Reduction in the risk of MACE with apabetalone in patients with recent acute coronary syndrome and diabetes according to NAFLD fibrosis score: exploratory analysis of the BETonMACE trial

Background/Introduction

Both major adverse cardiovascular events (MACE) and non-alcoholic fatty-liver disease (NAFLD) are highly prevalent in patients with high BMI and long-standing type 2 diabetes (T2DM). NAFLD is characterized by an augmented hepatic inflammation and fat deposition and is strongly associated with metabolic syndrome. Patients with NAFLD are at an increased risk of cardiovascular (CV) events, and MACE is the leading cause of death for patients with NAFLD. Apabetalone (APB) is a novel selective inhibitor of bromodomain and extra-terminal (BET) proteins, epigenetic regulators of gene expression. In the Phase 3 BETonMACE trial treatment of 2,425 T2DM patients post ACS with APB, resulted in hazard ratios (HR) of 0.82 (p=0.11) for the primary endpoint of ischemic MACE (CV death, non-fatal MI or stroke) and 0.59 (p=0.03) for the secondary endpoint of heart failure hospitalization (HFH) vs placebo (PBO). Transient elevations of alanine aminotransferase greater than 5xULN occurred in 3.3% of APB treated patients.

Purpose

In this exploratory post hoc analysis of BETonMACE we evaluated risk modification for a composite of MACE+HFH by APB based on the Angulo NAFLD fibrosis score (FS) using 6 variables (age, BMI, hyperglycemia/diabetes, AST/ALT ratio, platelet count, and albumin). The NAFLD FS categorizes individuals into groups that correlate with differing levels of fibrosis in biopsy studies: (FS F0-F2, no significant fibrosis; FS ID, indeterminant; and FS F3-F4, significant fibrosis).

Methods

Baseline characteristics and blood measurements were used to determine NAFLD FS at baseline. The incidence of MACE+HHF was compared between treatment groups.

Results

Based on FS, there were 618 pts were classified as FS F0-F2 (n=328 APB, n=290 PBO), 1,440 pts were classified as FS ID (n=708 APB, n=732 PBO) and 289 pts were classified as FS F3-F4 (n=144 APB, n=145). MACE+HHF in the PBO group was higher in FS ID and FS F3-F4 compared to FS F0-F2 (17.2% vs 15.0% vs 9.7%) and therefore the former two groups were combined into an elevated risk FS+ group. FS+ pts were older (63 vs 56), had longer duration of T2DM (9.0 vs 7.3 yrs), and higher BMI (30.8 vs 28.6) compared to FS- pts. Overall, APB was associated with fewer MACE+HHF (HR 0.78, 95% CI 0.60–1.01, p=0.06) compared to PBO in the FS+ pts with adjustment for age, duration of T2DM and BMI.

Conclusions

Patients with T2DM and ACS may share common risk factors with patients with NAFLD. Apabetalone appears to exert a favorable effect on MACE in patients with risk factors for NAFLD. Whether apabetalone has a modulatory effect on the development and progression of NAFLD is an important question requiring further investigation.

Funding Acknowledgement

Type of funding sources: Private company. Main funding source(s): Resverlogix Corp.

Fenofibrate-associated changes in renal function and relationship to clinical outcomes among individuals with type 2 diabetes: the Action to Control Cardiovascular Risk in Diabetes (ACCORD) experience

AIMS/HYPOTHESIS

Fenofibrate has been noted to cause an elevation in serum creatinine in some individuals. Participants in the Action to Control Cardiovascular Risk in Diabetes Lipid Study were studied to better characterise who is at risk of an increase in creatinine level and to determine whether those with creatinine elevation have a differential risk of adverse renal or cardiovascular outcomes.

METHODS

A fenofibrate-associated creatinine increase (FACI) was defined as an increase in serum creatinine of at least 20% from baseline to month 4 in participants assigned to fenofibrate. Baseline patient characteristics, and baseline and 4-month drug, clinical, laboratory characteristics and study outcomes were examined by FACI status.

RESULTS

Of the sample, 48% of those randomised to receive fenofibrate had at least a 20% increase in serum creatinine within 4 months. In multivariable analysis, participants who were older, male, used an ACE inhibitor at baseline, used a thiazolidinedione (TZD) at 4 months post-randomisation, had baseline CVD, and had lower baseline serum creatinine and LDL-cholesterol levels were all more likely to meet the criteria for FACI. Participants in the FACI group were also more likely to have a decrease in their serum triacylglycerol level from baseline to 4 months. No differences in study outcomes were seen by FACI criteria.

CONCLUSIONS/INTERPRETATION

Several characteristics predict a rapid rise in serum creatinine upon starting fenofibrate. Participants who met the criteria for FACI also had a greater change in triacylglycerol levels. In the setting of careful renal function surveillance and reduction of fenofibrate dose as indicated, no increase in renal disease or cardiovascular outcome was seen in those individuals demonstrating FACI.

TRIAL REGISTRATION

ClincalTrials.gov: NCT00000620.

FUNDING

The ACCORD Trial was supported by grants (N01-HC-95178, N01-HC-95179, N01-HC-95180, N01-HC-95181, N01-HC-95182, N01-HC-95183, N01-HC-95184, IAA-Y1-HC-9035 and IAA-Y1-HC-1010) from the National Heart, Lung, and Blood Institute; by the National Institute of Diabetes and Digestive and Kidney Diseases, the National Institute on Aging, and the National Eye Institute; by the Centers for Disease Control and Prevention; by General Clinical Research Centers and by the Clinical and Translational Science Awards. Abbott Laboratories, Amylin Pharmaceutical, AstraZeneca Pharmaceuticals LP, Bayer HealthCare LLC, Closer Healthcare, GlaxoSmithKline Pharmaceuticals, King Pharmaceuticals, Merck, Novartis Pharmaceuticals, Novo Nordisk, Omron Healthcare, sanofi-aventis US and Takeda Pharmaceuticals provided study medications, equipment or supplies.

Diabetes and dyslipidemia

Numerous prospective cohort studies have indicated that diabetes mellitus (DM), particularly type-2 DM (the type of diabetes associated with insulin resistance that usually strikes adults), is associated with a 3-4-fold increase in risk for coronary heart disease (CHD) [1-3]. The increase in risk is particularly evident in younger-age groups, and in women: females with type-2 DM appear to lose a great deal of the protection that characterizes non-diabetic females. Furthermore, patients with DM have a 50% greater in-hospital mortality, and a 2-fold increased rate of death within 2 years of surviving a myocardial infarction. Overall, CHD is the leading cause of death in individuals with DM who are >35 years old. Although a significant portion of this increased risk is associated with the presence of well-characterized risk factors for CHD, a significant proportion remains unexplained. Patients with DM, particularly those with type-2 DM, have abnormal plasma lipid and lipoprotein concentrations that are less commonly present in non-diabetics [4-6]. Patients with poorly controlled type-1 DM can also have a dyslipidemic pattern, but, in this review, we will focus on the dyslipidemia seen commonly in patients with type-2 DM. In particular, we will describe the pathophysiology underlying the increase in plasma very low-density lipoprotein triglyceride levels, the reductions in plasma high-density lipoprotein cholesterol levels, and the abnormal, small, dense low-density lipoproteins that are the central components of diabetic dyslipidemia. The dyslipidemia of DM clearly adds significantly to the high risk for CHD in this group, and must be treated aggressively with diet, weight loss and lipid-altering medications. Combinations of lipid-altering medications, particularly statins and fibrates, can markedly change plasma lipid levels, often bringing them all into the normal range.

