Dual effects on HDL metabolism by cholesteryl ester transfer protein inhibition in HepG2 cells

HDL Function and Atherosclerosis: Reactive Dicarbonyls as Promising Targets of Therapy

Epidemiologic studies detected an inverse relationship between HDL (high-density lipoprotein) cholesterol (HDL-C) levels and atherosclerotic cardiovascular disease (ASCVD), identifying HDL-C as a major risk factor for ASCVD and suggesting atheroprotective functions of HDL. However, the role of HDL-C as a mediator of risk for ASCVD has been called into question by the failure of HDL-C-raising drugs to reduce cardiovascular events in clinical trials. Progress in understanding the heterogeneous nature of HDL particles in terms of their protein, lipid, and small RNA composition has contributed to the realization that HDL-C levels do not necessarily reflect HDL function. The most examined atheroprotective function of HDL is reverse cholesterol transport, whereby HDL removes cholesterol from plaque macrophage foam cells and delivers it to the liver for processing and excretion into bile. Indeed, in several studies, HDL has shown inverse associations between HDL cholesterol efflux capacity and ASCVD in humans. Inflammation plays a key role in the pathogenesis of atherosclerosis and vulnerable plaque formation, and a fundamental function of HDL is suppression of inflammatory signaling in macrophages and other cells. Oxidation is also a critical process to ASCVD in promoting atherogenic oxidative modifications of LDL (low-density lipoprotein) and cellular inflammation. HDL and its proteins including apoAI (apolipoprotein AI) and PON1 (paraoxonase 1) prevent cellular oxidative stress and LDL modifications. Importantly, HDL in humans with ASCVD is oxidatively modified rendering HDL dysfunctional and proinflammatory. Modification of HDL with reactive carbonyl species, such as malondialdehyde and isolevuglandins, dramatically impairs the antiatherogenic functions of HDL. Importantly, treatment of murine models of atherosclerosis with scavengers of reactive dicarbonyls improves HDL function and reduces systemic inflammation, atherosclerosis development, and features of plaque instability. Here, we discuss the HDL antiatherogenic functions in relation to oxidative modifications and the potential of reactive dicarbonyl scavengers as a therapeutic approach for ASCVD.

Revealing the Role of High-Density Lipoprotein in Colorectal Cancer

Colorectal cancer (CRC) is a highly prevalent malignancy with multifactorial etiology, which includes metabolic alterations as contributors to disease development. Studies have shown that lipid status disorders are involved in colorectal carcinogenesis. In line with this, previous studies have also suggested that the serum high-density lipoprotein cholesterol (HDL-C) level decreases in patients with CRC, but more recently, the focus of investigations has shifted toward the exploration of qualitative properties of HDL in this malignancy. Herein, a comprehensive overview of available evidences regarding the putative role of HDL in CRC will be presented. We will analyze existing findings regarding alterations of HDL-C levels but also HDL particle structure and distribution in CRC. In addition, changes in HDL functionality in this malignancy will be discussed. Moreover, we will focus on the genetic regulation of HDL metabolism, as well as the involvement of HDL in disturbances of cholesterol trafficking in CRC. Finally, possible therapeutic implications related to HDL will be presented. Given the available evidence, future studies are needed to resolve all raised issues concerning the suggested protective role of HDL in CRC, its presumed function as a biomarker, and eventual therapeutic approaches based on HDL.

Particle number analysis of lipoprotein subclasses by gel permeation HPLC in patients with cholesteryl ester transfer protein deficiency

Objective

We previously reported that patients with cholesteryl ester transfer protein (CETP) deficiency (CETP-D) have a higher prevalence of atherosclerotic cardiovascular disease, in spite of increased HDL-C levels. However, characterization of HDL in CETP-D has not been well described. Therefore, we examined HDL particle number (PN) rather than HDL-C level.

Approach and results

Nine patients with CETP-D and 9 normolipidemic subjects were enrolled. We performed gel permeation high-performance liquid chromatography (GP-HPLC) analysis, determined the cholesterol and triglyceride composition of all lipoprotein subclasses, and calculated the PN of each subclass, which consisted of 3 VLDL (large, medium, and small), 4 LDL (large, medium, small, and very small), and 5 HDL (very large, large, medium, small, and very small) subclasses. The PNs of large and medium LDL were significantly lower in CETP-D than that in healthy subjects (0.66- and 0.63-fold decrease, respectively; p<0.001), whereas the PN of very small LDL, which is known to be atherogenic, was significantly higher (1.36-fold increase, p = 0.016). The PNs of very large and large HDL in CETP-D were markedly higher than that in healthy subjects (19.9- and 4.5-fold increase, respectively; p<0.001), whereas the PNs of small and very small HDL, which have more potent anti-atherogenic functions, were significantly lower (0.76- and 0.61-fold decrease, respectively; p<0.001).

Conclusion

We have assessed the PNs of detailed subclasses of patients with CETP-D for the first time. The PN of larger HDL was markedly increased, that of smaller HDL was decreased, and that of very small LDL was increased, suggesting that CETP-D has pro-atherogenic lipoprotein properties.

