Functional analysis of the missense APOC3 mutation Ala23Thr associated with human hypotriglyceridemia

Tea Polysaccharide Ameliorates Atherosclerosis by Inhibiting Insulin Resistance-Mediated Hepatic VLDL Overproduction

Hepatic VLDL overproduction, tightly modulated by insulin signaling, plays a pivotal role in the progression of atherosclerosis (AS). The present study aimed to investigate whether inhibition of hepatic VLDL overproduction is a novel therapeutic strategy for the homogeneous tea polysaccharide (TPS3A) to ameliorate AS under insulin resistance (IR) conditions and the potential molecular basis involved. Results showed that TPS3A supplementation effectively alleviated systemic IR and delayed atherosclerotic plaque progression in HFD-exposed ApoE-/- mice. Additionally, TPS3A markedly down-regulated the expression of TG synthesis markers (SREBP-1, ACC1, and FAS) and apoB lipidation markers (apoB, apoCIII, and MTP), while up-regulating the expression of apoB degradation maker (sortilin) and VLDL clearance maker (LDLR), thereby inhibiting VLDL overproduction in insulin-resistant ApoE-/- mice and HepG2 cells. The IRS-mediated PI3K-AKT-mTORC1/FoxO1 insulin signaling cascades are central pathways regulating VLDL production. We found that TPS3A significantly abolished insulin-induced activation of PI3K, AKT, mTORC1, and nuclear FoxO1 in vivo and in vitro. Moreover, the suppression effects of TPS3A on VLDL overproduction were synergistically strengthened by inhibitors targeting PI3K (Wortmannin), AKT (GSK690693), mTORC1 (Rapamycin), and FoxO1 (AS1842856). Overall, TPS3A holds promise in ameliorating AS by inhibiting hepatic VLDL overproduction through the IRS-mediated PI3K-AKT-mTORC1/FoxO1 insulin signaling pathways.

FoxO6-mediated ApoC3 upregulation promotes hepatic steatosis and hyperlipidemia in aged rats fed a high-fat diet

FoxO6, an identified factor, induces hyperlipidemia and hepatic steatosis during aging by activating hepatic lipoprotein secretion and lipogenesis leading to increased ApoC3 concentrations in the bloodstream. However, the intricate mechanisms underlying hepatic steatosis induced by elevated FoxO6 under hyperglycemic conditions remain intricate and require further elucidation. In order to delineate the regulatory pathway involving ApoC3 controlled by FoxO6 and its resultant functional impacts, we employed a spectrum of models including liver cell cultures, aged rats subjected to HFD, transgenic mice overexpressing FoxO6 (FoxO6-Tg), and FoxO6 knockout mice (FoxO6-KO). Our findings indicate that FoxO6 triggered ApoC3-driven lipid accumulation in the livers of aged rats on an HFD and in FoxO6-Tg, consequently leading to hepatic steatosis and hyperglycemia. Conversely, the absence of FoxO6 attenuated the expression of genes involved in lipogenesis, resulting in diminished hepatic lipid accumulation and mitigated hyperlipidemia in murine models. Additionally, the upregulation of FoxO6 due to elevated glucose levels led to increased ApoC3 expression, consequently instigating cellular triglyceride mediated lipid accumulation. The transcriptional activation of FoxO6 induced by both the HFD and high glucose levels resulted in hepatic steatosis by upregulating ApoC3 and genes associated with gluconeogenesis in aged rats and liver cell cultures. Our conclusions indicate that the upregulation of ApoC3 by FoxO6 promotes the development of hyperlipidemia, hyperglycemia, and hepatic steatosis in vivo, and in vitro. Taken together, our findings underscore the significance of FoxO6 in driving hyperlipidemia and hepatic steatosis specifically under hyperglycemic states by enhancing the expression of ApoC3 in aged rats.

ApoCIII: A multifaceted protein in cardiometabolic disease

ApoCIII has a well-recognized role in triglyceride-rich lipoproteins metabolism. A considerable amount of data has clearly highlighted that high levels of ApoCIII lead to hypertriglyceridemia and, thereby, may influence the risk of cardiovascular disease. However, recent findings indicate that ApoCIII might also act beyond lipid metabolism. Indeed, ApoCIII has been implicated in other physiological processes such as glucose homeostasis, monocyte adhesion, activation of inflammatory pathways, and modulation of the coagulation cascade. As the inhibition of ApoCIII is emerging as a new promising therapeutic strategy, the complete understanding of multifaceted pathophysiological role of this apoprotein may be relevant. Therefore, the purpose of this work is to review available evidences not only related to genetics and biochemistry of ApoCIII, but also highlighting the role of this apoprotein in triglyceride and glucose metabolism, in the inflammatory process and coagulation cascade as well as in cardiovascular disease.

Recent developments in pharmacotherapy for hypertriglyceridemia: what's the current state of the art?

Introduction: Hypertriglyceridemia is associated with both the development of cardiovascular disease (CVD) when mild-to-moderate and high risk of pancreatitis when more severe. The residual CVD risk after low-density lipoprotein cholesterol (LDL-C) lowering is, in part, attributed to high triglyceride (TG) levels. Therefore, there appears to be a need for effective TG-lowering agents.Areas covered: This review presents the most recent advances in hypertriglyceridemia treatment; specifically, it discusses the results of clinical trials and critically comments on apolipoprotein C-III inhibitors, angiopoietin-like 3 inhibitors, alipogene tiparvovec, pradigastat, pemafibrate and novel formulations of omega-3 fatty acids.Expert opinion: In the era of extreme lowering of LDL-C levels with several agents, there seems to be space for novel therapeutic options to combat parameters responsible for residual CVD risk, among which are elevated TGs. Furthermore, a significant number of individuals have very high TG levels and encounter the risk of acute pancreatitis. The most recently developed TG-lowering drugs appear to have a role in both conditions; the choice is mainly based on baseline TG levels. Dyslipidemia guidelines are likely to change in the near future to include some of these agents. Of course, long-term data regarding their safety and efficacy in terms of CVD outcomes and pancreatitis are warranted.

Lipid-Lowering Agents

Several new or emerging drugs for dyslipidemia owe their existence, in part, to human genetic evidence, such as observations in families with rare genetic disorders or in Mendelian randomization studies. Much effort has been directed to agents that reduce LDL (low-density lipoprotein) cholesterol, triglyceride, and Lp[a] (lipoprotein[a]), with some sustained programs on agents to raise HDL (high-density lipoprotein) cholesterol. Lomitapide, mipomersen, AAV8.TBG.hLDLR, inclisiran, bempedoic acid, and gemcabene primarily target LDL cholesterol. Alipogene tiparvovec, pradigastat, and volanesorsen primarily target elevated triglycerides, whereas evinacumab and IONIS-ANGPTL3-L_Rx target both LDL cholesterol and triglyceride. IONIS-APO(a)-L_Rx targets Lp(a).

Harnessing nucleic acid-based therapeutics for atherosclerotic cardiovascular disease: state of the art

Dyslipidemia is one of the major but modifiable risk factors for atherosclerotic cardiovascular disease (ACVD). Despite the accessibility of statins and other lipid-lowering drugs, the burden of ACVD is still high globally, highlighting the need for new therapeutic approaches. Nucleic acid-based technologies, including antisense oligonucleotides (ASOs), small interfering (si)RNAs, miRNAs, and decoys, are emerging therapeutic modalities for the treatment of ACVD. These technologies aim to degrade gene mRNA transcripts to decrease the levels of atherogenic lipoproteins. Using gene-silencing approaches, the levels of atherogenic lipoproteins can be decreased by targeting proteins that have key roles in lipoprotein metabolism. Here, we highlight preclinical and clinical findings using these approaches for the development of novel therapies against ACVD.

Identity-by-Descent Mapping Identifies Major Locus for Serum Triglycerides in Amerindians Largely Explained by an APOC3 Founder Mutation

BACKGROUND

Identity-by-descent mapping using empirical estimates of identity-by-descent allele sharing may be useful for studies of complex traits in founder populations, where hidden relationships may augment the inherent genetic information that can be used for localization.