Post-transcriptional stimulation of the assembly and secretion of triglyceride-rich apolipoprotein B lipoproteins in a mouse with selective deficiency of brown adipose tissue, obesity, and insulin resistance

A mouse model of insulin resistance and its associated dyslipidemia was generated by crossing mice expressing human apolipoprotein B (apoB) with mice lacking only brown adipose tissue (BATless). On a high fat diet, male apoB/BATless mice became obese, hypercholesterolemic, hypertriglyceridemic, and hyperinsulinemic compared with control apoB mice. Fast performance liquid chromatography revealed increased triglyceride concentrations in intermediate density lipoprotein/low density lipoprotein (LDL) and reduced high density lipoprotein cholesterol concentrations. Inhibition of lipolysis by the drug, tetrahydrolipostatin, demonstrated that very low density lipoprotein-sized particles were initially secreted. Metabolic studies employing Triton WR-1339 and either [(3)H]glycerol or [(3)H]palmitate showed that the hypertriglyceridemia in apoB/BATless mice was due to the increased synthesis and secretion of triglyceride. Furthermore, lipoprotein lipase and hepatic lipase activities were not defective. ApoB was also secreted at increased rates in the apoB/BATless mice. Similar levels of apoB mRNA in apoB and apoB/BATless mice indicated that apoB secretion was regulated post-transcriptionally. LDL receptor mRNA was increased in the apoB/BATless mice, indicating that the observed increase in apoB-lipoprotein secretion was not due to their decreased reuptake. Finally, mRNA levels of the large subunit of microsomal triglyceride transfer protein, a required component for very low density protein assembly, were not different between apoB and apoB/BATless mice. This rodent model should prove useful in exploring mechanisms underlying the regulation of apoB secretion in the context of insulin resistance.

HIV protease inhibitors protect apolipoprotein B from degradation by the proteasome: a potential mechanism for protease inhibitor-induced hyperlipidemia

Highly active anti-retroviral therapies, which incorporate HIV protease inhibitors, resolve many AIDS-defining illnesses. However, patients receiving protease inhibitors develop a marked lipodystrophy and hyperlipidemia. Using cultured human and rat hepatoma cells and primary hepatocytes from transgenic mice, we demonstrate that protease inhibitor treatment inhibits proteasomal degradation of nascent apolipoprotein B, the principal protein component of triglyceride and cholesterol-rich plasma lipoproteins. Unexpectedly, protease inhibitors also inhibited the secretion of apolipoprotein B. This was associated with inhibition of cholesteryl-ester synthesis and microsomal triglyceride transfer-protein activity. However, in the presence of oleic acid, which stimulates neutral-lipid biosynthesis, protease-inhibitor treatment increased secretion of apolipoprotein B-lipoproteins above controls. These findings suggest a molecular basis for protease-inhibitor-associated hyperlipidemia, a serious adverse effect of an otherwise efficacious treatment for HIV infection.

Overexpression of the A1 adenosine receptor in adipose tissue protects mice from obesity-related insulin resistance

In-vitro studies have implicated the A(1) adenosine receptor (A(1)AR) of adipocytes in inhibition of lipolysis, stimulation of lipogenesis and enhancement of the action of insulin on glucose metabolism. To determine whether any of these activities were physiologically relevant in an intact animal, A(1)AR was overexpressed in adipose tissue of transgenic mice. Lower plasma free fatty acid (FFA) levels were observed in the transgenic mice relative to the litter-matched controls, supporting a significant physiological role for adipocyte A(1)AR in the control of lipolysis. However, no differences were observed in body weights or body composition. On a high fat diet, both the transgenic mice and the litter matched controls, male and female, became equally obese. Unlike the control mice, however, the transgenic mice did not develop insulin resistance, as demonstrated by serum glucose and insulin levels and glucose and insulin tolerance tests. These findings demonstrate that adipocyte A(1)AR plays an important physiological role in the control of insulin sensitivity in an intact animal and therefore should be considered to be a potential therapeutic target for the treatment of obesity-related insulin resistance and type 2 diabetes.

Postprandial dyslipidemia: an atherogenic disorder common in patients with diabetes mellitus

The increased risk of coronary artery disease among patients with diabetes mellitus is attributable, in part, to specific disorders of lipoprotein metabolism that are common in this population. These include disordered metabolism of very-low-density lipoprotein and/or chylomicrons that may be proatherogenic. Elevated postprandial triglycerides, peak postprandial triglyceridemia, and late postprandial triglyceride levels have been associated in clinical trials with both early coronary artery and carotid artery atherosclerosis for persons with normal lipid profiles and those with mild-to-moderate hyperlipidemia, independently of established risk factors. If hyperlipidemia cannot be managed through better glycemic control, diet, and exercise, then hepatic 3-hydroxy-3-methylglutaryl coenzyme A reductase inhibitors, fibric acid derivatives, and omega-3 fatty acids are safe and effective lipid-altering agents that can be used to correct these disorders.

Age-related effects of genetic variation on lipid levels: The Columbia University BioMarkers Study

OBJECTIVES

To examine the genotype:phenotype association in children compared with their parents.

METHODS

Variations at 4 key gene loci, namely lipoprotein lipase (LPL S447X), hepatic lipase (HL -480C>T), cholesteryl ester transfer protein (CETP TaqIB), and apolipoprotein CIII (APOC3 -455T>C and -482C>T), were examined in children (n = 495) and their parents (n = 353) in the Columbia University BioMarkers Study, 1994 to 1998.

RESULTS

The frequencies of the rare alleles of the HL -480C>T and APOC3 -455T>C and -482C>T (but not LPL S447X or CETP TaqIB) were significantly lower in non-Hispanic white participants compared with Hispanics. Overall, genotype effects seen in the adults were weaker in the children, although similar trends were seen. In an examination of the effect of body fat on the genotypic effects in the children, there was significant HL -480C>T:sum of skinfold interaction.

CONCLUSIONS

All genotypes were associated with clear relationships to plasma lipid levels in adults, but the effects were weaker in their children, unless stressed by body fat. atherosclerosis, cardiovascular disease, child, lipids, genetics.

Microsomal triglyceride transfer protein binding and lipid transfer activities are independent of each other, but both are required for secretion of apolipoprotein B lipoproteins from liver cells

Recent studies indicate that microsomal triglyceride transfer protein (MTP) and apolipoprotein B (apoB) interact physically via two specific binding sites located within the amino-terminal globular region of apoB100. The first site is thought to be within the first 5.8% of the amino-terminal sequence, and the second site is between 9 and 16% of the amino-terminal sequence. It is not clear from prior studies whether these sites have unique or overlapping functions. Furthermore, there are no data differentiating between lipid transfer and potential chaperone functions of MTP. In the present study we have attempted to further characterize the physiologic interaction between apoB and MTP and to determine the relationship between the binding and lipid transfer aspects of the interaction. HepG2 cells were transiently transfected with apoB cDNAs, and MTP binding to apoB polypeptides was determined by two-step immunoprecipitation. MTP bound equally well to apoB polypeptides with (apoB13, 16,beta, apoB34, and apoB42) or without (apoB16, apoB13, and 16 or apoB13, 13, and 16) beta sheet domains. When proteasomal degradation of newly synthesized apoB polypeptides was blocked, MTP binding to all of the apoB polypeptides was only modestly affected by lipid availability and was independent of MTP-associated lipid transfer. Furthermore, MTP did not bind directly to a portion of the first beta sheet domain. We created two apoB constructs (apoB16del and apoB34del) by deleting the first 210 amino acids of apoB16 and apoB34. These apoB polypeptides, therefore, lacked the putative first MTP binding site. MTP binding to apoB16del and apoB34del was decreased significantly. However, the secretion of apoB16del was not different from apoB16, whereas the secretion of apoB34del was impaired significantly. Our results indicate that the interaction between MTP and apoB involves independent binding and lipid transfer activities but that both activities are required for the secretion of apolipoprotein B from liver cells.

Variation in the human ApoB signal peptide modulates ApoB17 translocation

The functional effects of the common 27- or 24-amino-acid (aa) variants in the human apoB signal peptide (SP) on intracellular and secreted apoB17 were investigated in vitro. Only in the presence of oleate was a significant difference in intracellular and secreted SP27-B17 compared to SP24-B17 observed (P = 0.01 and P < 0.0007, respectively), although in the presence or absence of oleate mRNA levels from the two constructs were similar. After fractionation, oleate treatment enhanced microsomal SP27-B17 by 150% (P < 0.0005) with a modest but significant effect on SP24-B17 (32% P = 0.007). Oleate stimulated SP24-B17 accumulation in the nonmicrosomal fraction. The data suggest that the presence of oleate leads to inefficient translocation of the 24-amino-acid signal peptide, possibly resulting in increased retrograde translocation into the cytoplasm and reduced intracellular and secreted levels compared to the "wildtype" 27 aa SP. This implies a direct role of the SP variants in the regulation of apoB intracellular metabolism.