Re-evaluation of cholesteryl ester transfer protein function in atherosclerosis based upon genetics and pharmacological manipulation

PURPOSE OF REVIEW

To re-evaluate the functions of plasma cholesteryl ester transfer protein (CETP) in atherosclerosis based upon recent findings from human genetics and pharmacological CETP manipulation.

RECENT FINDINGS

CETP is involved in the transfer of cholesteryl ester from HDL to apolipoprotein B-containing lipoproteins, a key step of reverse cholesterol transport (RCT). CETP inhibitors have been developed to raise serum HDL-cholesterol (HDL-C) levels and reduce cardiovascular events. However, outcome studies of three CETP inhibitors (torcetrapib, dalcetrapib and evacetrapib) were prematurely terminated because of increased mortality or futility despite marked increases in HDL-cholesterol and decreases in LDL-cholesterol except for dalcetrapib. Patients with CETP deficiency show remarkable changes in HDL and LDL and are sometimes accompanied by atherosclerotic cardiovascular diseases. Recent prospective epidemiological studies demonstrated atheroprotective roles of CETP. CETP inhibition induces formation of small dense LDL and possibly dysfunctional HDL and downregulates hepatic scavenger receptor class B type I (SR-BI). Therefore, CETP inhibitors may interrupt LDL receptor and SR-BI-mediated cholesterol delivery back to the liver.

SUMMARY

For future drug development, the opposite strategy, namely enhancers of RCT via CETP and SR-BI activation as well as the inducers of apolipoprotein A-I or HDL production might be a better approach rather than delaying HDL metabolism by inhibiting a main stream of RCT in vivo.

Defective triglyceride biosynthesis in CETP-deficient SW872 cells

We previously reported that reducing the expression of cholesteryl ester transfer protein (CETP) disrupts cholesterol homeostasis in SW872 cells and causes an ∼50% reduction in TG. The causes of this reduced TG content, investigated here, could not be attributed to changes in the differentiation status of CETP-deficient cells, nor was there evidence of endoplasmic reticulum (ER) stress. In short-term studies, the total flux of oleate through the TG biosynthetic pathway was not altered in CETP-deficient cells, although mRNA levels of some pathway enzymes were different. However, the conversion of diglyceride (DG) to TG was impaired. In longer-term studies, newly synthesized TG was not effectively transported to lipid droplets, yet this lipid did not accumulate in the ER, apparently due to elevated lipase activity in this organelle. DG, shown to be a novel CETP substrate, was also inefficiently transferred to lipid droplets. This may reduce TG synthesis on droplets by resident diacylglycerol acyltransferase. Overall, these data suggest that the decreased TG content of CETP-deficient cells arises from the reduced conversion of DG to TG in the ER and/or on the lipid droplet surface, and enhanced TG degradation in the ER due to its ineffective transport from this organelle.

Overexpression of full-length cholesteryl ester transfer protein in SW872 cells reduces lipid accumulation

Cells produce two cholesteryl ester transfer protein (CETP) isoforms, full-length and a shorter variant produced by alternative splicing. Blocking synthesis of both isoforms disrupts lipid metabolism and storage. To further define the role of CETP in cellular lipid metabolism, we stably overexpressed full-length CETP in SW872 cells. These CETP(+) cells had several-fold higher intracellular CETP and accumulated 50% less TG due to a 26% decrease in TG synthesis and 2.5-fold higher TG turnover rate. Reduced TG synthesis was due to decreased fatty acid uptake and impaired conversion of diglyceride to TG even though diacylglycerol acyltransferase activity was normal. Sterol-regulatory element binding protein 1 mRNA levels were normal, and although PPARγ expression was reduced, the expression of several of its target genes including adipocyte triglyceride lipase, FASN, and APOE was normal. CETP(+) cells contained smaller lipid droplets, consistent with their higher levels of perilipin protein family (PLIN) 3 compared with PLIN1 and PLIN2. Intracellular CETP was mostly associated with the endoplasmic reticulum, although CETP near lipid droplets poorly colocalized with this membrane. A small pool of CETP resided in the cytoplasm, and a subfraction coisolated with lipid droplets. These data show that overexpression of full-length CETP disrupts lipid homeostasis resulting in the formation of smaller, more metabolically active lipid droplets.