METHODS AND RESULTS

Through identity-by-descent mapping, using ≈400 000 single-nucleotide polymorphisms (SNPs), of serum lipid profiles, we identified a major linkage signal for triglycerides in 1007 Pima Indians (LOD=9.23; P=3.5×10^-11 on chromosome 11q). In subsequent fine-mapping and replication association studies in ≈7500 Amerindians, we determined that this signal reflects effects of a loss-of-function Ala43Thr substitution in APOC3 (rs147210663) and 3 established functional SNPs in APOA5. The association with rs147210663 was particularly strong; each copy of the Thr allele conferred 42% lower triglycerides (β=-0.92±0.059 SD unit; P=9.6×10^-55 in 4668 Pimas and 2793 Southwest Amerindians combined). The Thr allele is extremely rare in most global populations but has a frequency of 2.5% in Pimas. We further demonstrated that 3 APOA5 SNPs with established functional impact could explain the association with the most well-replicated SNP (rs964184) for triglycerides identified by genome-wide association studies. Collectively, these 4 SNPs account for 6.9% of variation in triglycerides in Pimas (and 4.1% in Southwest Amerindians), and their inclusion in the original linkage model reduced the linkage signal to virtually null.

CONCLUSIONS

APOC3/APOA5 constitutes a major locus for serum triglycerides in Amerindians, especially the Pimas, and these results provide an empirical example for the concept that population-based linkage analysis is a useful strategy to identify complex trait variants.

New medications targeting triglyceride-rich lipoproteins: Can inhibition of ANGPTL3 or apoC-III reduce the residual cardiovascular risk?

Remarkably good results have been achieved in the treatment of atherosclerotic cardiovascular diseases (CVD) by using statin, ezetimibe, antihypertensive, antithrombotic, and PCSK9 inhibitor therapies and their proper combinations. However, despite this success, the remaining CVD risk is still high. To target this residual risk and to treat patients who are statin-intolerant or have an exceptionally high CVD risk for instance due to familial hypercholesterolemia (FH), new therapies are intensively sought. One pathway of drug development is targeting the circulating triglyceride-rich lipoproteins (TRL) and their lipolytic remnants, which, according to the current view, confer a major CVD risk. Angiopoietin-like protein 3 (ANGPTL3) and apolipoprotein C-III (apoC-III) are at present the central molecular targets for therapies designed to reduce TRL, and there are new drugs emerging that suppress their expression or inhibit the function of these two key proteins. The medications targeting these components are biological, either human monoclonal antibodies or antisense oligonucleotides. In this article, we briefly review the mechanisms of action of ANGPTL3 and apoC-III, the reasons why they have been considered promising targets of novel therapies for CVD, as well as the current status and the most important results of their clinical trials.

Update on the laboratory investigation of dyslipidemias

The role of the clinical laboratory is evolving to provide more information to clinicians to assess cardiovascular disease (CVD) risk and target therapy more effectively. Current routine methods to measure LDL-cholesterol (LDL-C), the Friedewald calculation, ultracentrifugation, electrophoresis and homogeneous direct methods have established limitations. Studies suggest that LDL and HDL size or particle concentration are alternative methods to predict future CVD risk. At this time there is no consensus role for lipoprotein particle or subclasses in CVD risk assessment. LDL and HDL particle concentration are measured by several methods, namely gradient gel electrophoresis, ultracentrifugation-vertical auto profile, nuclear magnetic resonance and ion mobility. It has been suggested that HDL functional assays may be better predictors of CVD risk. To assess the issue of lipoprotein subclasses/particles and HDL function as potential CVD risk markers robust, simple, validated analytical methods are required. In patients with small dense LDL particles, even a perfect measure of LDL-C will not reflect LDL particle concentration. Non-HDL-C is an alternative measurement and includes VLDL and CM remnant cholesterol and LDL-C. However, apolipoprotein B measurement may more accurately reflect LDL particle numbers. Non-fasting lipid measurements have many practical advantages. Defining thresholds for treatment with new measurements of CVD risk remain a challenge. In families with genetic variants, ApoCIII and lipoprotein (a) may be additional risk factors. Recognition of familial causes of dyslipidemias and diagnosis in childhood will result in early treatment. This review discusses the limitations in current laboratory technologies to predict CVD risk and reviews the evidence for emergent approaches using newer biomarkers in clinical practice.

Farnesoid X receptor: A "homeostat" for hepatic nutrient metabolism

The Farnesoid X receptor (FXR) is a nuclear receptor activated by bile acids (BAs). BAs are amphipathic molecules that serve as fat solubilizers in the intestine under postprandial conditions. In the post-absorptive state, BAs bind FXR in the hepatocytes, which in turn provides feedback signals on BA synthesis and transport and regulates lipid, glucose and amino acid metabolism. Therefore, FXR acts as a homeostat of all three classes of nutrients, fats, sugars and proteins. Here we re-analyze the function of FXR in the perspective of nutritional metabolism, and discuss the role of FXR in liver energy homeostasis in postprandial, post-absorptive and fasting/starvation states. FXR, by regulating nutritional metabolism, represses autophagy in conditions of nutrient abundance, and controls the metabolic needs of proliferative cells. In addition, FXR regulates inflammation via direct effects and via its impact on nutrient metabolism. These functions indicate that FXR is an attractive therapeutic target for liver diseases.

The apolipoprotein C-III (Gln38Lys) variant associated with human hypertriglyceridemia is a gain-of-function mutation

Recent cell culture and animal studies have suggested that expression of human apo C-III in the liver has a profound impact on the triacylglycerol (TAG)-rich VLDL₁ production under lipid-rich conditions. The apoC-III Gln38Lys variant was identified in subjects of Mexican origin with moderate hypertriglyceridemia. We postulated that Gln38Lys (C3^QK), being a gain-of-function mutation, promotes hepatic VLDL₁ assembly/secretion. To test this hypothesis, we expressed C3^QK in McA-RH7777 cells and apoc3-null mice to contrast its effect with WT apoC-III (C3^WT). In both model systems, C3^QK expression increased the secretion of VLDL₁-TAG (by 230%) under lipid-rich conditions. Metabolic labeling experiments with C3^QK cells showed an increase in de novo lipogenesis (DNL). Fasting plasma concentration of TAG, cholesterol, cholesteryl ester, and FA were increased in C3^QK mice as compared with C3^WT mice. Liver of C3^QK mice also displayed an increase in DNL and expression of lipogenic genes as compared with that in C3^WT mice. These results suggest that C3^QK variant is a gain-of-function mutation that can stimulate VLDL₁ production, through enhanced DNL.

A human APOC3 missense variant and monoclonal antibody accelerate apoC-III clearance and lower triglyceride-rich lipoprotein levels

Recent large-scale genetic sequencing efforts have identified rare coding variants in genes in the triglyceride-rich lipoprotein (TRL) clearance pathway that are protective against coronary heart disease (CHD), independently of LDL cholesterol (LDL-C) levels. Insight into the mechanisms of protection of these variants may facilitate the development of new therapies for lowering TRL levels. The gene APOC3 encodes apoC-III, a critical inhibitor of triglyceride (TG) lipolysis and remnant TRL clearance. Here we report a detailed interrogation of the mechanism of TRL lowering by the APOC3 Ala43Thr (A43T) variant, the only missense (rather than protein-truncating) variant in APOC3 reported to be TG lowering and protective against CHD. We found that both human APOC3 A43T heterozygotes and mice expressing human APOC3 A43T display markedly reduced circulating apoC-III levels. In mice, this reduction is due to impaired binding of A43T apoC-III to lipoproteins and accelerated renal catabolism of free apoC-III. Moreover, the reduced content of apoC-III in TRLs resulted in accelerated clearance of circulating TRLs. On the basis of this protective mechanism, we developed a monoclonal antibody targeting lipoprotein-bound human apoC-III that promotes circulating apoC-III clearance in mice expressing human APOC3 and enhances TRL catabolism in vivo. These data reveal the molecular mechanism by which a missense variant in APOC3 causes reduced circulating TG levels and, hence, protects from CHD. This protective mechanism has the potential to be exploited as a new therapeutic approach to reduce apoC-III levels and circulating TRL burden.