Co-translational interactions of apoprotein B with the ribosome and translocon during lipoprotein assembly or targeting to the proteasome

Hepatic lipoprotein assembly and secretion can be regulated by proteasomal degradation of newly synthesized apoB, especially if lipid synthesis or lipid transfer is low. Our previous studies in HepG2 cells showed that, under these conditions, newly synthesized apoB remains stably associated with the endoplasmic reticulum (ER) membrane (Mitchell, D. M., Zhou, M., Pariyarath, R., Wang, H., Aitchison, J. D., Ginsberg, H. N., and Fisher, E. A. (1998) Proc. Natl. Acad. Sci. U. S. A. 95, 14733-14738). We now show that independent of lipid synthesis, apoB chains that appear full-length are, in fact, incompletely translated polypeptides still engaged by the ribosome and associated with the ER translocon. In the presence of active lipid synthesis and transfer, translation and lipoprotein assembly are completed, and the complexes exit the ER. Upon omitting fatty acids from, or adding a microsomal triglyceride transfer protein inhibitor to, culture media to reduce lipid synthesis or transfer, respectively, apoB was degraded while it remained associated with the ER and complexed with cytosolic hsp70 and proteasomes. Thus, unlike other ER substrates of the proteasome, such as major histocompatibility complex class I molecules, apoB does not fully retrotranslocate to the cytosol before entering the ubiquitin-proteasome pathway. Although, upon immunofluorescence, apoB in proteasome-inhibited cells accumulated in punctate structures similar in appearance to aggresomes (cytosolic structures containing molecules irreversibly lost from the secretory pathway), these apoB molecules could be secreted when lipid synthesis was stimulated. The results suggest a model in which 1) apoB translation does not complete until lipoprotein assembly terminates, and 2) assembly with lipids or entry into the ubiquitin-proteasome pathway occurs while apoB polypeptides remain associated with the translocon and attached to the ribosome.

Nonpharmacologic management of low levels of high-density lipoprotein cholesterol

Several nonpharmacologic approaches can effectively increase low serum levels of high-density lipoprotein cholesterol (HDL-C), including weight control, specific nutritional choices, exercise, alcohol consumption, and smoking cessation. Increased visceral fat is inversely associated with HDL-C in both men and women. During weight reduction, HDL-C, HDL2-C, and apolipoprotein A-1 (apo A-1) tend to decrease, but levels increase with sustained weight loss. Overall, weight cycling is not detrimental in terms of serum lipids. Increasing monounsaturated fat intake and reducing carbohydrates increases HDL-C levels. Lowering trans-fatty acid intake also improves serum lipids. A very low-fat diet combined with stress-lowering lifestyle changes has been shown to cause regression of coronary artery disease. Moderate alcohol consumption, even in diabetic patients, and smoking cessation can increase serum HDL-C levels.

Predictors of postprandial triacylglycerol response in children: the Columbia University Biomarkers Study

BACKGROUND

Predictors of postprandial lipemia have not been explored in children.

OBJECTIVE

Our objective was to determine whether the postprandial triacylglycerol response is associated with low HDL-cholesterol and high fasting triacylglycerol concentrations and family history of early-onset ischemic heart disease (IHD) in children.

DESIGN

We administered a standardized fat load (52.5 g fat/m(2)) to 60 children (mean age: 14.0 y), 20 with and 40 without a family history of early-onset IHD, and to 29 mothers, all recruited from families enrolled in the Columbia University Biomarkers Study. Plasma lipid and retinyl palmitate concentrations were measured in the fasting state and 3, 6, and 8 h after the oral fat load.

RESULTS

In children, postprandial lipemia, as indicated by the incremental area under the triacylglycerol response curve, was associated with elevated fasting triacylglycerol concentrations (>/=1.13 mmol/L; P: < 0.01), with low fasting HDL-cholesterol concentrations (</=0.91 mmol/L; P: < 0.01), and with the combination of low HDL-cholesterol and high triacylglycerol concentrations (P: < 0.05). Family history of IHD, baseline LDL-cholesterol concentration, and apolipoprotein E genotype were not associated with the postprandial triacylglycerol or retinyl palmitate response. The mothers had fasting triacylglycerol concentrations similar to those of their children but a more prolonged response with higher triacylglycerol concentrations at 6 and 8 h (P: < 0.01 and P: < 0.05, respectively).

CONCLUSIONS

In children, a delayed postprandial triacylglycerol response to a fat load is associated with the combination of high fasting triacylglycerol and low HDL-cholesterol concentrations. Predictors of postprandial triacylglycerol concentrations may be similar in children and adults.

The amino-terminal domain of apolipoprotein B does not undergo retrograde translocation from the endoplasmic reticulum to the cytosol. Proteasomal degradation of nascent apolipoprotein B begins at the carboxyl terminus of the protein, while apolipoprotein B is still in its original translocon

We studied the sequential topology of the NH(2) and COOH termini of apoB during translocation by expressing, in Chinese hamster ovary (CHO) and HepG2 cells, an apoB42 construct with c-Myc and hemagglutinin (HA) tags at 2 and 41% (relative to apoB100) of its amino acid sequence. We conducted similar studies using monoclonal antibodies against the NH(2) and COOH termini of apoB100 in HepG2 cells. After radiolabeling, microsomes were immunoisolated from transfected CHO cells using anti-c-Myc or anti-HA antibodies. Throughout a 60-min chase in the presence of N-acetyl-leucyl-norleucinal, more than 90% of microsomes were isolated by anti-HA antibodies, whereas less than 10% were isolated by anti-c-Myc antibodies. Proteinase K digestion of total microsomes consistently generated two fragments ( approximately 70 and approximately 120 kDa) of apoB42 containing the NH(2) terminus throughout the chase; no fragments containing the COOH terminus were detected. Immunofluorescent studies of transfected CHO cells were consistent with results from the labeling studies. Essentially identical results were obtained from pulse-chase studies in both native and apoB42-transfected HepG2 cells. The present studies support a model in which, in the absence of adequate core lipid synthesis, there is partial translocation of apoB leading to cytosolic exposure, ubiquitination, and proteasomal degradation directly from the original translocation channel.

The insulin resistance syndrome: impact on lipoprotein metabolism and atherothrombosis

Insulin resistance is a common metabolic abnormality that is associated with an increased risk of both atherosclerosis and type 2 diabetes. The phenotype of insulin resistance includes a dyslipidemia characterized by an elevation of very low-density lipoprotein triglyceride, a reduction in high-density lipoprotein cholesterol, and the presence of small, triglyceride-enriched low-density lipoproteins. The underlying metabolic abnormality driving this dylipidemia is an increased assembly and secretion of very low-density lipoprotein particles, leading to an increased plasma level of triglyceride. Hypertriglyceridemia, in turn, results in a reduction in the high-density lipoprotein level and the generation of small, dense low-density lipoproteins; these events are mediated by cholesteryl ester transfer protein. In addition, hypertension, obesity, and a prothrombotic state are also integral components of the insulin resistance syndrome. In this review, we will provide a pathophysiologic basis, based on studies on humans and in tissue culture, for the dyslipidemia of insulin resistance. We will also review the effects of insulin resistance on the coagulation and fibrinolytic pathways. It is hoped that this review will allow health professionals better to evaluate and treat their patients with insulin resistance, thereby reducing the very much increased risk of atherosclerotic cardiovascular disease carried by these individuals.