Expression of human hepatic lipase negatively impacts apolipoprotein A-I production in primary hepatocytes from Lipc-null mice

This study aimed to examine whether expression of human hepatic lipase (hHL) exerted an intracellular effect on hepatic production of apolipoprotein (apo) A-I. The levels of secreted and cell-associated apoA-I were contrasted between primary hepatocytes isolated from Lipc-null and C57BL/6 mice, and between Lipc-null hepatocytes transfected with either hHL-encoding or control adenovirus. An HSPG-binding deficient hHL protein (hHLmt) was used to determine the impact of cell surface binding on HL action. Accumulation of apoA-I in conditioned media of primary hepatocytes isolated from Lipc-null mice was increased as compared to that from C57BL/6 mice. Metabolic labeling experiments showed that secretion of ³⁵S-apoA-I from Lipc-null cells was significantly higher than that from C57BL/6 cells. Expression of hHL in Lipc-null hepatocytes, through adenovirus-mediated gene transfer, resulted in decreased synthesis and secretion of ³⁵S-apoA-I, but not ³⁵S-apoE, as compared with cells transfected with control adenovirus. Expression of HSPG-binding deficient hHLmt in Lipc-null cells also exerted an inhibitory effect on apoA-I production, even though hHLmt displayed impaired exit from the endoplasmic reticulum as compared with hHL. Subcellular fractionation revealed that expression of hHL or hHLmt led to increased microsome-association of apoA-I relative to non-transfected control. Expression of hHL negatively impacts hepatic production of apoA-I.

Cholesteryl ester transfer proteins from different species do not have equivalent activities

Site-specific changes in the amino acid composition of human cholesteryl ester transfer protein (CETP) modify its preference for triglyceride (TG) versus cholesteryl ester (CE) as substrate. CETP homologs are found in many species but little is known about their activity. Here, we examined the lipid transfer properties of CETP species with 80-96% amino acid identity to human CETP. TG/CE transfer ratios for recombinant rabbit, monkey, and hamster CETPs were 1.40-, 1.44-, and 6.08-fold higher than human CETP, respectively. In transfer assays between VLDL and HDL, net transfers of CE into VLDL by human and monkey CETPs were offset by equimolar net transfers of TG toward HDL. For hamster CETP this process was not equimolar but resulted in a net flow of lipid (TG) into HDL. When assayed for the ability to transfer lipid to an acceptor particle lacking CE and TG, monkey and hamster CETPs were most effective, although all CETP species were able to promote this one-way movement of neutral lipid. We conclude that CETPs from human, monkey, rabbit, and hamster are not functionally equivalent. Most unique was hamster CETP, which strongly prefers TG as a substrate and promotes the net flow of lipid from VLDL to HDL.

Identification of Novel Human High-Density Lipoprotein Receptor Up-regulators Using a Cell-Based High-Throughput Screening Assay

Scavenger receptor class B type I (SR-BI) is the high-affinity high-density lipoprotein (HDL) receptor, and CLA-1 is the human homologue of the murine SR-BI. CLA-1/SR-BI receptor has been suggested as a new preventative and/or therapeutic target for atherosclerosis due to its pivotal role in overall HDL cholesterol (HDL-C) metabolism and its antiatherogenic activity in vivo. To search for active compounds that can increase CLA-1 transcription, a novel cell-based assay was developed for application in high-throughput screening (HTS). Human hepatoma HepG2 cells were transfected with a CLA-1-promoter-luciferase reporter gene construct, and the stable transfected cell line was selected and named CLAp-LUC HepG2. With rosiglitazone as a positive control, this stable cell line was used to establish a specific CLA-1 gene expression assay in a 96-well microplate format. The evaluating parameter Z' value of 0.64 showed that this cell-based HTS assay was robust and reliable. Screening of 6000 microbial secondary metabolite crude extracts identified 8 positive strains. Between 2 identified CLA-1 up-regulators produced by actinomycete strain 04-4776, 4776B may stimulate not only the expression of CLA-1 on the transcriptional and translational levels but also the activity of CLA-1 to uptake the HDL-C in HepG2 cells. The active compounds originated from this HTS assay may be developed to drug candidates or lead compounds for new antiatherosclerosis agents.

Cholesteryl ester transfer protein (CETP) inhibition beyond raising high-density lipoprotein cholesterol levels: pathways by which modulation of CETP activity may alter atherogenesis

Raising high-density lipoprotein cholesterol (HDL-C) is a promising strategy in the struggle to prevent cardiovascular disease, and cholesteryl ester transfer protein (CETP) inhibitors have been developed to accomplish this. The first results are encouraging, and, in fact, in rabbits, inhibition of CETP reduces atherosclerosis. Because human data regarding the reduction of atheroma burden require more time, the biochemical mechanisms underlying the putative atheroprotection of CETP inhibitors are currently dissected, and several pathways have emerged. First, CETP inhibition increases HDL-C and reduces low-density lipoprotein cholesterol (LDL-C) levels consistent with CETP lipid transfer activity and its role in reverse cholesterol transport (RCT). This coincides with putative beneficial increases in both HDL and LDL size. However, many aspects regarding the impact of CETP inhibition on the RCT pathway remain elusive, in particular whether the first step concerning cholesterol efflux from peripheral tissues to HDL is influenced. Moreover, the relevance of scavenger receptor BI and consequently the central role of HDL in human RCT is still unclear. Second, CETP inhibition was shown recently to increase antioxidant enzymes associated with HDL, in turn associated with decreased oxidation of LDL. Atheroprotection in man is currently anticipated based on the improvement of these biochemical parameters known to influence atherosclerosis, but final confirmation regarding the impact of CETP inhibition on cardiovascular outcome will have to come from trials evaluating clinical end points.