Targeting ApoC-III to Reduce Coronary Disease Risk

Triglyceride-rich lipoproteins (TRLs) are causal contributors to the risk of developing coronary artery disease (CAD). Apolipoprotein C-III (apoC-III) is a component of TRLs that elevates plasma triglycerides (TGs) through delaying the lipolysis of TGs and the catabolism of TRL remnants. Recent human genetics approaches have shown that heterozygous loss-of-function mutations in APOC3, the gene encoding apoC-III, lower plasma TGs and protect from CAD. This observation has spawned new interest in therapeutic efforts to target apoC-III. Here, we briefly review both currently available as well as developing therapies for reducing apoC-III levels and function to lower TGs and cardiovascular risk. These therapies include existing options including statins, fibrates, thiazolidinediones, omega-3-fatty acids, and niacin, as well as an antisense oligonucleotide targeting APOC3 currently in clinical development. We review the mechanisms of action by which these drugs reduce apoC-III and the current understanding of how reduction in apoC-III may impact CAD risk.

A Novel Role for RARα Agonists as Apolipoprotein CIII Inhibitors Identified from High Throughput Screening

Elevated triglyceride (TG) levels are well-correlated with the risk for cardiovascular disease (CVD). Apolipoprotein CIII (ApoC-III) is a key regulator of plasma TG levels through regulation of lipolysis and lipid synthesis. To identify novel regulators of TG levels, we carried out a high throughput screen (HTS) using an ApoC-III homogenous time resolved fluorescence (HTRF) assay. We identified several retinoic acid receptor (RAR) agonists that reduced secreted ApoC-III levels in human hepatic cell lines. The RARα specific agonist AM580 inhibited secreted ApoC-III by >80% in Hep3B cells with an EC₅₀ ~2.9 nM. In high-fat diet induced fatty-liver mice, AM580 reduced ApoC-III levels in liver as well as in plasma (~60%). In addition, AM580 treatment effectively reduced body weight, hepatic and plasma TG, and total cholesterol (TC) levels. Mechanistically, AM580 suppresses ApoC-III synthesis by downregulation of HNF4α and upregulation of SHP1 expression. Collectively, these studies suggest that an RARα specific agonist may afford a new strategy for lipid-lowering and CVD risk reduction.

Alleviating VLDL overproduction is an important mechanism for Laminaria japonica polysaccharide to inhibit atherosclerosis in LDLr-/- mice with diet-induced insulin resistance

SCOPE

The overproduction of very low density lipoprotein (VLDL) is an important cause for initiation and development of atherosclerosis, which is highly associated with insulin signaling. The aim of this work is to verify whether the inhibition of VLDL overproduction is an underlying mechanism for a Laminaria japonica polysaccharide (LJP61A (where LJP is L. japonica)) to resist atherosclerosis.

METHODS AND RESULTS

LJP61A (50 and 200 mg/kg/day) was orally administered to a high-fat diet (HFD)-fed LDL receptor deficient mice for 14 weeks. LJP61A significantly attenuated insulin resistance, hepatic steatosis, atherosclerosis, and dyslipidemia. Meanwhile, LJP61A ameliorated the HFD-induced impairment of hepatic insulin signaling and reduced VLDL overproduction via regulating the expression of genes involved in the assembly and secretion of VLDL. To study the possibility that the inhibition of mammalian target of rapamycin complex 1 and stimulation of Forkhead box protein O1 (Foxo1) nuclear exclusion is a result of LJP61A via regulating insulin signaling, LJP61A was administrated to HepG2 cells in the presence or absence of mTOR inhibitor and Foxo1 inhibitor. Results showed that LJP61A alleviated VLDL overproduction via regulating insulin receptor substrate mediated phosphatidylinositide 3-kinase AKT mammalian target of rapamycin complex 1 and phosphatidylinositide 3-kinase AKT-Foxo1 signaling pathways.

CONCLUSION

These results suggested that LJP61A ameliorated HFD-induced insulin resistance to attenuate VLDL overproduction possibly via regulating insulin signaling, leading to the inhibition of atherosclerosis.

Aromatic residues in the C terminus of apolipoprotein C-III mediate lipid binding and LPL inhibition

Plasma apoC-III levels correlate with triglyceride (TG) levels and are a strong predictor of CVD outcomes. ApoC-III elevates TG in part by inhibiting LPL. ApoC-III likely inhibits LPL by competing for lipid binding. To probe this, we used oil-drop tensiometry to characterize binding of six apoC-III variants to lipid/water interfaces. This technique monitors the dependence of lipid binding on surface pressure, which increases during TG hydrolysis by LPL. ApoC-III adsorption increased surface pressure by upward of 18 mN/m at phospholipid/TG/water interfaces. ApoC-III was retained to high pressures at these interfaces, desorbing at 21-25 mN/m. Point mutants, which substituted alanine for aromatic residues, impaired the lipid binding of apoC-III. Adsorption and retention pressures decreased by 1-6 mN/m in point mutants, with the magnitude determined by the location of alanine substitutions. Trp42 was most critical to mediating lipid binding. These results strongly correlate with our previous results, linking apoC-III point mutants to increased LPL binding and activity at lipid surfaces. We propose that aromatic residues in the C-terminal half of apoC-III mediate binding to TG-rich lipoproteins. Increased apoC-III expression in the hypertriglyceridemic state allows apoC-III to accumulate on lipoproteins and inhibit LPL by preventing binding and/or access to substrate.

APOC3 Protein Is Not a Predisposing Factor for Fat-induced Nonalcoholic Fatty Liver Disease in Mice

Nonalcoholic fatty liver disease (NAFLD), characterized by excessive fat accumulation in liver, is prevalent in obesity. Genetic factors that link obesity to NAFLD remain obscure. Apolipoprotein C3 (APOC3) is a lipid-binding protein with a pivotal role in triglyceride metabolism. Humans with APOC3 gain-of-function mutations and mice with APOC3 overproduction are associated with hypertriglyceridemia. Nonetheless, it remains controversial whether APOC3 is culpable for diet-induced NAFLD. To address this fundamental issue, we fed APOC3-transgenic and wild-type littermates a high fructose diet or high fat diet, followed by determination of the effect of APOC3 on hepatic lipid metabolism and inflammation and the progression of NAFLD. To gain mechanistic insight into NAFLD, we determined the impact of APOC3 on hepatic triglyceride synthesis and secretion versus fatty acid oxidation. APOC3-transgenic mice were hypertriglyceridemic, culminating in marked elevation of triglycerides, cholesterols, and non-esterified fatty acids in plasma. Despite the prevailing hypertriglyceridemia, APOC3-transgenic mice, relative to wild-type littermates, had similar weight gain and hepatic lipid content without alterations in hepatic expression of key genes involved in triglyceride synthesis and secretion and fatty acid oxidation. APOC3-transgenic and wild-type mice had similar Kupffer cell content without alterations in hepatic expression of pro- and anti-inflammatory cytokines. APOC3 neither exacerbated diet-induced adiposity nor aggravated the degree of steatosis in high fructose or high fat-fed APOC3-transgenic mice. These effects ensued independently of weight gain even after 10-month high fat feeding. We concluded that APOC3, whose dysregulation is liable for hypertriglyceridemia, is not a predisposing factor for linking overnutrition to NAFLD in obesity.

Metabolic Characterization of a Rare Genetic Variation Within APOC3 and Its Lipoprotein Lipase-Independent Effects

BACKGROUND

Plasma triglyceride levels have been implicated in atherosclerosis and coronary heart disease. Apolipoprotein C-III (APOC3) plays a key role in the hydrolysis of triglyceride-rich lipoproteins to remnant particles by lipoprotein lipase (LPL) and their uptake by the liver. A rare variant in APOC3(rs138326449) has been associated with triglyceride, very low-density lipoprotein, and high-density lipoprotein levels, as well as risk of coronary heart disease. We aimed to characterize the impact of this locus across a broad set of mainly lipids-focused metabolic measures.

METHODS AND RESULTS

A high-throughput serum nuclear magnetic resonance metabolomics platform was used to quantify 225 metabolic measures in 13 285 participants from 2 European population cohorts. We analyzed the effect of the APOC3 variant on the metabolic measures and used the common LPL(rs12678919) polymorphism to test for LPL-independent effects. Eighty-one metabolic measures showed evidence of association with APOC3(rs138326449). In addition to previously reported triglyceride and high-density lipoprotein associations, the variant was also associated with very low-density lipoprotein and high-density lipoprotein composition measures, other cholesterol measures, and fatty acids. Comparison of the APOC3 and LPL associations revealed that APOC3 association results for medium and very large very low-density lipoprotein composition are unlikely to be solely predictable by the action of APOC3 through LPL.