Acute elevations of plasma asymmetric dimethylarginine and impaired endothelial function in response to a high-fat meal in patients with type 2 diabetes

Asymmetric dimethylarginine (ADMA), a compound detectable in human plasma, is an endogenous inhibitor of NO synthase. Endothelial dysfunction is an early event in atherogenesis, and large-vessel atherosclerosis is a major cause of morbidity and mortality in patients with type 2 diabetes mellitus. Fifty patients with type 2 diabetes mellitus were studied at baseline and 5 hours after ingestion of a high-fat meal. Plasma ADMA measured by using high-performance liquid chromatography increased from 1.04+/-0.99 to 2.51+/-2.27 micromol/L (P:<0.0005). Brachial arterial vasodilation after reactive hyperemia, a NO-dependent function, measured by high-resolution ultrasound, decreased from 6.9+/-3.9% at baseline to 1.3+/-4.5% (P:<0.0001). These changes occurred in association with increased plasma levels of triglycerides and very low density lipoprotein triglycerides, with reduced low density lipoprotein cholesterol and high density lipoprotein cholesterol, and with no changes in total cholesterol. The increase in plasma ADMA in response to a high-fat meal was significantly and inversely related to the decrease in percent vasodilation. In 10 of the subjects studied with a similar protocol on another day, no significant changes in the brachial artery flow responses or in plasma ADMA were observed 5 hours after ingestion of a nonfat isocaloric meal. The data suggest that ADMA may contribute to abnormal blood flow responses and to atherogenesis in type 2 diabetics.

Inhibition of translocation of nascent apolipoprotein B across the endoplasmic reticulum membrane is associated with selective inhibition of the synthesis of apolipoprotein B

In HepG2 cells, inhibition of apolipoprotein B100 (apoB) translocation across the endoplasmic reticulum by an microsomal triglyceride transfer protein (MTP) inhibitor (CP-10447) in the presence of N-acetyl-leucinyl-norleucinal, a proteasomal inhibitor, results in accumulation of newly synthesized apoB in the translocation channel. Here we demonstrated that such accumulation led to a specific reduction of apoB synthesis. ApoB mRNA levels remained unchanged, but we observed reduced rates of elongation of nascent apoB in puromycin-synchronized cells pretreated with MTP inhibitor. This observation was consistent with a longer half-ribosome transit time for the synthesis of apoB in MTP-inhibited cells. Initiation of translation of apoB mRNA was not impaired by MTP inhibition. Overall, these findings suggest that translocation arrest of apoB in the endoplasmic reticulum channel can exert a selective and negative effect on the synthesis of apoB at the stage of elongation.

Circulating lipoprotein profiles are modulated differently by lipoprotein lipase in obese humans

BACKGROUND

Several genetic analyses have suggested that lipoprotein lipase (LpL) genotypes causing decreased LpL activity correlate with increased triglyceride concentrations and risk for coronary artery disease. In contrast, in some other studies LpL activity was positively correlated with plasma low-density lipoprotein (LDL) cholesterol concentrations.

OBJECTIVE

To assess whether these different associations represent physiologic differences in lipoprotein metabolism.

METHODS

We correlated postheparin lipase activities, postprandial lipemia, and fasting lipoprotein concentrations in obese (BMI > or = 30 kg/m2, n = 26) and non-obese (BMI < or = 30 kg/m2, n = 57) individuals. LpL was measured using specific inhibitory antibodies.

RESULTS

Surprisingly, LpL activity was significantly correlated with triglyceride area under the curve after a fat load in the non-obese, but not the entire group. Moreover, in non-obese individuals, LpL activity correlated directly (r = 0.40) and hepatic lipase activity correlated inversely (r = -0.32) with high-density lipoprotein (HDL) cholesterol concentrations. These relationships were not found in the obese group, in whom LpL correlated with LDL cholesterol concentrations (r = 0.54).

CONCLUSIONS

We conclude that postheparin LpL activity relates to different lipoproteins in obese and non-obese individuals. In obesity, greater LpL activity may enhance conversion of very-low-density lipoprotein cholesterol to LDL cholesterol, whereas in non-obese individuals the correlation is with HDL cholesterol. Whether this is due to differences in the source of LpL (muscle or fat), or to other associated alterations in lipoprotein metabolism is unknown. These results may explain the non-uniformity of correlations between LpL and atherogenic lipoproteins in different populations.

HDL-subpopulation patterns in response to reductions in dietary total and saturated fat intakes in healthy subjects

BACKGROUND

Little information is available about HDL subpopulations during dietary changes.

OBJECTIVE

The objective was to investigate the effect of reductions in total and saturated fat intakes on HDL subpopulations.

DESIGN

Multiracial, young and elderly men and women (n = 103) participating in the double-blind, randomized DELTA (Dietary Effects on Lipoproteins and Thrombogenic Activities) Study consumed 3 different diets, each for 8 wk: an average American diet (AAD: 34.3% total fat,15.0% saturated fat), the American Heart Association Step I diet (28.6% total fat, 9.0% saturated fat), and a diet low in saturated fat (25.3% total fat, 6.1% saturated fat).

RESULTS

HDL(2)-cholesterol concentrations, by differential precipitation, decreased (P < 0.001) in a stepwise fashion after the reduction of total and saturated fat: 0.58 +/- 0.21, 0.53 +/- 0.19, and 0.48 +/- 0.18 mmol/L with the AAD, Step I, and low-fat diets, respectively. HDL(3) cholesterol decreased (P < 0.01) less: 0.76 +/- 0.13, 0.73 +/- 0.12, and 0.72 +/- 0.11 mmol/L with the AAD, Step I, and low-fat diets, respectively. As measured by nondenaturing gradient gel electrophoresis, the larger-size HDL(2b) subpopulation decreased with the reduction in dietary fat, and a corresponding relative increase was seen for the smaller-sized HDL(3a, 3b), and (3c) subpopulations (P < 0.01). HDL(2)-cholesterol concentrations correlated negatively with serum triacylglycerol concentrations on all 3 diets: r = -0.46, -0.37, and -0.45 with the AAD, Step I, and low-fat diets, respectively (P < 0.0001). A similar negative correlation was seen for HDL(2b), whereas HDL(3a, 3b), and (3c) correlated positively with triacylglycerol concentrations. Diet-induced changes in serum triacylglycerol were negatively correlated with changes in HDL(2) and HDL(2b) cholesterol.

CONCLUSIONS

A reduction in dietary total and saturated fat decreased both large (HDL(2) and HDL(2b)) and small, dense HDL subpopulations, although decreases in HDL(2) and HDL(2b) were most pronounced.

Identification and treatment of hypertriglyceridemia as a risk factor for coronary heart disease

Recent publications, including new population-based studies and a meta-analysis of prospective, population-based studies, provide strong evidence for an elevated triglyceride level as an independent risk factor for coronary heart disease. Pathophysiologic relationships between elevated triglyceride levels and both reduced high-density lipoprotein levels and an increase in the proportion of low density lipoproteins that are small and dense support the epidemiologic data, and suggest that an elevated triglyceride level should constitute a target for lipid-lowering therapy. There are no clear recommendations for management of patients with hypertriglyceridemia available in the current treatment guidelines. Treatment options include life-style measures and, if drug therapy is required, nicotinic acid, fibrates, more potent 3-hydroxy-3-methylglutaryl coenzyme A reductase inhibitors (statins), or combination therapy with statin plus fibrate or nicotinic acid.

Apoprotein B100 has a prolonged interaction with the translocon during which its lipidation and translocation change from dependence on the microsomal triglyceride transfer protein to independence

When lipid synthesis is limited in HepG2 cells, apoprotein B100 (apoB100) is not secreted but rapidly degraded by the ubiquitin-proteasome pathway. To investigate apoB100 biosynthesis and secretion further, the physical and functional states of apoB100 destined for either degradation or lipoprotein assembly were studied under conditions in which lipid synthesis, proteasomal activity, and microsomal triglyceride transfer protein (MTP) lipid-transfer activity were varied. Cells were pretreated with a proteasomal inhibitor (which remained with the cells throughout the experiment) and radiolabeled for 15 min. During the chase period, labeled apoB100 remained associated with the microsomes. Furthermore, by crosslinking sec61beta to apoB100, we showed that apoB100 remained close to the translocon at the same time apoB100-ubiquitin conjugates could be detected. When lipid synthesis and lipoprotein assembly/secretion were stimulated by adding oleic acid (OA) to the chase medium, apoB100 was deubiquitinated, and its interaction with sec61beta was disrupted, signifying completion of translocation concomitant with the formation of lipoprotein particles. MTP participates in apoB100 translocation and lipoprotein assembly. In the presence of OA, when MTP lipid-transfer activity was inhibited at the end of pulse labeling, apoB100 secretion was abolished. In contrast, when the labeled apoB100 was allowed to accumulate in the cell for 60 min before adding OA and the inhibitor, apoB100 lipidation and secretion were no longer impaired. Overall, the data imply that during most of its association with the endoplasmic reticulum, apoB100 is close to or within the translocon and is accessible to both the ubiquitin-proteasome and lipoprotein-assembly pathways. Furthermore, MTP lipid-transfer activity seems to be necessary only for early translocation and lipidation events.

