 (3 mg/kg per day). GW1516 significantly attenuated pre-established fasting hyperlipidemia, hyperglycemia, and hyperinsulinemia, as well as glucose and insulin intolerance. GW1516 intervention markedly reduced aortic sinus lesions and lesion macrophages, whereas smooth muscle &agr;-actin was unchanged and collagen deposition enhanced. In aortae, GW1516 increased the expression of the PPAR&dgr;-specific gene Adfp but not PPAR&agr;- or &ggr;-specific genes. GW1516 intervention decreased the expression of aortic proinflammatory M1 cytokines, increased the expression of the anti-inflammatory M2 cytokine Arg1, and attenuated the iNos/Arg1 ratio. Enhanced mitogen-activated protein kinase signaling, known to induce inflammatory cytokine expression in vitro, was enhanced in aortae of HFHC-fed mice. Furthermore, the HFHC diet impaired aortic insulin signaling through Akt and forkhead box O1, which was associated with elevated endoplasmic reticulum stress markers CCAAT-enhancer-binding protein homologous protein and 78kDa glucose regulated protein. GW1516 intervention normalized mitogen-activated protein kinase activation, insulin signaling, and endoplasmic reticulum stress. Conclusions— Intervention with a PPAR&dgr; agonist inhibits aortic inflammation and attenuates the progression of pre-established atherosclerosis.

Citrus Flavonoids as Regulators of Lipoprotein Metabolism and Atherosclerosis

Citrus flavonoids are polyphenolic compounds with significant biological properties. This review summarizes recent advances in understanding the ability of citrus flavonoids to modulate lipid metabolism, other metabolic parameters related to the metabolic syndrome, and atherosclerosis. Citrus flavonoids, including naringenin, hesperitin, nobiletin, and tangeretin, have emerged as potential therapeutics for the treatment of metabolic dysregulation. Epidemiological studies reveal an association between the intake of citrus flavonoid-containing foods and a decreased incidence of cardiovascular disease. Studies in cell culture and animal models, as well as a limited number of clinical studies, reveal the lipid-lowering, insulin-sensitizing, antihypertensive, and anti-inflammatory properties of citrus flavonoids. In animal models, supplementation of rodent diets with citrus flavonoids prevents hepatic steatosis, dyslipidemia, and insulin resistance primarily through inhibition of hepatic fatty acid synthesis and increased fatty acid oxidation. Citrus flavonoids blunt the inflammatory response in metabolically important tissues including liver, adipose, kidney, and the aorta. The mechanisms underlying flavonoid-induced metabolic regulation have not been completely established, although several potential targets have been identified. In mouse models, citrus flavonoids show marked suppression of atherogenesis through improved metabolic parameters as well as through direct impact on the vessel wall. Recent studies support a role for citrus flavonoids in the treatment of dyslipidemia, insulin resistance, hepatic steatosis, obesity, and atherosclerosis. Larger human studies examining dose, bioavailability, efficacy, and safety are required to promote the development of these promising therapeutic agents.

PCSK9: Regulation and Target for Drug Development for Dyslipidemia

Proprotein convertase subtilisin/kexin type-9 (PCSK9) is a secreted zymogen expressed primarily in the liver. PCSK9 circulates in plasma, binds to cell surface low-density lipoprotein (LDL) receptors, is internalized, and then targets the receptors to lysosomal degradation. Studies of naturally occurring PCSK9 gene variants that caused extreme plasma LDL cholesterol (LDL-C) deviations and altered atherosclerosis risk unleashed a torrent of biological and pharmacological research. Rapid progress in understanding the physiological regulation of PCSK9 was soon translated into commercially available biological inhibitors of PCSK9 that reduced LDL-C levels and likely also cardiovascular outcomes. Here we review the swift evolution of PCSK9 from novel gene to drug target, to animal and human testing, and finally to outcome trials and clinical applications. In addition, we explore how the genetics-guided path to PCSK9 inhibitor development exemplifies a new paradigm in pharmacology. Finally, we consider some potential challenges as PCSK9 inhibition becomes established in the clinic.