CONCLUSIONS

We characterized the effects of the rare APOC3(rs138326449) loss of function mutation in lipoprotein metabolism, as well as the effects of LPL(rs12678919). Our results improve our understanding of the role of APOC3 in triglyceride metabolism, its LPL independent action, and the complex and correlated nature of human metabolites.

Sugar consumption, metabolic disease and obesity: The state of the controversy

The impact of sugar consumption on health continues to be a controversial topic. The objective of this review is to discuss the evidence and lack of evidence that allows the controversy to continue, and why resolution of the controversy is important. There are plausible mechanisms and research evidence that supports the suggestion that consumption of excess sugar promotes the development of cardiovascular disease (CVD) and type 2 diabetes (T2DM) both directly and indirectly. The direct pathway involves the unregulated hepatic uptake and metabolism of fructose, leading to liver lipid accumulation, dyslipidemia, decreased insulin sensitivity and increased uric acid levels. The epidemiological data suggest that these direct effects of fructose are pertinent to the consumption of the fructose-containing sugars, sucrose and high fructose corn syrup (HFCS), which are the predominant added sugars. Consumption of added sugar is associated with development and/or prevalence of fatty liver, dyslipidemia, insulin resistance, hyperuricemia, CVD and T2DM, often independent of body weight gain or total energy intake. There are diet intervention studies in which human subjects exhibited increased circulating lipids and decreased insulin sensitivity when consuming high sugar compared with control diets. Most recently, our group has reported that supplementing the ad libitum diets of young adults with beverages containing 0%, 10%, 17.5% or 25% of daily energy requirement (Ereq) as HFCS increased lipid/lipoprotein risk factors for CVD and uric acid in a dose-response manner. However, un-confounded studies conducted in healthy humans under a controlled, energy-balanced diet protocol that enables determination of the effects of sugar with diets that do not allow for body weight gain are lacking. Furthermore, recent reports conclude that there are no adverse effects of consuming beverages containing up to 30% Ereq sucrose or HFCS, and the conclusions from several meta-analyses suggest that fructose has no specific adverse effects relative to any other carbohydrate. Consumption of excess sugar may also promote the development of CVD and T2DM indirectly by causing increased body weight and fat gain, but this is also a topic of controversy. Mechanistically, it is plausible that fructose consumption causes increased energy intake and reduced energy expenditure due to its failure to stimulate leptin production. Functional magnetic resonance imaging (fMRI) of the brain demonstrates that the brain responds differently to fructose or fructose-containing sugars compared with glucose or aspartame. Some epidemiological studies show that sugar consumption is associated with body weight gain, and there are intervention studies in which consumption of ad libitum high-sugar diets promoted increased body weight gain compared with consumption of ad libitum low- sugar diets. However, there are no studies in which energy intake and weight gain were compared in subjects consuming high or low sugar, blinded, ad libitum diets formulated to ensure both groups consumed a comparable macronutrient distribution and the same amounts of fiber. There is also little data to determine whether the form in which added sugar is consumed, as beverage or as solid food, affects its potential to promote weight gain. It will be very challenging to obtain the funding to conduct the clinical diet studies needed to address these evidence gaps, especially at the levels of added sugar that are commonly consumed. Yet, filling these evidence gaps may be necessary for supporting the policy changes that will help to turn the food environment into one that does not promote the development of obesity and metabolic disease.

Aggravated restenosis and atherogenesis in ApoCIII transgenic mice but lack of protection in ApoCIII knockouts: the effect of authentic triglyceride-rich lipoproteins with and without ApoCIII

AIM

Previously, our group and others have demonstrated a causative relationship between severe hypertriglyceridaemia and atherogenesis in mice. Furthermore, clinical investigations have shown high levels of plasma Apolipoprotein C-III (ApoCIII) associated with hypertriglyceridaemia and even cardiovascular disease. However, it remains unclear whether ApoCIII affects restenosis in vivo, and whether such an effect is mediated by ApoCIII alone, or in combination with hypertriglyceridaemia. We sought to investigate ApoCIII in restenosis and clarify how smooth muscle cells (SMCs) respond to authentic triglyceride-rich lipoproteins (TRLs) with or without ApoCIII (TRLs ± ApoCIII).

METHODS AND RESULTS

ApoCIII transgenic (ApoCIIItg) and knockout (ApoCIII-/-) mice underwent endothelial denudation to model restenosis. Here, ApoCIIItg mice displayed severe hypertriglyceridaemia and increased neointimal formation compared with wild-type (WT) or ApoCIII-/- mice. Furthermore, increased proliferating cell nuclear antigen (PCNA)-positive cells, Mac-3, and vascular cell adhesion protein-1 (VCAM-1) expression, and 4-hydroxynonenal (4HNE) production were found in lesion sites. ApoCIIItg and ApoCIII-/- mice were then crossed to low-density lipoprotein receptor-deficient (Ldlr-/-) mice and fed an atherogenic diet. ApoCIIItg/Ldlr-/- mice had significantly increased atherosclerotic lesions. However, there was no statistical difference in restenosis between ApoCIII-/- and WT mice, and in atherosclerosis between ApoCIII/Ldlr double knockout and Ldlr-/- mice. SMCs were then incubated in vitro with authentic TRLs ± ApoCIII isolated from extreme hypertriglyceridaemia glycosylphosphatidylinositol-anchored high-density lipoprotein-binding protein 1-deficient (GPIHBP1-/-) mice crossed with ApoCIIItg or ApoCIII-/- mice. It was shown that TRLs + ApoCIII promoted SMC proliferation, VCAM-1 expression, and reactive oxygen species (ROS) production, and activated the Akt pathway. Scavenging ROS significantly reduced SMC activation caused by TRLs + ApoCIII.

CONCLUSIONS

Severe hypertriglyceridaemia resulting from ApoCIII overexpression promotes restenosis and atherosclerosis. Furthermore, we demonstrated that TRLs + ApoCIII promotes SMC proliferation.

Recent advances in physiological lipoprotein metabolism

Research into lipoprotein metabolism has developed because understanding lipoprotein metabolism has important clinical indications. Lipoproteins are risk factors for cardiovascular disease. Recent advances include the identification of factors in the synthesis and secretion of triglyceride rich lipoproteins, chylomicrons (CM) and very low density lipoproteins (VLDL). These included the identification of microsomal transfer protein, the cotranslational targeting of apoproteinB (apoB) for degradation regulated by the availability of lipids, and the characterization of transport vesicles transporting primordial apoB containing particles to the Golgi. The lipase maturation factor 1, glycosylphosphatidylinositol-anchored high density lipoprotein binding protein 1 and an angiopoietin-like protein play a role in lipoprotein lipase (LPL)-mediated hydrolysis of secreted CMs and VLDL so that the right amount of fatty acid is delivered to the right tissue at the right time. Expression of the low density lipoprotein (LDL) receptor is regulated at both transcriptional and post-transcriptional level. Proprotein convertase subtilisin/kexin type 9 (PCSK9) has a pivotal role in the degradation of LDL receptor. Plasma remnant lipoproteins bind to specific receptors in the liver, the LDL receptor, VLDL receptor and LDL receptor-like proteins prior to removal from the plasma. Reverse cholesterol transport occurs when lipid free apoAI recruits cholesterol and phospholipid to assemble high density lipoprotein (HDL) particles. The discovery of ABC transporters (ABCA1 and ABCG1) and scavenger receptor class B type I (SR-BI) provided further information on the biogenesis of HDL. In humans HDL-cholesterol can be returned to the liver either by direct uptake by SR-BI or through cholesteryl ester transfer protein exchange of cholesteryl ester for triglycerides in apoB lipoproteins, followed by hepatic uptake of apoB containing particles. Cholesterol content in cells is regulated by several transcription factors, including the liver X receptor and sterol regulatory element binding protein. This review summarizes recent advances in knowledge of the molecular mechanisms regulating lipoprotein metabolism.