Glucose does not stimulate apoprotein B secretion from HepG2 cells because of insufficient stimulation of triglyceride synthesis

We have previously demonstrated a close relationship between oleic acid (OA)-stimulated triglyceride (TG) synthesis and apolipoprotein B (apoB) secretion in HepG2 cells. However, other investigators studying the association between glucose-stimulated TG synthesis and apoB secretion have reported variable results. The present study was carried out to answer the question: does TG derived from glucose have different effects on apoB secretion from HepG2 cells compared to TG derived from oleate? We observed that incubations of HepG2 cells for as long as 48 h in 30 mm glucose did not increase apoB secretion. We then demonstrated that the failure of glucose to stimulate apoB secretion from HepG2 cells results from insufficient stimulation of TG synthesis by glucose. Thus, incorporation of [3H]glycerol into [3H]TG in the presence of 30 mm glucose for up till 48 h was not increased compared to basal conditions. The inability of glucose to stimulate TG synthesis was also evidenced by the inability of both 8 -h and 24 -h incubations with 30 mm glucose to increase cell TG mass; similar incubations with OA increased TG mass 50-100%. Additional studies demonstrated that glucose conversion to either glycerol or fatty acids was minimal; this accounted for the lack of stimulation of TG synthesis. We conclude that in HepG2 cells, availability of high glucose levels in the media for as long as 48 h does not stimulate triglyceride synthesis, and that this is the basis for the failure of glucose to stimulate apoB secretion.

Regulated Co-translational ubiquitination of apolipoprotein B100. A new paradigm for proteasomal degradation of a secretory protein

Presentation of a wild-type secretory protein, apolipoprotein B100 (apoB), to the cytosol for ubiquitin-proteasome proteolysis has been observed in HepG2 cells. A currently accepted model for proteasomal degradation of secretory proteins is retrograde translocation of the substrate polypeptides from the lumen of endoplasmic reticulum (ER) back to the cytosol. In this report, we present evidence that newly synthesized apoB becomes exposed to the cytosol and targeted to the proteasomes in a co-translational manner. Thus, after protein translation was synchronized with puromycin, partially synthesized apoB polypeptides were found to be conjugated to ubiquitin. The magnitude of co-translational ubiquitination and subsequent degradation of apoB was increased when cells were pretreated with either herbimycin A to induce cytosolic Hsp70 or with an inhibitor of microsomal triglyceride transfer protein; both treatments impede translocation of nascent apoB across the ER membrane. These treatments also decreased secretion of apoB and increased its degradation via the ubiquitin-proteasome pathway. We suggest that translocation arrest with subsequent co-translational exposure to the cytosol provides an alternative model to explain how mammalian secretory proteins can overcome topological segregation by the ER membrane and undergo degradation by the ubiquitin-proteasome pathway.

Lipoprotein physiology

Lipoproteins are spherical macromolecular complexes in which hydrophobic molecules of triglyceride and cholesteryl ester are enveloped within a monolayer of amphipathic molecules of phospholipids, free cholesterol, and apoproteins. The major lipoprotein classes include intestinally derived chylomicrons that transport dietary fats and cholesterol, hepatic-derived VLDL, IDL, and LDL that can be atherogenic, and hepatic- and intestinally derived HDL that are anti-atherogenic. Apoprotein B is necessary for the secretion of chylomicrons (apo B48) and VLDL, IDL, and LDL (apo B100). Post-translational regulation of the assembly of apo B-containing lipoproteins by core lipid availability seems to be the major mechanism for variations in secretion. Plasma levels of VLDL triglycerides are determined mainly by rates of secretion and LPL lipolytic activity; plasma levels of LDL cholesterol are determined mainly by the secretion of apo B100 into plasma, the efficacy with which VLDL are converted to LDL and by LDL receptor-mediated clearance. Regulation of HDL cholesterol levels is complex and is affected by rates of synthesis of its apoproteins, rates of esterification of free cholesterol to cholesteryl ester by LCAT, levels of triglyceride-rich lipoproteins and CETP-mediated transfer of cholesteryl esters from HDL, and clearance from plasma of HDL lipids and apoproteins. Normal lipoprotein transport is associated with low levels of triglycerides and LDL cholesterol and high levels of HDL cholesterol. When lipoprotein transport is abnormal, lipoproteins levels can change in ways that predispose individuals to atherosclerosis.

LPL promoter -93T/G transition influences fasting and postprandial plasma triglycerides response in African-Americans and Hispanics

The lipoprotein lipase (LPL) promoter -93T/G transition has previously been reported as having a triglyceride (Tg)-lowering effect, whereas the D9N variant has been shown to have a Tg-raising effect. These two variants were studied in 66 healthy subjects of Hispanic and 42 subjects of African-American origin, who had participated in a study of postprandial lipemia. While the allele frequency of the -93G was significantly different in the Hispanics and African Americans (0.09: 95% CI 0.04-0.13 and 0.28: 95% CI 0.19-0.38; P=0.0001, respectively), the N9 allele frequency was not different (0.06: 95% CI 0.02-0.1 and 0.05: 95% CI 0.002-0.093, respectively). Linkage disequilibrium between the -93T/G and D9N was highly significant in Hispanics (delta=0.67. P=0.0001), compared to delta=0.09 (NS) in African-Americans. In the combined group, compared to individuals with the common genotype (TT/DD; n=71) with fasting plasma Tg of 1.34 (+/-4.5% SEM) mmol/l, carriers of the G/D haplotype (TG/DD + GG/DD; n=25) had significantly lower plasma Tg levels of 1.08 (+/-10% SEM) mmol/l (P < 0.02). After the fat meal, compared to individuals with neither mutation, TT/DD, the effect of the G/D haplotype was to reduce significantly postprandial Tg (P < 0.036). Retinyl palmitate concentration at 5 hrs was significantly lower in G/D carriers than TT/DD individuals (P < 0.05). The lipid-raising effect of the N9 allele in carriers of the -93G (TG/DN + GG/DN) and effect on postprandial Tg clearance was not significant in this group. Thus carriers of the G/D haplotype have lower fasting plasma Tg and reduced alimentary lipemia. This allele may be associated with reduced risk of coronary artery disease.

Three-fold effect of lovastatin treatment on low density lipoprotein metabolism in subjects with hyperlipidemia: increase in receptor activity, decrease in apoB production, and decrease in particle affinity for the receptor. Results from a novel triple-tracer approach

To differentiate effects of lovastatin on low density lipoprotein (LDL) receptor activity from effects on LDL metabolic properties, LDL apolipoprotein B (apoB) turnover was studied in eight hyperlipidemic subjects during baseline and lovastatin treatment, in the latter case with LDL tracers isolated during both baseline (CLDL) and drug treatment (Rx-LDL) conditions. Lovastatin (40 mg/day) significantly lowered total plasma and LDL cholesterol levels (27% and 25%, respectively) as well as plasma triglyceride levels (30%). Using contemporaneous tracers (C-LDL before and Rx-LDL during treatment), lovastatin caused a modest increase in LDL fractional catabolic rate (FCR) (0.410+/-0.113 vs. 0.339+/-0.108 pools/day, P < 0.04 by paired t). The increase in LDL tracer FCR was higher when C-LDL tracer isolated during the untreated period was injected during lovastatin treatment (0.496+/-0.177 vs. 0.339+/-0.108 pools/day, P < 0.02). These in vivo studies in humans were confirmed by injecting LDL tracers from two patients into five guinea pigs. The C-LDL tracer was cleared consistently faster than the Rx-LDL tracer (0.082+/-0.018 vs. 0.057+/-0.015 pools/h, P< 0.001). The results demonstrate three important outcomes of lovastatin treatment in these subjects: LDL receptor activity increased by 49% (P < 0.02); LDL apoB production rate decreased by 17% (P < 0.03), and LDL particle in vivo affinity for the LDL receptor decreased by 15% (P < 0.01). The decrease in LDL particle affinity partially negated the expected effect of increased LDL receptors on LDL clearance. The present study provides an explanation for earlier observations by several investigators using contemporaneous tracers that treatment with HMG-CoA reductase inhibitors resulted in only modest increases in low density lipoprotein functional catabolic rate.

