Effects on apoB-100 secretion and bile acid synthesis by redirecting cholesterol efflux from HepG2 cells

Hepatic cholesterol homeostasis: is the low-density lipoprotein pathway a regulatory or a shunt pathway?

OBJECTIVE

The hypothesis that cholesterol that enters the cell within low-density lipoprotein (LDL) particles rapidly equilibrates with the regulatory pool of intracellular cholesterol and maintains cholesterol homeostasis by reducing cholesterol and LDL receptor synthesis was validated in the fibroblast but not in the hepatocyte. Accordingly, the present studies were designed to compare the effects of cholesterol that enters the hepatocyte within an LDL particle with those of cholesterol that enters via other lipoprotein particles.

APPROACH AND RESULTS

We measured cholesterol synthesis and esterification in hamster hepatocytes treated with LDL and other lipoprotein particles, including chylomicron remnants and VLDL. Endogenous cholesterol synthesis was not significantly reduced by uptake of LDL, but cholesterol esterification (280%) and acyl CoA:cholesterol acyltransferase 2 expression (870%) were increased. In contrast, cholesterol synthesis was significantly reduced (70% decrease) with other lipoprotein particles. Furthermore, more cholesterol that entered the hepatocyte within LDL particles was secreted within VLDL particles (480%) compared with cholesterol from other sources.

CONCLUSIONS

Much of the cholesterol that enters the hepatocyte within LDL particles is shunted through the cell and resecreted within VLDL particles without reaching equilibrium with the regulatory pool.

Synthesis of apolipoprotein B lipoparticles to deliver hydrophobic/amphiphilic materials

To develop a drug delivery system (DDS), it is critical to address challenging tasks such as the delivery of hydrophobic and amphiphilic compounds, cell uptake, and the metabolic fate of the drug delivery carrier. Low-density lipoprotein (LDL) has been acknowledged as the human serum transporter of natively abundant lipoparticles such as cholesterol, triacylglycerides, and lipids. Apolipoprotein B (apo B) is the only protein contained in LDL, and possesses a binding moiety for the LDL receptor that can be internalized and degraded naturally by the cell. Therefore, synthetic/reconstituting apoB lipoparticle (rABL) could be an excellent delivery carrier for hydrophobic or amphiphilic materials. Here, we synthesized rABL in vitro, using full-length apoB through a five-step solvent exchange method, and addressed its potential as a DDS. Our rABL exhibited good biocompatibility when evaluated with cytotoxicity and cell metabolic response assays, and was stable during storage in phosphate-buffered saline at 4 °C for several months. Furthermore, hydrophobic superparamagnetic iron oxide nanoparticles (SPIONPs) and the anticancer drug M4N (tetra-O-methyl nordihydroguaiaretic acid), used as an imaging enhancer and lipophilic drug model, respectively, were incorporated into the rABL, leading to the formation of SPIONPs- and M4N- containing rABL (SPIO@rABL and M4N@rABL, respectively). Fourier transform infrared spectroscopy suggested that rABL has a similar composition to that of LDL, and successfully incorporated SPIONPs or M4N. SPIO@rABL presented significant hepatic contrast enhancement in T2-weighted magnetic resonance imaging in BALB/c mice, suggesting its potential application as a medical imaging contrast agent. M4N@rABL could reduce the viability of the cancer cell line A549. Interestingly, we developed solution-phase high-resolution transmission electron microscopy to observe both LDL and SPIO@rABL in the liquid state. In summary, our LDL-based DDS, rABL, has significant potential as a novel DDS for hydrophobic and amphiphilic materials, with good cell internalization properties and metabolicity.

Cholesterol lowering with bile salt hydrolase-active probiotic bacteria, mechanism of action, clinical evidence, and future direction for heart health applications

INTRODUCTION

Cardiovascular diseases (CVD) are the leading cause of global mortality and morbidity. Current CVD treatment methods include dietary intervention, statins, fibrates, niacin, cholesterol absorption inhibitors, and bile acid sequestrants. These formulations have limitations and, thus, additional treatment modalities are needed. Probiotic bacteria, especially bile salt hydrolase (BSH)-active probiotic bacteria, have demonstrated cholesterol-lowering efficacy in randomized controlled trials.

AREAS COVERED

This review describes the current treatments for CVD and the need for additional therapeutics. Gut microbiota etiology of CVD, cholesterol metabolism, and the role of probiotic formulations as therapeutics for the treatment and prevention of CVD are described. Specifically, we review studies using BSH-active bacteria as cholesterol-lowering agents with emphasis on their cholesterol-lowering mechanisms of action. Potential limitations and future directions are also highlighted.