Associations of the APOC3 rs5128 polymorphism with plasma APOC3 and lipid levels: a meta-analysis

BACKGROUND

Studies of the association between the apolipoprotein C3 gene (APOC3) rs5128 polymorphism and plasma levels of apolipoprotein C3 (APOC3) and lipids have reported apparently conflicting findings. This meta-analysis aimed to investigate the associations of the rs5128 polymorphism with fasting APOC3 and lipid levels.

METHODS

The following information was abstracted for each study: ethnicity, age, sex, health condition, sample size, genotyping and lipid assay methods, mean and standard deviation or standard error by genotypes for APOC3 and lipid variables. There were 42 eligible studies with 23846 subjects included in this meta-analysis. A dominant model was used for this meta-analysis.

RESULTS

The results showed that the carriers of the variant allele G had higher levels of APOC3 [standardized mean difference (SMD): 0.22, 95% confidence interval (CI): 0.12-0.31, P<0.00001], triglycerides (TG) (SMD: 0.33, 95% CI: 0.23-0.44, P<0.00001), total cholesterol (TC) (SMD: 0.15, 95% CI: 0.09-0.22, P<0.00001), and low-density lipoprotein cholesterol (LDL-C) (SMD: 0.11, 95% CI: 0.04-0.17, P=0.001) than the non-carriers. No significant association between the APOC3 rs5128 polymorphism and lower levels of high-density lipoprotein cholesterol (HDL-C) was detected under the dominant model (SMD: -0.03, 95% CI: -0.06-0.01, P=0.156).

CONCLUSIONS

The results from the present meta-analysis demonstrate a significant association between the APOC3 rs5128 polymorphism and higher levels of APOC3, TG, TC and LDL-C, but further studies are needed to elucidate the underlying mechanisms.

Apolipoprotein C-III: a potent modulator of hypertriglyceridemia and cardiovascular disease

PURPOSE OF REVIEW

The purpose of this article is to summarize the recent epidemiological, basic science, and pharmaceutical research linking apolipoprotein C-III (apoC-III) with the development and treatment of cardiovascular disease (CVD).

RECENT FINDINGS

ApoC-III is an important emerging target linking hypertriglyceridemia with CVD. ApoC-III is a potent modulator of many established CVD risk factors, and is found on chylomicrons, very-low density lipoprotein, low-density lipoprotein, and high-density lipoprotein particles. Recent studies show that in humans, apoC-III levels are an independent risk factor for CVD, and its presence on lipoproteins may promote their atherogenicity. This year, two large-scale epidemiological studies have linked mutations in apoC-III with increased incidence of CVD and hypertriglyceridemia. ApoC-III raises plasma triglycerides through inhibition of lipoprotein lipase, stimulation of very-low density lipoprotein secretion, and is a novel factor in modulating intestinal triglyceride trafficking. ApoC-III also stimulates inflammatory processes in the vasculature and the pancreas. The combination of raising plasma triglycerides and independently stimulating inflammatory processes makes apoC-III a valuable target for reducing the residual CVD risk in patients already on statin therapy, or for whom triglycerides are poorly controlled. Clinical trials on apoC-III antisense oligonucleotides are in progress.

SUMMARY

ApoC-III is a potent direct modulator of established CVD risk factors: plasma triglycerides and inflammation. Recent findings show that changes in apoC-III levels are directly associated with changes in cardiovascular risk and the atherogenicity of the lipoproteins on which apoC-III resides. Emerging roles of apoC-III include a role in directing the atherogenicity of high-density lipoprotein, intestinal dietary triglyceride trafficking, and modulating pancreatic β-cell survival. The combination of these roles makes apoC-III an important therapeutic target for the management and prevention of CVD.

New insights into the pathophysiology of dyslipidemia in type 2 diabetes

Cardiovascular disease (CVD) remains the leading cause of morbidity and mortality for patients with type 2 diabetes, despite recent significant advances in management strategies to lessen CVD risk factors. A major cause is the atherogenic dyslipidemia, which consists of elevated plasma concentrations of both fasting and postprandial triglyceride-rich lipoproteins (TRLs), small dense low-density lipoprotein (LDL) and low high-density lipoprotein (HDL) cholesterol. The different components of diabetic dyslipidemia are not isolated abnormalities but closely linked to each other metabolically. The underlying disturbances are hepatic overproduction and delayed clearance of TRLs. Recent results have unequivocally shown that triglyceride-rich lipoproteins and their remnants are atherogenic. To develop novel strategies for the prevention and treatment of dyslipidaemia, it is essential to understand the pathophysiology of dyslipoproteinaemia in humans. Here, we review recent advances in our understanding of the pathophysiology of diabetic dyslipidemia.

Recent advances in pharmacotherapy for hypertriglyceridemia

Elevated plasma triglyceride (TG) concentrations are associated with an increased risk of atherosclerotic cardiovascular disease (CVD), hepatic steatosis and pancreatitis. Existing pharmacotherapies, such as fibrates, n-3 polyunsaturated fatty acids (PUFAs) and niacin, are partially efficacious in correcting elevated plasma TG. However, several new TG-lowering agents are in development that can regulate the transport of triglyceride-rich lipoproteins (TRLs) by modulating key enzymes, receptors or ligands involved in their metabolism. Balanced dual peroxisome proliferator-activated receptor (PPAR) α/γ agonists, inhibitors of microsomal triglyceride transfer protein (MTTP) and acyl-CoA:diacylglycerol acyltransferase-1 (DGAT-1), incretin mimetics, and apolipoprotein (apo) B-targeted antisense oligonucleotides (ASOs) can all decrease the production and secretion of TRLs; inhibitors of cholesteryl ester transfer protein (CETP) and angiopoietin-like proteins (ANGPTLs) 3 and 4, monoclonal antibodies (Mabs) against proprotein convertase subtilisin/kexin type 9 (PCSK9), apoC-III-targeted ASOs, selective peroxisome proliferator-activated receptor modulators (SPPARMs), and lipoprotein lipase (LPL) gene replacement therapy (alipogene tiparvovec) enhance the catabolism and clearance of TRLs; dual PPAR-α/δ agonists and n-3 polyunsaturated fatty acids can lower plasma TG by regulating both TRL secretion and catabolism. Varying degrees of TG reduction have been reported with the use of these therapies, and for some agents such as CETP inhibitors and PCSK9 Mabs findings have not been consistent. Whether they reduce CVD events has not been established. Trials investigating the effect of CETP inhibitors (anacetrapib and evacetrapib) and PCSK9 Mabs (AMG-145 and REGN727/SAR236553) on CVD outcomes are currently in progress, although these agents also regulate LDL metabolism and, in the case of CETP inhibitors, HDL metabolism. Further to CVD risk reduction, these new treatments might also have a potential role in the management of diabetes and non-alcoholic fatty liver disease owing to their insulin-sensitizing action (PPAR-α/γ agonists) and potential capacity to decrease hepatic TG accumulation (PPAR-α/δ agonists and DGAT-1 inhibitors), but this needs to be tested in future trials. We summarize the clinical trial findings regarding the efficacy and safety of these novel therapies for hypertriglyceridemia.

Loss-of-function mutations in APOC3, triglycerides, and coronary disease

BACKGROUND

Plasma triglyceride levels are heritable and are correlated with the risk of coronary heart disease. Sequencing of the protein-coding regions of the human genome (the exome) has the potential to identify rare mutations that have a large effect on phenotype.

METHODS

We sequenced the protein-coding regions of 18,666 genes in each of 3734 participants of European or African ancestry in the Exome Sequencing Project. We conducted tests to determine whether rare mutations in coding sequence, individually or in aggregate within a gene, were associated with plasma triglyceride levels. For mutations associated with triglyceride levels, we subsequently evaluated their association with the risk of coronary heart disease in 110,970 persons.