Effects of reducing dietary saturated fatty acids on plasma lipids and lipoproteins in healthy subjects: the DELTA Study, protocol 1

Few well-controlled diet studies have investigated the effects of reducing dietary saturated fatty acid (SFA) intake in premenopausal and postmenopausal women or in blacks. We conducted a multicenter, randomized, crossover-design trial of the effects of reducing dietary SFA on plasma lipids and lipoproteins in 103 healthy adults 22 to 67 years old. There were 46 men and 57 women, of whom 26 were black, 18 were postmenopausal women, and 16 were men > or =40 years old. All meals and snacks, except Saturday dinner, were prepared and served by the research centers. The study was designed to compare three diets: an average American diet (AAD), a Step 1 diet, and a low-SFA (Low-Sat) diet. Dietary cholesterol was constant. Diet composition was validated and monitored by a central laboratory. Each diet was consumed for 8 weeks, and blood samples were obtained during weeks 5 through 8. The compositions of the three diets were as follows: AAD, 34.3% kcal fat and 15.0% kcal SFA; Step 1, 28.6% kcal fat and 9.0% kcal SFA; and Low-Sat, 25.3% kcal fat and 6.1% kcal SFA. Each diet provided approximately 275 mg cholesterol/d. Compared with AAD, plasma total cholesterol in the whole group fell 5% on Step 1 and 9% on Low-Sat. LDL cholesterol was 7% lower on Step 1 and 11% lower on Low-Sat than on the AAD (both P<.01). Similar responses were seen in each subgroup. HDL cholesterol fell 7% on Step 1 and 11% on Low-Sat (both P<.01). Reductions in HDL cholesterol were seen in all subgroups except blacks and older men. Plasma triglyceride levels increased approximately 9% between AAD and Step 1 but did not increase further from Step 1 to Low-Sat. Changes in triglyceride levels were not significant in most subgroups. Surprisingly, plasma Lp(a) concentrations increased in a stepwise fashion as SFA was reduced. In a well-controlled feeding study, stepwise reductions in SFA resulted in parallel reductions in plasma total and LDL cholesterol levels. Diet effects were remarkably similar in several subgroups of men and women and in blacks. The reductions in total and LDL cholesterol achieved in these different subgroups indicate that diet can have a significant impact on risk for atherosclerotic cardiovascular disease in the total population.

Effects of statins on triglyceride metabolism

Hepatic 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase inhibitors, or "statins," have been extremely efficacious in decreasing low-density lipoprotein (LDL) cholesterol levels in patients with hypercholesterolemia and in treating patients with dysbetalipoproteinemia, a relatively rare form of hyperlipidemia. Clinical trials have indicated that statins can significantly lower very-low-density lipoprotein (VLDL) triglyceride levels, although the central mechanism of action of statins-i.e., increasing the number of LDL receptors-would appear to suggest that they would have no significant effect on VLDL levels. Through a review of published data from animal and human studies, this article addresses the important clinical question of how drugs that inhibit the rate-limiting enzyme for cholesterol can affect triglyceride metabolism.

Demonstration of biphasic effects of docosahexaenoic acid on apolipoprotein B secretion in HepG2 cells

Oleic acid (OA) stimulates apolipoprotein B (apoB) secretion from HepG2 cells by protecting the nascent protein from rapid intracellular degradation. In contrast, the n-3 fatty acids, docosahexaenoic acid (DHA) and eicosapentaenoic acid, have been shown to reduce apoB secretion by increasing its intracellular degradation in rat hepatocytes. We attempted to determine if OA and DHA have these opposite effects at the same point in the secretory pathway for apoB or if they act at different points in HepG2 cells. Unexpectedly, we found that when DHA (0.2 mmol/L) was incubated with HepG2 cells for 2 hours, it stimulated both triglyceride (TG) synthesis and apoB secretion significantly (the "stimulatory effect"). The stimulatory effect of DHA on apoB secretion was associated with decreased intracellular degradation of newly synthesized apoB. These acute effects of DHA on TG synthesis and apoB secretion paralleled those previously demonstrated with OA. After DHA was removed from the medium, however, both TG synthesis and apoB secretion rapidly decreased to a level that was significantly less than the control level (the "inhibitory effect"). At the same time, intracellular apoB degradation was significantly increased, and this degradation was efficiently prevented by proteasome inhibitors. Removal of DHA from the incubation resulted in inhibition of the incorporation of endogenous fatty acids into TG. In contrast, removal of OA from the media was not associated with any such inhibitory effect. The inhibitory effect of DHA on basal apoB secretion persisted at least 8 hours. These studies suggest that incubation of HepG2 cells with DHA has biphasic effects on TG synthesis and apoB secretion: an initial stimulation of TG synthesis is followed by inhibition of TG synthesis and increased apoB degradation. Although the stimulatory effect of DHA is apparent during short incubations of HepG2 cells, both effects would be expected to occur during long incubations, since fatty acid uptake by cells is rapid and efficient. Thus, long incubations of HepG2 cells with DHA could result in overall reduced apoB secretion compared with cells incubated in bovine serum albumin. If these findings are extrapolated to the in vivo situation, they can explain the ability of dietary n-3 fatty acids, which would be delivered to the liver intermittently, to reduce very low density lipoprotein secretion.

ApoE genotype does not predict lipid response to changes in dietary saturated fatty acids in a heterogeneous normolipidemic population. The DELTA Research Group. Dietary Effects on Lipoproteins and Thrombogenic Activity

Recent studies have suggested that variations in apoE genotypes may influence the magnitude of plasma lipid changes in response to dietary interventions. We examined the ability of apoE genotype to predict plasma lipid response to reductions in percent of calories from total fat (TF) and saturated fat (SF) in a normolipidemic study population (n = 103) heterogeneous with respect to age, gender, race, and menopausal status. Three diets, an average American diet (34.3% TF, 15.0% SF), an AHA Step 1 diet (28.6% TF, 9.0% SF), and a low saturated fat (Low-Sat) diet (25.3% TF, 6.1% SF) were each fed for a period of 8 weeks in a three-way crossover design. Cholesterol was kept constant at 275 mg/d; monounsaturated and polyunsaturated fat were kept constant at approximately 13% and 6.5% of calories, respectively. Fasting lipid levels were measured during each of the final 4 weeks of each diet period. Participants were grouped by apoE genotype: E2 (E2/2, E2/3, E2/4); E3 (E3/3); E4 (E3/4, E4/4). Relative to the average American diet, both the Step 1 and Low-Sat diets significantly reduced total cholesterol, LDL cholesterol, and HDL cholesterol in all three apoE genotype groups. No evidence of a significant diet by genotype interaction, however, could be identified for any of the measured lipid and lipoprotein end points. Additional analysis of the data within individual population subgroup (men and women, blacks and whites) likewise provided no evidence of a significant diet by genotype interaction. Thus, in a heterogeneous, normolipidemic study population, apoE genotype does not predict the magnitude of lipid response to reductions in dietary saturated fat.

Safety and efficacy of Omacor in severe hypertriglyceridemia

BACKGROUND

Severe hypertriglyceridemia is a risk factor for acute pancreatitis, therefore decreasing serum triglyceride concentrations is an important component of risk management. Omega-3 fatty acids are well known hypotriglyceridemic agents, but their efficacy in severe forms of the disorder is not well documented. Our objective was to examine the effects of Omacor, a drug composed of 85% omega-3 fatty acid ethyl esters.

METHODS

Forty-two patients with triglyceride concentrations between 5.65 and 22.60 mmol/l (500 and 2000 mg/dl) were studied in a prospective, double-blind, placebo-controlled trial of Omacor (4 g/day for 4 months).