EXPERT OPINION

Numerous clinical studies have concluded that BSH-active probiotic bacteria, or products containing them, are efficient in lowering total and low-density lipoprotein cholesterol. However, the mechanisms of action of BSH-active probiotic bacteria need to be further supported. There is also the need for a meta-analysis to provide better information regarding the therapeutic use of BSH-active probiotic bacteria. The future of BSH-active probiotic bacteria most likely lies as a combination therapy with already existing treatment options.

ApoB versus non-HDL-C: what to do when they disagree

The high correlation between apolipoprotein B (apoB) and non-high-density lipoprotein cholesterol (non-HDL-C) is the chief argument employed against introducing apoB into clinical practice. However, high correlation does mean that non-HDL-C and apoB will often yield similar clinical information. Nevertheless, the critical issue is not how often the two tests agree, but how often, and how substantially, they differ. In other words, how often would an apoB result change a clinical decision based on a value for non-HDL-C? This article presents a series of examples from prominent published studies in which apoB and non-HDL-C differ so dramatically that diagnosis and therapy would truly differ depending on which index was used by the physician. These examples establish that apoB and non-HDL-C are not clinical equivalents.

Adiponectin accelerates reverse cholesterol transport by increasing high density lipoprotein assembly in the liver

Plasma high density lipoprotein (HDL)-cholesterol levels are negatively correlated with the incidence of coronary artery disease. HDL plays an important role in protecting against atherosclerosis by removing cholesterol from atheroma and transporting it back to the liver. The ATP-binding cassette transporters (ABCA1 and ABCG1) and scavenger receptor BI (SR-BI) are thought to be one of the rate-limiting factors to generate HDL in the liver. Adiponectin (APN) secreted from adipocytes is also one of the important molecules to inhibit the development of atherosclerosis. Recently, it has been reported that plasma HDL-cholesterol levels are positively correlated with plasma APN concentrations in humans. Therefore, we investigated the association of APN with HDL assembly in the liver. Human hepatoma cell line, HepG2 cells, were incubated for 24h in the culture medium with the indicated concentrations of recombinant APN. APN enhanced the mRNA level of apolipoprotein A-I (apoA-I) in HepG2 cells and increased the secretion of apoA-I from the cells to the medium. Furthermore, APN increased both mRNA and protein levels of ABCA1, but not ABCG1 and SR-BI, in HepG2 cells. Taken together, the current study demonstrates that APN might protect against atherosclerosis by increasing HDL assembly through enhancing ABCA1 pathway and apoA-1 synthesis in the liver.

Regulation of postprandial lipemia: an update on current trends

People spend a large percentage of their waking hours in the postprandial state. Postprandial lipemia is associated with disruptions in lipoprotein metabolism and inflammatory factors, cardiovascular disease, MetS, and diabetes. Commonly, the dietary sources of fat exceed the actual needs and the tissues are faced with the excess, with accumulation of chylomicrons and remnant particles. This review will summarize recent findings in postprandial lipemia research with a focus on human studies. The effects of dietary factors and other meal components on postprandial lipemia leads to the following question: do we need a standardized oral lipid tolerance test (OLTT)? An overview of recent findings on FABP2, MTP, LPL, apoAV, and ASP and the effects of body habitus (sex influence and body size), as well as exercise and weight loss, on postprandial lipemia will be summarized.

Assessing the effects of LXR agonists on cellular cholesterol handling: a stable isotope tracer study

The liver X receptors (LXRs) alpha and beta are responsible for the transcriptional regulation of a number of genes involved in cholesterol efflux from cells and therefore may be molecular targets for the treatment of cardiovascular disease. However, the effects of LXR ligands on cholesterol turnover in cells has not been examined comprehensively. In this study, cellular cholesterol handling (e.g., synthesis, catabolism, influx, and efflux) was examined using a stable isotope labeling study and a two-compartment modeling scheme. In HepG2 cells, the incorporation of 13C into cholesterol from [1-13C]acetate was analyzed by mass isotopomer distribution analysis in conjunction with nonsteady state, multicompartment kinetic analysis to calculate the cholesterol fluxes. Incubation with synthetic, nonsteroidal LXR agonists (GW3965, T0901317, and SB742881) increased cholesterol synthesis (approximately 10-fold), decreased cellular cholesterol influx (71-82%), and increased cellular cholesterol efflux (1.7- to 1.9-fold) by 96 h. As a consequence of these altered cholesterol fluxes, cellular cholesterol decreased (36-39%) by 96 h. The increased cellular cholesterol turnover was associated with increased expression of the LXR-activated genes ABCA1, ABCG1, FAS, and sterol-regulatory element binding protein 1c. In summary, the mathematical model presented allows time-dependent calculations of cellular cholesterol fluxes. These data demonstrate that all of the cellular cholesterol fluxes were altered by LXR activation and that the increase in cholesterol synthesis did not compensate for the increased cellular cholesterol efflux, resulting in a net cellular cholesterol loss.