RESULTS

An aggregate of rare mutations in the gene encoding apolipoprotein C3 (APOC3) was associated with lower plasma triglyceride levels. Among the four mutations that drove this result, three were loss-of-function mutations: a nonsense mutation (R19X) and two splice-site mutations (IVS2+1G→A and IVS3+1G→T). The fourth was a missense mutation (A43T). Approximately 1 in 150 persons in the study was a heterozygous carrier of at least one of these four mutations. Triglyceride levels in the carriers were 39% lower than levels in noncarriers (P<1×10(-20)), and circulating levels of APOC3 in carriers were 46% lower than levels in noncarriers (P=8×10(-10)). The risk of coronary heart disease among 498 carriers of any rare APOC3 mutation was 40% lower than the risk among 110,472 noncarriers (odds ratio, 0.60; 95% confidence interval, 0.47 to 0.75; P=4×10(-6)).

CONCLUSIONS

Rare mutations that disrupt APOC3 function were associated with lower levels of plasma triglycerides and APOC3. Carriers of these mutations were found to have a reduced risk of coronary heart disease. (Funded by the National Heart, Lung, and Blood Institute and others.).

Loss-of-function mutations in APOC3 and risk of ischemic vascular disease

BACKGROUND

High plasma levels of nonfasting triglycerides are associated with an increased risk of ischemic cardiovascular disease. Whether lifelong low levels of nonfasting triglycerides owing to mutations in the gene encoding apolipoprotein C3 (APOC3) are associated with a reduced risk of ischemic cardiovascular disease in the general population is unknown.

METHODS

Using data from 75,725 participants in two general-population studies, we first tested whether low levels of nonfasting triglycerides were associated with reduced risks of ischemic vascular disease and ischemic heart disease. Second, we tested whether loss-of-function mutations in APOC3, which were associated with reduced levels of nonfasting triglycerides, were also associated with reduced risks of ischemic vascular disease and ischemic heart disease. During follow-up, ischemic vascular disease developed in 10,797 participants, and ischemic heart disease developed in 7557 of these 10,797 participants.

RESULTS

Participants with nonfasting triglyceride levels of less than 1.00 mmol per liter (90 mg per deciliter) had a significantly lower incidence of cardiovascular disease than those with levels of 4.00 mmol per liter (350 mg per deciliter) or more (hazard ratio for ischemic vascular disease, 0.43; 95% confidence interval [CI], 0.35 to 0.54; hazard ratio for ischemic heart disease, 0.40; 95% CI, 0.31 to 0.52). Heterozygosity for loss-of-function mutations in APOC3, as compared with no APOC3 mutations, was associated with a mean reduction in nonfasting triglyceride levels of 44% (P<0.001). The cumulative incidences of ischemic vascular disease and ischemic heart disease were reduced in heterozygotes as compared with noncarriers of APOC3 mutations (P=0.009 and P=0.05, respectively), with corresponding risk reductions of 41% (hazard ratio, 0.59; 95% CI, 0.41 to 0.86; P=0.007) and 36% (hazard ratio, 0.64; 95% CI, 0.41 to 0.99; P=0.04).

CONCLUSIONS

Loss-of-function mutations in APOC3 were associated with low levels of triglycerides and a reduced risk of ischemic cardiovascular disease. (Funded by the European Union and others.).

Analysis of lipid droplets in hepatocytes

The liver plays an important role in triacylglycerol (TG) metabolism. It can store large amounts of TG in cytosolic lipid droplets (CLDs), or it can package TG into very-low density lipoproteins (VLDL) that are secreted from the cell. TG packaged into VLDL is derived from TG stored within the endoplasmic reticulum in lumenal lipid droplets (LLDs). Therefore, the liver contains at least three kinds of LDs that differ in their protein composition, subcellular localization, and function. Hepatic LDs undergo tremendous changes in their size and protein composition depending on the energetic (fasting/feeding) and pathological (viral infection, nonalcoholic fatty liver disease, etc.) states. It is crucial to develop methodologies that allow the isolation and analyses of the various hepatic LDs in order to gain insight into the differential metabolism of these important lipid storage/transport particles in health and disease. Here, we present detailed protocols for the isolation and analysis of CLDs and LLDs and for monitoring CLD dynamics.

Overexpression of apolipoprotein C-III decreases secretion of dietary triglyceride into lymph

Apolipoprotein C‐III (apoC‐III) is not only predominantly synthesized by the liver but also by the small intestine. Because apoC‐III is secreted from the intestine on the chylomicron along with lipid absorption, we questioned whether apoC‐III might play a role in intestinal lipid absorption and/or transport. Using both wild‐type (WT) and apoC‐III transgenic (apoC‐III Tg) mice, we showed that apoC‐III Tg mice have decreased lymphatic lipid transport compared with WT mice in response to an intraduodenal infusion of radiolabeled lipid. This is associated with accumulation of radiolabeled lipids in the luminal compartment of the apoC‐III Tg mice, indicating delayed lipid uptake from the lumen. The total amount of radioactive lipids in the mucosal compartment did not differ between apoC‐III Tg and WT mice, but the lipid distribution analysis indicated a predominance of free fatty acids and monoacylglycerol in the mucosa of apoC‐III Tg mice, implying impaired esterification capacity. Thus, the mechanisms underlying the reduced lymphatic lipid transport in apoC‐III Tg mice involve both a delayed lipid uptake into enterocytes, as well as impaired esterification to form triglyceride in the mucosa. These data document a novel role for apoC‐III in the uptake, re‐esterification, and lymphatic transport of dietary lipids in the intestine.

Apolipoproteins C-I and C-III inhibit lipoprotein lipase activity by displacement of the enzyme from lipid droplets

Apolipoproteins (apo) C-I and C-III are known to inhibit lipoprotein lipase (LPL) activity, but the molecular mechanisms for this remain obscure. We present evidence that either apoC-I or apoC-III, when bound to triglyceride-rich lipoproteins, prevent binding of LPL to the lipid/water interface. This results in decreased lipolytic activity of the enzyme. Site-directed mutagenesis revealed that hydrophobic amino acid residues centrally located in the apoC-III molecule are critical for attachment to lipid emulsion particles and consequently inhibition of LPL activity. Triglyceride-rich lipoproteins stabilize LPL and protect the enzyme from inactivating factors such as angiopoietin-like protein 4 (angptl4). The addition of either apoC-I or apoC-III to triglyceride-rich particles severely diminished their protective effect on LPL and rendered the enzyme more susceptible to inactivation by angptl4. These observations were seen using chylomicrons as well as the synthetic lipid emulsion Intralipid. In the presence of the LPL activator protein apoC-II, more of apoC-I or apoC-III was needed for displacement of LPL from the lipid/water interface. In conclusion, we show that apoC-I and apoC-III inhibit lipolysis by displacing LPL from lipid emulsion particles. We also propose a role for these apolipoproteins in the irreversible inactivation of LPL by factors such as angptl4.

The regulation of ApoB metabolism by insulin

The leading cause of death in diabetic patients is cardiovascular disease. Apolipoprotein B (ApoB)-containing lipoprotein particles, which are secreted and cleared by the liver, are essential for the development of atherosclerosis. Insulin plays a key role in the regulation of ApoB. Insulin decreases ApoB secretion by promoting ApoB degradation in the hepatocyte. In parallel, insulin promotes clearance of circulating ApoB particles by the liver via the low-density lipoprotein receptor (LDLR), LDLR-related protein 1 (LRP1), and heparan sulfate proteoglycans (HSPGs). Consequently, the insulin-resistant state of type 2 diabetes (T2D) is associated with increased secretion and decreased clearance of ApoB. Here, we review the mechanisms by which insulin controls the secretion and uptake of ApoB in normal and diabetic livers.