RESULTS

Compared with baseline values, Omacor significantly reduced mean triglyceride concentrations by 45% (P<0.00001), cholesterol by 15% (P< 0.001), very-low-density lipoprotein cholesterol by 32% (P< 0.0001) and cholesterol:high density lipoprotein (HDL) cholesterol ratio by 20% (P=0.0013), and increased HDL cholesterol by 13% (P=0.014) and low-density lipoprotein cholesterol by 31% (P=0.0014). The placebo had no effect on these parameters. Omacor was well tolerated and no patient discontinued medication because of side effects.

CONCLUSIONS

Four capsules of Omacor per day markedly decreased triglyceride concentrations in patients with severe hypertriglyceridemia. The availability of a potent and safe omega-3 fatty acid preparation for this patient population should diminish the risk for acute pancreatitis, and may also reduce the long-term risk for cardiovascular disease.

The degradation of apolipoprotein B100 is mediated by the ubiquitin-proteasome pathway and involves heat shock protein 70

Apolipoprotein B (apoB) is the major protein component of atherogenic lipoproteins of hepatic origin. In HepG2 cells, the standard cell culture model of human hepatic lipoprotein metabolism, there is a limited availability of core lipids in the endoplasmic reticulum for association with nascent apoB. Under these conditions, apoB is partially translocated, interacts with cytosolic Hsp70, and undergoes rapid degradation. We show that increasing the expression of Hsp70 in HepG2 cells promotes apoB degradation. In addition, apoB is polyubiquitinated and its degradation both normally and after Hsp70 induction is blocked by inhibitors of the proteasome. The apoB that accumulates after proteasome inhibition is endoplasmic reticulum-associated and can be assembled into lipoproteins and secreted if new lipid synthesis is stimulated. Thus, apoB is the first example of a wild-type mammalian protein whose secretion is regulated by degradation in the cytosol via the ubiquitin-proteasome pathway. Furthermore, targeting of this secretory protein to the proteasome is regulated by the molecular chaperone Hsp70 and the availability of apoB's lipid-ligands.

Role of lipid synthesis, chaperone proteins and proteasomes in the assembly and secretion of apoprotein B-containing lipoproteins from cultured liver cells

1. Apolipoprotein B (apoB) is necessary for the assembly and secretion of both chylomicrons from the small intestine and very low-density lipoproteins (VLDL) from the liver. ApoB is also the major protein in low-density lipoproteins (LDL) and is the ligand for the LDL receptor. Studies in humans suggest that increased production of apoB-containing lipoproteins, particularly VLDL, is a common abnormality in dyslipidaemias. 2. Studies in primary and long-term cultures of hepatocytes and hepatoma cells indicate that a significant proportion of newly synthesized apoB is rapidly degraded and that this is the major mechanism for regulation of apoB secretion. The availability of newly synthesized lipids, particularly triglyceride and cholesteryl ester, appears to be a critical factor in targeting apoB for secretion rather than degradation. 3. ApoB is an atypical secretory protein in that cotranslational translocation across the endoplasmic reticulum membrane, a feature of all secretory proteins, seems to slow or stop in the absence of adequate lipid availability (or in the absence of microsomal triglyceride transfer protein), allowing for rapid degradation of apoB. 4. The degradation of apoB seems to be facilitated by the association of nascent apoB with the major cytosolic chaperone protein, heat shock protein 70. Additionally, degradation of nascent apoB appears to occur, to a large degree, via the proteasomal pathway for degradation of cytosolic proteins.

A two-site model for ApoB degradation in HepG2 cells

Newly synthesized apolipoprotein B (apoB) undergoes rapid degradation in a pre-Golgi compartment in HepG2 cells. A major site of this early degradation seems to be on the cytosolic side of the endoplasmic reticulum (ER) membrane and is sensitive to N-acetyl-leucinyl-leucinyl-norleucinal (ALLN), which can inhibit neutral cysteine proteases and/or proteasome activity. Oleate (OA) treatment, which facilitates translocation of nascent apoB across the ER membrane, also reduces early degradation. In the present studies, we have used brefeldin A (BFA), which inhibits vesicular transport from the ER to the Golgi, to demonstrate that apoB can also be degraded by an ER luminal proteolytic activity that is distinct from the ALLN-sensitive proteases. Thus, when BFA-treated HepG2 cells were co-treated with ALLN, which protects apoB but does not facilitate its translocation into the ER lumen, degradation of newly synthesized apoB was significantly reduced compared with cells incubated with BFA alone. However, apoB degradation was rapid and complete when OA was added to media containing either BFA or ALLN/BFA. These results suggested that OA, by increasing translocation of nascent apoB into the ER lumen, exposed apoB to an ALLN-resistant proteolytic pathway. When we incubated HepG2 cells with dithiothreitol (DTT)/OA/BFA or DTT/OA/ALLN/BFA, degradation of apoB was inhibited. Furthermore, addition of DTT resulted in the accumulation of a 70-kDa amino-terminal fragment of apoB. Both full-length and amino-terminal apoB were degraded if DTT was removed from the incubation media; both were secreted if only BFA was removed. Thus, even after apoB is translocated into the ER lumen (thereby avoiding the initial proteolytic pathway), it can potentially be degraded by a lumenal proteolytic process that is ALLN-resistant but DTT-sensitive. The present results, together with previous studies, suggest that at least two distinct steps may be involved in the posttranslational degradation of apoB: 1) the first occurs while apoB is partially translocated and is ALLN-sensitive; and 2) the second occurs in the ER lumen and is DTT-sensitive. Finally, our results support the hypothesis that degradation of partially translocated apoB generates a 70-kDa amino-terminal fragment that is mainly degraded in the ER lumen by a DTT-sensitive pathway.

Intestinal transcription and synthesis of apolipoprotein AI is regulated by five natural polymorphisms upstream of the apolipoprotein CIII gene

To understand the factors contributing to the synthesis of human apolipoprotein AI (apoAI), relative apoAI synthesis was measured from endoscopic biopsy samples obtained from 18 healthy volunteers. The relative amount of apoAI synthesis was directly correlated with steady state intestinal apoAI mRNA levels and a 10-fold within-group variability was observed. Analysis of genomic DNA from the subjects revealed five polymorphic sites which defined two haplotypes in the intestinal enhancer region of the apoAI gene located upstream of the apolipoprotein CIII gene transcriptional start site (+ 1): (-641 C to A, -630 G to A, -625 T to deletion, -482 C to T, and -455 T to C). The population frequencies of the wild-type and mutant alleles were 0.53 and 0.44, respectively. Mean steady state apoAI mRNA levels and mean relative apoAI synthesis were 49 and 37% lower, respectively, in homozygotes for the mutant allele and 28 and 41% lower, respectively, in heterozygotes than in homozygotes for the wild-type allele (P < 0.05 for both). Site-directed mutants of apoAI gene promoter/reporter constructs containing the above mutations were transfected into Caco-2 cells and showed a 46% decrease in transcriptional activity compared with the wild type (P < 0.001); however, no significant differences were observed in HepG2 cells. Electrophoretic mobility shift assays showed that the mutated sequences from -655 to -610 bound Caco-2 cell nuclear protein(s) while the wild type did not. These results indicate that intestinal apoAI gene transcription and protein synthesis are genetically determined and are reduced in the presence of common mutations which induced binding of nuclear protein(s), possibly a transcriptional repressor.