ApoA-I Lipidation in Primary Mouse Hepatocytes

The liver is the major site of both apolipoprotein A-I (apoA-I) synthesis and ATP-binding cassette transporter A1 (ABCA1) expression. Here, we compare the lipidation with cholesterol and phospholipid of newly synthesized human apoA-I (hapoA-I) using adenoviral vector-mediated endogenous expression or exogenously added hapoA-I in wild type and ABCA1-null hepatocytes. Hepatocytes were labeled with [3H]cholesterol (delivered with LDL or methyl-beta-cyclodextrin), [3H]mevalonate, or [3H]choline. ABCA1 deficiency decreased apoA-I phospholipidation by 80%, but acquisition of de novo synthesized and exogenous cholesterol only decreased by 40-60%. The transfer of de novo synthesized cholesterol to apoA-I was decreased at all time points, but that of exogenously delivered cholesterol was independent of ABCA1 activity at the early time points. Progesterone does not affect apoA-I synthesis or its lipidation but inhibited the early phase of apoA-I cholesterol lipidation in both wild type and ABCA1-null hepatocytes. Fast protein liquid chromatography analysis of medium lipoproteins confirmed that with ABCA1 deficiency, the proportion of secreted high density lipoprotein-associated apoA-I and cholesterol decreased by about 50%. The very low density lipoprotein (VLDL)/LDL size fraction also contained a significant level of cholesterol in ABCA1 deficiency, consistent with the result of immunoprecipitations showing the presence of lipoproteins with both apoA-I and murine apoB. ApoA-I lipidation with newly synthesized cholesterol in ABCA1-null hepatocytes was significantly decreased by brefeldin A and monensin. In conclusion, we demonstrate that: (i) whereas most hepatic phospholipidation of apoA-I is mediated by ABCA1, acquisition of cholesterol depends on active transfer from intracellular compartments by ABCA1-dependent and -independent pathways, both sensitive to progesterone and (ii) there is separate regulation of phospholipid and cholesterol lipidation of apoA-I in hepatocytes.

ABCA1-dependent lipid efflux to apolipoprotein A-I mediates HDL particle formation and decreases VLDL secretion from murine hepatocytes

High levels of expression of the ATP binding cassette transporter A1 (ABCA1) in the liver and the need to over- or underexpress hepatic ABCA1 to impact plasma HDL levels in mice suggest a major role of the liver in HDL formation and in determining circulating HDL levels. Cultured murine hepatocytes were used to examine the role of hepatic ABCA1 in mediating the lipidation of apolipoprotein A-I (apoA-I) for HDL particle formation. Exogenous apoA-I stimulated cholesterol efflux to the medium from wild-type hepatocytes, but not from ABCA1-deficient (abca1(-/-)) hepatocytes. ApoA-I induced the formation of new HDL particles and enhanced the lipidation of endogenously secreted murine apoA-I in ABCA1-expressing but not abca1(-/-) hepatocytes. ABCA1-dependent cholesterol mobilization to apoA-I increased new cholesterol synthesis, indicating depletion of the regulatory pool of hepatocyte cholesterol during HDL formation. Secretion of triacylglycerol and apoB was decreased following apoA-I incubation with ABCA1-expressing but not abca1(-/-) hepatocytes. These results support a major role for hepatocyte ABCA1 in generating a critical pool of HDL precursor particles that enhance further HDL generation and passive cholesterol mobilization in the periphery. The results also suggest that diversion of hepatocyte cholesterol into the "reverse" cholesterol transport pathway diminishes cholesterol availability for apoB-containing lipoprotein secretion by the liver.

Scavenger receptor class B type I-mediated protection against atherosclerosis in LDL receptor-negative mice involves its expression in bone marrow-derived cells

OBJECTIVE

Scavenger receptor class B type I (SR-BI) is a cell-surface HDL receptor that is implicated in reverse cholesterol transport and protection against atherosclerosis. We have previously demonstrated that SR-BI/apolipoprotein E double-knockout mice develop severe occlusive coronary artery disease and myocardial infarction and die at approximately 6 weeks of age. To determine if this is a general effect of a lack of SR-BI, we generated mice deficient in both SR-BI and the LDL receptor.

METHODS AND RESULTS

Complete ablation of SR-BI expression in LDL receptor knockout mice resulted in increased plasma cholesterol associated with HDL particles of abnormally large size and a 6-fold increase in diet-induced aortic atherosclerosis but no macroscopic evidence of early-onset coronary artery disease, cardiac pathology, or early death. Furthermore, selective elimination of SR-BI expression in bone marrow-derived cells resulted in increased diet-induced atherosclerosis in LDL receptor knockout mice without concomitant alterations in the distributions of plasma lipoprotein cholesterol.

CONCLUSIONS

SR-BI expression protects against atherosclerosis in LDL receptor-deficient as well as apolipoprotein E-deficient mice, and its expression in bone marrow-derived cells contributes to this protection.