Two novel mutations in apolipoprotein C3 underlie atheroprotective lipid profiles in families

Apolipoprotein C3 (APOC3) mutations carriers typically display high plasma high-density lipoprotein cholesterol (HDL-C) and low triglycerides (TGs). We set out to investigate the prevalence and clinical consequences of APOC3 mutations in individuals with hyperalphalipoproteinemia. Two novel mutations (c.-13-2A>G and c.55+1G>A) and one known mutation (c.127G>A;p.Ala43Thr) were found. Lipid profiles and apoCIII isoform distributions were measured. c.55+1G>A mutation carriers displayed higher HDL-C percentiles (35.6 ± 35.8 vs 99.0 ± 0, p = 0.002) and lower TGs (0.51 (0.37-0.61) vs 1.42 (1.12-1.81) mmol/l, p = 0.007) and apoCIII levels (4.24 ± 1.57 vs 7.33 ± 3.61 mg/dl, p = 0.18). c.-13-2A>G mutation carriers did not display significantly different HDL-C levels (84.0 ± 30.0 vs 63.7 ± 45.7, p = 0.50), a trend towards lower TGs [0.71 (0.54 to 0.78) vs 0.85 (0.85 to -) mmol/l, p = 0.06] and significantly lower apoCIII levels (3.09 ± 1.08 vs 11.45 ± 1.06 mg/dl, p = 0.003). p.Ala43Thr mutation carriers displayed a trend towards higher HDL-C percentiles (91.2 ± 31.8 vs 41.0 ± 29.7 mmol/l, p = 0.06) and significantly lower TGs [0.58 (0.36-0.63) vs 0.95 (0.71-1.20) mmol/l, p = 0.02] and apoCIII levels (4.92 ± 2.33 vs 6.60 ± 1.60, p = 0.25). Heterozygosity for APOC3 mutations results in high HDL-C and low TGs and apoCIII levels. This favourable lipid profile in patients with genetically low apoCIII levels holds promise for current studies investigating the potential of apoCIII inhibition as a novel therapeutic in cardiovascular disease prevention.

Changes in helical content or net charge of apolipoprotein C-I alter its affinity for lipid/water interfaces

Amphipathic α-helices mediate binding of exchangeable apolipoproteins to lipoproteins. To probe the role of α-helical structure in protein-lipid interactions, we used oil-drop tensiometry to characterize the interfacial behavior of apolipoprotein C-I (apoC-I) variants at triolein/water (TO/W) and 1-palmitoyl-2-oleoylphosphatidylcholine/triolein/water (POPC/TO/W) interfaces. ApoC-I, the smallest apolipoprotein, has two amphipathic α-helices. Mutants had single Pro or Ala substitutions that resulted in large differences in helical content in solution and on phospholipids. The ability of apoC-I to bind TO/W and POPC/TO/W interfaces correlated strongly with α-helical propensity. On binding these interfaces, peptides with higher helical propensity increased surface pressure to a greater extent. Likewise, peptide exclusion pressure at POPC/TO/W interfaces increased with greater helical propensity. ApoC-I retention on TO/W and POPC/TO/W interfaces correlated strongly with phospholipid-bound helical content. On compression of these interfaces, peptides with higher helical content were ejected at higher pressures. Substitution of Arg for Pro in the N-terminal α-helix altered net charge and reduced apoC-I affinity for POPC/TO/W interfaces. Our results suggest that peptide-lipid interactions drive α-helix binding to and retention on lipoproteins. Point mutations in small apolipoproteins could significantly change α-helical propensity or charge, thereby disrupting protein-lipid interactions and preventing the proteins from regulating lipoprotein catabolism at high surface pressures.

Microsome-associated lumenal lipid droplets in the regulation of lipoprotein secretion

PURPOSE OF REVIEW

Liver is the major organ in mammals that possesses the capacity to release triglyceride within VLDL. VLDL assembly requires apolipoprotein (apo) B-100 with the assistance of microsomal triglyceride transfer protein (MTP), which facilitates the mobilization of triglyceride into the microsomal lumen. Recent experimental evidence has suggested that the lumenal triglyceride associated with endoplasmic reticulum (ER)/Golgi may represent an entity serving as precursors for large VLDL1.

RECENT FINDINGS

Under lipid-rich conditions, discrete triglyceride-rich lipidic bodies, termed lumenal lipid droplets, are accumulated in association with ER/Golgi microsomes. Formation of the microsome-associated lumenal lipid droplets (MALD) is dependent on the activity of MTP, and the resulting apoB-free lipidic body is associated with a variety of proteins including apolipoproteins that are components of VLDL. Formation and utilization of MALD during the assembly and secretion of VLDL1 have a profound influence on hepatic cell physiology, such as ER stress responses.

SUMMARY

This review summarizes current understanding of hepatic triglyceride homeostasis in general, and highlights the functional significance of triglyceride compartmentalization between cytosol and microsomes in particular. Understanding of MALD metabolism may shed new light on the prevention and treatment of liver diseases associated with abnormally elevated intracellular triglycerides.

Lipoprotein metabolism in nonalcoholic fatty liver disease

Nonalcoholic fatty liver disease (NAFLD), an escalating health problem worldwide, covers a spectrum of pathologies characterized by fatty accumulation in hepatocytes in early stages, with potential progression to liver inflammation, fibrosis, and failure. A close, yet poorly understood link exists between NAFLD and dyslipidemia, a constellation of abnormalities in plasma lipoproteins including triglyceride-rich very low density lipoproteins. Apolipoproteins are a group of primarily liver-derived proteins found in serum lipoproteins; they not only play an extracellular role in lipid transport between vital organs through circulation, but also play an important intracellular role in hepatic lipoprotein assembly and secretion. The liver functions as the central hub for lipoprotein metabolism, as it dictates lipoprotein production and to a significant extent modulates lipoprotein clearance. Lipoprotein metabolism is an integral component of hepatocellular lipid homeostasis and is implicated in the pathogenesis, potential diagnosis, and treatment of NAFLD.

Intrahepatic role of exchangeable apolipoproteins in lipoprotein assembly and secretion

Exchangeable apolipoproteins, composed mainly of amphipathic α-helices, are associated with various plasma lipoproteins and play an important role in the metabolism of those lipoproteins to which they bind. Accumulating experimental evidence suggests that exchangeable apolipoproteins, such as apoE, apoA-IV, and apoC-III, also play a role intracellularly in facilitating lipid recruitment at different stages of very low-density lipoprotein assembly and trafficking through the endoplasmic reticulum-Golgi secretory compartments. Experimental evidence also suggests that apoA-I may become lipidated intracellularly through mechanisms dependent on or independent of ATP-binding cassette transporter A1. Thus, expression of these secretory proteins may exert an impact on hepatic triglyceride and cholesterol homeostasis during their transit from the endoplasmic reticulum through the Golgi apparatus. This review summarizes findings related to the modulation of intracellular assembly of very low-density lipoprotein and high-density lipoprotein by exchangeable apolipoproteins.

Apolipoprotein C-III and hepatic triglyceride-rich lipoprotein production

PURPOSE OF REVIEW

A strong positive correlation between plasma apolipoprotein (apo) C-III and triglyceride concentrations has been invariably observed in human and animal studies. The hypertriglyceridemic effect of apo C-III has been conventionally explained by its extracellular roles in inhibiting lipolysis catalysed by lipoprotein lipase and attenuating triglyceride-rich lipoprotein clearance through receptor-dependent and/or independent mechanisms. However, recent experimental evidence suggests that apo C-III may also play an intracellular role in promoting hepatic triglyceride-rich lipoprotein production.

RECENT FINDINGS

Kinetic studies with humans and genetically modified mice have shown that apo C-III is linked with increased production of triglyceride-rich lipoproteins, such as very-low-density lipoprotein 1 (VLDL1). Mutational studies on human apo C-III variants (originally identified in humans with hypotriglyceridemia or hyperalphalipoproteinemia) provide the structure-function analysis of human apo C-III, demonstrating that loss-of-function mutations within human apo C-III impair the assembly and secretion of triglyceride-rich VLDL1 under lipid-rich conditions.

SUMMARY

The current review summarizes recent experimental evidence for an intrahepatic role of human apo C-III in promoting mobilization and utilization of triglyceride during VLDL1 assembly/secretion. Understanding mechanisms by which hepatic apo C-III expression is regulated under insulin resistance and diabetic conditions will lead to better and more rational strategies for the prevention and treatment of diabetic hypertriglyceridemia that is closely related to premature atherosclerosis.