Evidence that a rapidly turning over protein, normally degraded by proteasomes, regulates hsp72 gene transcription in HepG2 cells

Heat shock protein 72/73 (Hsp70) is a cytosolic molecular chaperone that carries out fundamental roles under both normal and stress situations. There is great interest in delineating the mechanisms whereby Hsp70 levels are regulated. We observed that N-acetyl-leucyl-leucyl-norleucinal (ALLN), a synthetic aldehydic tripeptide that inhibits proteasomes, markedly induced Hsp70 levels (up to 30-fold above base line in HepG2 cells and human endothelial cells). Induction of Hsp70 by ALLN was dose-dependent and not related to cell toxicity. ALLN selectively increased Hsp70 levels without affecting Hsp25, Hsp27, Hsp60, Hsp86, Hsp90, Hsp104, or Bip (immunoglobulin heavy chain binding protein) in HepG2 cells. ALLN induced Hsp70 not only by stabilizing the protein but also by dramatically increasing its synthesis. The modulation of Hsp70 synthesis by ALLN resulted from a rapid and marked increase in transcription of the hsp72 gene, since the induction of hsp72 mRNA was blocked in cells co-treated with actinomycin D. hsp72 mRNA levels were affected in a time-dependent manner by exposure to ALLN; significant elevations occurred within 60 min of treatment, and a decline to background levels was observed by 7 h of recovery. The ALLN-induced increase in hsp72 gene expression was associated with trimerization of the heat shock transcriptional factor (HSF1). ALLN did not affect the steady-state level of HSF1 protein. The effects of ALLN appeared to require de novo protein synthesis, since the induction of both HSF1 trimerization and hsp72 transcription was blocked by co-treatment with cycloheximide. When we tested a series of protease inhibitors, only the related aldehydic tripeptides, N-acetyl-leucyl-leucyl-methioninal and the proteasome inhibitor, Cbz-leucyl-leucyl-leucinal, induced Hsp70 levels. The specific proteasome inhibitor, lactacystin, which has a different structure, also induced Hsp70 levels. Overall, our results suggest that a rapidly turning over protein that is normally degraded by proteasomes may be involved in the regulation of Hsp70 synthesis via effects on the hsp70 transcriptional factor, HSF1.

Combined hyperlipidemia in transgenic mice overexpressing human apolipoprotein Cl

We have generated transgenic mice over-expressing human apolipoprotein CI (apo CI) using the native gene joined to the downstream 154-bp liver-specific enhancer that we defined for apo E. Human apo CI (HuCI)-transgenic mice showed elevation of plasma triglycerides (mg/dl) compared to controls in both the fasted (211 +/- 81 vs 123 +/- 52, P = 0.0001) and fed (265 +/- 105 vs 146 +/- 68, P < 0.0001) states. Unlike the human apo CII (HuCII)- and apo CIII (HuCIII)-transgenic mouse models of hypertriglyceridemia, plasma cholesterol was disproportionately elevated (95 +/- 23 vs 73 +/- 23, P = 0.002, fasted and 90 +/- 24 vs 61 +/- 14, P < 0.0001, fed). Lipoprotein fractionation showed increased VLDL and IDL + LDL with an increased cholesterol/triglyceride ratio (0.114 vs 0.065, P = 0.02, in VLDL). The VLDL apo E/apo B ratio was decreased 3.4-fold (P = 0.05) and apo CII and apo CIII decreased in proportion to apo E. Triglyceride and apo B production rates were normal, but clearance rates of VLDL triglycerides and postlipolysis lipoprotein "remnants" were significantly slowed. Plasma apo B was significantly elevated. Unlike HuCII- and HuCIII-transgenic mice, VLDL from HuCI transgenic mice bound heparin-Sepharose, a model for cell-surface glycosaminoglycans, normally. In summary, apo CI overexpression is associated with decreased particulate uptake of apo B-containing lipoproteins, leading to increased levels of several potentially atherogenic species, including cholesterol-enriched VLDL, IDL, and LDL.

Diabetic dyslipidemia: basic mechanisms underlying the common hypertriglyceridemia and low HDL cholesterol levels

Elevated levels of plasma triglycerides (TG) and reduced concentrations of HDL cholesterol are very common in patients with diabetes, particularly NIDDM. Although regulation of the plasma concentrations of VLDL, the major TG-rich lipoprotein is extremely complex, it is clear from in vivo kinetic studies that increased rates of secretion of VLDL into plasma is almost uniformly present in patients with NIDDM and hypertriglyceridemia. Recent studies at the cellular level indicate that increased fatty acid flux to the liver, also common in NIDDM (and other insulin-resistant states associated with elevated plasma TG levels), will stimulate the assembly and secretion of apoprotein (apo) B-containing lipoproteins by targeting apoB for secretion rather than intracellular degradation. Increased rates of secretion of VLDL into plasma appears to drive the exchange of TG from these lipoproteins for HDL cholesteryl ester. This exchange, which occurs in plasma, is facilitated by cholesteryl ester transfer protein, and generates a TG-enriched HDL that is a substrate for either hepatic lipase or lipoprotein lipase. When the TG in HDL is hydrolyzed, the resultant particle is smaller, and this appears to affect the binding of the major HDL protein, apoA-I. ApoA-I dissociates from the smaller, lipid-poor HDL, and the free apoA-I (molecular weight 28,000) can be filtered by the glomerulus in the kidney and most likely is degraded in renal tubular cells after reabsorption. Thus, increased free fatty acid transport in plasma, a common abnormality in insulin-resistant states, may be the underlying driving force for the two common lipid abnormalities seen in diabetes.

Low rates of apoB secretion from HepG2 cells result from reduced delivery of newly synthesized triglyceride to a "secretion-coupled" pool

The present study was aimed at defining the roles of intracellular triglyceride pools in apolipoprotein B secretion from HepG2 cells. Oleic acid (0.2 mmol/L) in the medium stimulated both triglyceride synthesis and apolipoprotein B secretion. Stimulation of apolipoprotein B secretion was lost about 30-40 min after oleic acid was removed from the medium, despite the finding that most newly synthesized triglyceride was still present in the cells. This suggested that only a small fraction of newly synthesized triglyceride was transferred to a pool available for assembly of nascent apoB into lipoproteins. Using cell fractionation, we analyzed two triglyceride pools in HepG2 cells: a microsomal pool and a cytoplasmic pool. Oleic acid-induced increases in the microsomal pool were small and short-lived due to secretion; this pool, therefore, is a "secretion-coupled" pool. The large majority of newly synthesized triglyceride was in a cytosolic pool that was not associated with secretion of apoB. Dibutyryl cAMP treatment was associated with a 3-fold increase in the mobilization of the triglyceride droplets. Apolipoprotein B secretion, however, was not increased, suggesting that the amount of triglyceride that entered the "secretion-coupled" pool after hydrolysis and re-esterification of cytoplasmic triglyceride was inadequate to stimulate apolipoprotein B secretion. In summary, the majority of newly synthesized triglyceride, whether derived from exogenous or endogenous fatty acids, is rapidly shifted to a cytoplasmic pool that does not play a regulatory role in apolipoprotein B secretion. The presence of a very small "secretion-coupled" pool of triglyceride in HepG2 cells likely explains the high rates of degradation of nascent apolipoprotein B, and the low rates of secretion of lipid-poor lipoproteins.

Demonstration of a physical interaction between microsomal triglyceride transfer protein and apolipoprotein B during the assembly of ApoB-containing lipoproteins

Microsomal triglyceride (TG) transfer protein (MTP) is an endoplasmic reticulum lumenal protein consisting of a 97-kDa subunit and protein disulfide isomerase. It is believed that MTP delivers TG to nascent apoB molecules during the assembly of lipoprotein particles in the secretory pathway. Although in vitro studies have established the mechanism of TG transfer between donor and acceptor membranes, the mechanism of action of MTP in vivo remains unknown. The present studies were undertaken to examine whether or not the transfer of TG to nascent apoB in the endoplasmic reticulum involves the physical interaction between MTP and apoB. HepG2 cells were labeled with [3H]leucine, lysed in a nondenaturing homogenizing buffer, and immunoprecipitated with anti-MTP antiserum. We found that labeled apoB and protein disulfide isomerase were co-immunoprecipitated by this procedure. In addition, we were able to detect the 97-kDa subunit of MTP in these immunoprecipitates by immunoblot. The association of MTP and apoB, as assessed in pulse-labeled cells by co-immunoprecipitation, was transient; apoB was prominent on fluorgraphy at 10 min of chase but minimal thereafter. Oleic acid treatment, which protects apoB from rapid intracellular degradation by increasing TG availability, increased both the degree and the duration of association between MTP and apoB dramatically. Inhibition of TG synthesis by Triacsin D, on the other hand, significantly decreased the MTP-apoB binding. N-Acetyl-leucyl-leucyl-norleucinal, a cysteine protease inhibitor, which directly protects apoB from rapid intracellular degradation but does not affect TG synthesis, increased the interaction between MTP and apoB only slightly, although it did prolong it. Our results suggest that direct interaction between MTP and apoB occurs during the assembly of apoB-containing lipoproteins in HepG2 cells.