Prevention of Diet-Induced Metabolic Dysregulation, Inflammation, and Atherosclerosis in Ldlr−/− Mice by Treatment With the ATP-Citrate Lyase Inhibitor Bempedoic Acid

Objective— Bempedoic acid (ETC-1002, 8-hydroxy-2,2,14,14-tetramethylpentadecanedioic acid) is a novel low-density lipoprotein cholesterol–lowering compound. In animals, bempedoic acid targets the liver where it inhibits cholesterol and fatty acid synthesis through inhibition of ATP-citrate lyase and through activation of AMP-activated protein kinase. In this study, we tested the hypothesis that bempedoic acid would prevent diet-induced metabolic dysregulation, inflammation, and atherosclerosis. Approach and Results— Ldlr−/− mice were fed a high-fat, high-cholesterol diet (42% kcal fat, 0.2% cholesterol) supplemented with bempedoic acid at 0, 3, 10 and 30 mg/kg body weight/day. Treatment for 12 weeks dose-dependently attenuated diet-induced hypercholesterolemia, hypertriglyceridemia, hyperglycemia, hyperinsulinemia, fatty liver and obesity. Compared to high-fat, high-cholesterol alone, the addition of bempedoic acid decreased plasma triglyceride (up to 64%) and cholesterol (up to 50%) concentrations, and improved glucose tolerance. Adiposity was significantly reduced with treatment. In liver, bempedoic acid prevented cholesterol and triglyceride accumulation, which was associated with increased fatty acid oxidation and reduced fatty acid synthesis. Hepatic gene expression analysis revealed that treatment significantly increased expression of genes involved in fatty acid oxidation while suppressing inflammatory gene expression. In full-length aorta, bempedoic acid markedly suppressed cholesteryl ester accumulation, attenuated the expression of proinflammatory M1 genes and attenuated the iNos/Arg1 ratio. Treatment robustly attenuated atherosclerotic lesion development in the aortic sinus by 44%, with beneficial changes in morphology, characteristic of earlier-stage lesions. Conclusions— Bempedoic acid effectively prevents plasma and tissue lipid elevations and attenuates the onset of inflammation, leading to the prevention of atherosclerotic lesion development in a mouse model of metabolic dysregulation.

ATP-citrate lyase: genetics, molecular biology and therapeutic target for dyslipidemia

Purpose of review ATP-citrate lyase (ACLY) has re-emerged as a drug target for LDL cholesterol (LDL-C) lowering. We review ACLY as a therapeutic strategy, its genetics, its molecular and cellular biology, and also its inhibition. Recent findings ACLY is a critical enzyme linking glucose catabolism to lipogenesis by providing acetyl-CoA from mitochondrial citrate for fatty acid and cholesterol biosynthesis. Human genetic variants have been associated with enhanced growth and survival of several cancers, and with attenuated plasma triglyceride responses to dietary fish oil. In mice, liver-specific Acly deficiency protects from hepatic steatosis and dyslipidemia, whereas adipose tissue-specific Acly deletion has no phenotype, supporting therapeutic inhibition of ACLY. A lipid-regulating compound, bempedoic acid, was discovered to potently inhibit ACLY, and in animal models, it prevents dyslipidemia and attenuates atherosclerosis. Phase 2 clinical trials revealed that bempedoic acid effectively lowers LDL-C as monotherapy, combined with ezetimibe, added to statin therapy and in statin-intolerant hypercholesterolemic patients. Summary The efficacy of bempedoic acid as an LDL-C-lowering agent has validated ACLY inhibition as a therapeutic strategy. Positive results of phase 3 patient studies, together with long-term cardiovascular disease outcome trials, are required to establish ACLY as a major new target in cardiovascular medicine.