Hepatic ABCA1 and VLDL triglyceride production

Elevated plasma triglyceride (TG) and reduced high density lipoprotein (HDL) concentrations are prominent features of metabolic syndrome (MS) and type 2 diabetes (T2D). Individuals with Tangier disease also have elevated plasma TG concentrations and a near absence of HDL, resulting from mutations in ATP binding cassette transporter A1 (ABCA1), which facilitates the efflux of cellular phospholipid and free cholesterol to assemble with apolipoprotein A-I (apoA-I), forming nascent HDL particles. In this review, we summarize studies focused on the regulation of hepatic very low density lipoprotein (VLDL) TG production, with particular attention on recent evidence connecting hepatic ABCA1 expression to VLDL, LDL, and HDL metabolism. Silencing ABCA1 in McArdle rat hepatoma cells results in diminished assembly of large (>10nm) nascent HDL particles, diminished PI3 kinase activation, and increased secretion of large, TG-enriched VLDL1 particles. Hepatocyte-specific ABCA1 knockout (HSKO) mice have a similar plasma lipid phenotype as Tangier disease subjects, with a two-fold elevation of plasma VLDL TG, 50% lower LDL, and 80% reduction in HDL concentrations. This lipid phenotype arises from increased hepatic secretion of VLDL1 particles, increased hepatic uptake of plasma LDL by the LDL receptor, elimination of nascent HDL particle assembly by the liver, and hypercatabolism of apoA-I by the kidney. These studies highlight a novel role for hepatic ABCA1 in the metabolism of all three major classes of plasma lipoproteins and provide a metabolic link between elevated TG and reduced HDL levels that are a common feature of Tangier disease, MS, and T2D. This article is part of a Special Issue entitled: Triglyceride Metabolism and Disease.

Low triglyceride levels are associated with a better metabolic control in patients with type 1 diabetes

BACKGROUND

Although it is well known in the literature that high triglyceride serum (TG) levels can jeopardize the metabolic control, little is known about the influence of low TG on type 1 diabetes patients (T1D). The aim of this study is to investigate the distribution of TG serum levels in individuals with T1D and its relationship with metabolic control.

FINDINGS

We reviewed the medical charts of 180 patients with T1D, who were classified in groups according to TG levels: 1) low (below 50 mg/dL); 2) normal (50-150 mg/dL); 3) high (above 150 mg/dL). TG were low in 21.1% (n = 38; group 1), normal in 68.6% (n = 123; group 2) and high in 10.6% (n = 19; group 3). High TG was associated with a poor metabolic control (p < 0.001). Patients with TG lower than 50 mg/dL had a lower HbA1c than those with TG between 50 and 150 mg/dL (7.41+/-1.50% vs 8.56%+/-1.94%; p = 0.002).

CONCLUSION

TG lower than 50 mg/dL was common and might be associated with a better metabolic control in patients with T1D, although it is not clear whether the former is the cause or consequence for the latter.

Partner in fat metabolism: role of KLFs in fat burning and reproductive behavior

The abnormalities caused by excess fat accumulation can result in pathological conditions which are linked to several interrelated diseases, such as cardiovascular disease and obesity. This set of conditions, known as metabolic syndrome, is a global pandemic of enormous medical, economic, and social concern affecting a significant portion of the world’s population. Although genetics, physiology and environmental components play a major role in the onset of disease caused by excessive fat accumulation, little is known about how or to what extent each of these factors contributes to it. The worm, Caenorhabditis elegans offers an opportunity to study disease related to metabolic disorder in a developmental system that provides anatomical and genomic simplicity relative to the vertebrate animals and is an excellent eukaryotic genetic model which enable us to answer the questions concerning fat accumulation which remain unresolved. The stored triglycerides (TG) provide the primary source of energy during periods of food deficiency. In nature, lipid stored as TGs are hydrolyzed into fatty acids which are broken down through β-oxidation to yield acetyl-CoA. Our recent study suggests that a member of C. elegans Krüppel-like factor, klf-3 regulates lipid metabolism by promoting FA β-oxidation and in parallel may contribute in normal reproduction and fecundity. Genetic and epigenetic factors that influence this pathway may have considerable impact on fat related diseases in human. Increasing number of studies suggest the role of mammalian KLFs in adipogenesis. This functional conservation should guide our further effort to explore C. elegans as a legitimate model system for studying the role of KLFs in many pathway components of lipid metabolism.

Missense mutation in APOC3 within the C-terminal lipid binding domain of human ApoC-III results in impaired assembly and secretion of triacylglycerol-rich very low density lipoproteins: evidence that ApoC-III plays a major role in the formation of lipid precursors within the microsomal lumen

Hepatic assembly of triacylglycerol (TAG)-rich very low density lipoproteins (VLDL) is achieved through recruitment of bulk TAG (presumably in the form of lipid droplets within the microsomal lumen) into VLDL precursor containing apolipoprotein (apo) B-100. We determined protein/lipid components of lumenal lipid droplets (LLD) in cells expressing recombinant human apoC-III (C3wt) or a mutant form (K58E, C3KE) initially identified in humans that displayed hypotriglyceridemia. Although expression of C3wt markedly stimulated secretion of TAG and apoB-100 as VLDL(1), the K58E mutation (located at the C-terminal lipid binding domain) abolished the effect in transfected McA-RH7777 cells and in apoc3-null mice. Metabolic labeling studies revealed that accumulation of TAG in LLD was decreased (by 50%) in cells expressing C3KE. A Fat Western lipid protein overlay assay showed drastically reduced lipid binding of the mutant protein. Substituting Lys(58) with Arg demonstrated that the positive charge at position 58 is crucial for apoC-III binding to lipid and for promoting TAG secretion. On the other hand, substituting both Lys(58) and Lys(60) with Glu resulted in almost entire elimination of lipid binding and loss of function in promoting TAG secretion. Thus, the lipid binding domain of apoC-III plays a key role in the formation of LLD for hepatic VLDL assembly and secretion.

Secretion of triacylglycerol-poor VLDL particles from McA-RH7777 cells expressing human hepatic lipase

Hepatic lipase (HL) plays a role in the catabolism of apolipoprotein (apo)B-containing lipoproteins through its lipolytic and ligand-binding properties. We describe a potential intracellular role of HL in the assembly and secretion of VLDL. Transient or stable expression of HL in McA-RH7777 cells resulted in decreased (by 40%) incorporation of [(3)H]glycerol into cell-associated and secreted triacylglycerol (TAG) relative to control cells. However, incorporation of [(35)S]methionine/cysteine into cell and medium apoB-100 was not decreased by HL expression. The decreased (3)H-TAG synthesis/secretion in HL expressing cells was not attributable to decreased expression of genes involved in lipogenesis. Fractionation of medium revealed that the decreased [(3)H]TAG from HL expressing cells was mainly attributable to decreased VLDL. Expression of catalytically-inactive HL (HL(SG)) (Ser-145 at the catalytic site was substituted with Gly) in the cells also resulted in decreased secretion of VLDL-[(3)H]TAG. Examination of lumenal contents of microsomes showed a 40% decrease in [(3)H]TAG associated with lumenal lipid droplets in HL or HL(SG) expressing cells as compared with control. The microsomal membrane-associated [(3)H]TAG was decreased by 50% in HL expressing cells but not in HL(SG) expressing cells. Thus, expression of HL, irrespective of its lipolytic function, impairs formation of VLDL precursor [(3)H]TAG in the form of lumenal lipid droplets. These results suggest that HL expression in McA-RH7777 cells result in secretion of [(3)H]TAG-poor VLDL.

Genetic determinants of plasma triglycerides

Plasma triglyceride (TG) concentration is reemerging as an important cardiovascular disease risk factor. More complete understanding of the genes and variants that modulate plasma TG should enable development of markers for risk prediction, diagnosis, prognosis, and response to therapies and might help specify new directions for therapeutic interventions. Recent genome-wide association studies (GWAS) have identified both known and novel loci associated with plasma TG concentration. However, genetic variation at these loci explains only ∼10% of overall TG variation within the population. As the GWAS approach may be reaching its limit for discovering genetic determinants of TG, alternative genetic strategies, such as rare variant sequencing studies and evaluation of animal models, may provide complementary information to flesh out knowledge of clinically and biologically important pathways in TG metabolism. Herein, we review genes recently implicated in TG metabolism and describe how some of these genes likely modulate plasma TG concentration. We also discuss lessons regarding plasma TG metabolism learned from various genomic and genetic experimental approaches. Treatment of patients with moderate to severe hypertriglyceridemia with existing therapies is often challenging; thus, gene products and pathways found in recent genetic research studies provide hope for development of more effective clinical strategies.