Inefficient degradation of triglyceride-rich lipoprotein by HepG2 cells is due to a retarded transport to the lysosomal compartment

Wbox2: A clathrin terminal domain-derived peptide inhibitor of clathrin-mediated endocytosis

Clathrin-mediated endocytosis (CME) occurs via the formation of clathrin-coated vesicles from clathrin-coated pits (CCPs). Clathrin is recruited to CCPs through interactions between the AP2 complex and its N-terminal domain, which in turn recruits endocytic accessory proteins. Inhibitors of CME that interfere with clathrin function have been described, but their specificity and mechanisms of action are unclear. Here we show that overexpression of the N-terminal domain with (TDD) or without (TD) the distal leg inhibits CME and CCP dynamics by perturbing clathrin interactions with AP2 and SNX9. TDD overexpression does not affect clathrin-independent endocytosis or, surprisingly, AP1-dependent lysosomal trafficking from the Golgi. We designed small membrane–permeant peptides that encode key functional residues within the four known binding sites on the TD. One peptide, Wbox2, encoding residues along the W-box motif binding surface, binds to SNX9 and AP2 and potently and acutely inhibits CME.

S-nitrosylation of ARH is required for LDL uptake by the LDL receptor

The LDL receptor (LDLR) relies upon endocytic adaptor proteins for internalization of lipoproteins. The results of this study show that the LDLR adaptor autosomal recessive hypercholesterolemia protein (ARH) requires nitric oxide to support LDL uptake. Nitric oxide nitrosylates ARH at C199 and C286, and these posttranslational modifications are necessary for association of ARH with the adaptor protein 2 (AP-2) component of clathrin-coated pits. In the absence of nitrosylation, ARH is unable to target LDL-LDLR complexes to coated pits, resulting in poor LDL uptake. The role of nitric oxide on LDLR function is specific for ARH because inhibition of nitric oxide synthase activity impairs ARH-supported LDL uptake but has no effect on other LDLR-dependent lipoprotein uptake processes, including VLDL remnant uptake and dab2-supported LDL uptake. These findings suggest that cells that depend upon ARH for LDL uptake can control which lipoproteins are internalized by their LDLRs through changes in nitric oxide.

Quantitative fluorescence imaging reveals point of release for lipoproteins during LDLR-dependent uptake

The LDL receptor (LDLR) supports efficient uptake of both LDL and VLDL remnants by binding lipoprotein at the cell surface, internalizing lipoprotein through coated pits, and releasing lipoprotein in endocytic compartments before returning to the surface for further rounds of uptake. While many aspects of lipoprotein binding and receptor entry are well understood, it is less clear where, when, and how the LDLR releases lipoprotein. To address these questions, the current study employed quantitative fluorescence imaging to visualize the uptake and endosomal processing of LDL and the VLDL remnant β-VLDL. We find that lipoprotein release is rapid, with most release occurring prior to entry of lipoprotein into early endosomes. Published biochemical studies have identified two mechanisms of lipoprotein release: one that involves the β-propeller module of the LDLR and a second that is independent of this module. Quantitative imaging comparing uptake supported by the normal LDLR or by an LDLR variant incapable of β-propeller-dependent release shows that the β-propeller-independent process is sufficient for release for both lipoproteins but that the β-propeller process accelerates both LDL and β-VLDL release. Together these findings define where, when, and how lipoprotein release occurs and provide a generalizable methodology for visualizing endocytic handling in situ.

Shiga toxins: intracellular trafficking to the ER leading to activation of host cell stress responses

Despite efforts to improve hygenic conditions and regulate food and drinking water safety, the enteric pathogens, Shiga toxin-producing Escherichia coli (STEC) and Shigella dysenteriae serotype 1 remain major public health concerns due to widespread outbreaks and the severity of extra-intestinal diseases they cause, including acute renal failure and central nervous system complications. Shiga toxins are the key virulence factors expressed by these pathogens mediating extra-intestinal disease. Delivery of the toxins to the endoplasmic reticulum (ER) results in host cell protein synthesis inhibition, activation of the ribotoxic stress response, the ER stress response, and in some cases, the induction of apoptosis. Intrinsic and/or extrinsic apoptosis inducing pathways are involved in executing cell death following intoxication. In this review we provide an overview of the current understanding Shiga toxin intracellular trafficking, host cellular responses to the toxin and ER stress-induced apoptosis with an emphasis on recent findings.

The epidermal growth factor homology domain of the LDL receptor drives lipoprotein release through an allosteric mechanism involving H190, H562, and H586

The low density lipoprotein (LDL) receptor (LDLR) mediates efficient endocytosis of VLDL, VLDL remnants, and LDL. As part of the endocytic process, the LDLR releases lipoproteins in endosomes. The release process correlates with an acid-dependent conformational change in the receptor from an extended, "open" state to a compact, "closed" state. The closed state has an intramolecular contact involving H190, H562, and H586. The current model for lipoprotein release holds that protonation of these histidines drives the conformational change that is associated with release. We tested the roles of H190, H562, and H586 on LDLR conformation and on lipoprotein binding, uptake, and release using variants in which the three histidines were replaced with alanine (AAA variant) or in which the histidines were replaced with charged residues that can form ionic contacts at neutral pH (DRK variant). Contrary to expectation, both the AAA and the DRK variants exhibited normal acid-dependent transitions from open to closed conformations. Despite this similarity, both the AAA and DRK mutations modulated lipoprotein release, indicating that H190, H562, and H586 act subsequent to the conformational transition. These observations also suggest that the intramolecular contact does not drive release through a competitive mechanism. In support of this possibility, mutagenesis experiments showed that beta-VLDL binding was inhibited by mutations at D203 and E208, which are exposed in the closed conformation of the LDLR. We propose that H190, H562, and H586 are part of an allosteric mechanism that drives lipoprotein release.

Identification of a VLDL-induced, FDNPVY-independent internalization mechanism for the LDLR

The low-density lipoprotein (LDL) receptor (LDLR) binds to and internalizes lipoproteins that contain apolipoproteinB100 (apoB100) or apolipoproteinE (apoE). Internalization of the apoB100 lipoprotein ligand, LDL, requires the FDNPVY(807) sequence on the LDLR cytoplasmic domain, which binds to the endocytic machinery of coated pits. We show here that inactivation of the FDNPVY(807) sequence by mutation of Y807 to cysteine prevented the uptake of LDL; however, this mutation did not prevent LDLR-dependent uptake of the apoE lipoprotein ligand, beta-VLDL. Comparison of the surface localization of the LDLR-Y807C using LDLR-immunogold, LDL-gold and beta-VLDL-gold probes revealed enrichment of LDLR-Y807C-bound beta-VLDL in coated pits, suggesting that beta-VLDL binding promoted the internalization of the LDLR-Y807C. Consistent with this possibility, treatment with monensin, which traps internalized LDLR in endosomes, resulted in the loss of surface LDLR-Y807C only when beta-VLDL was present. Reconstitution experiments in which LDLR variants were introduced into LDLR-deficient cells showed that the HIC(818) sequence is involved in beta-VLDL uptake by the LDLR-Y807C. Together, these experiments demonstrate that the LDLR has a very low-density lipoprotein (VLDL)-induced, FDNPVY-independent internalization mechanism.

Endocytosis and intracellular trafficking of fatty acid esters of phenylaminopropanediol, the putative etiologic agents of the toxic oil syndrome (TOS)

The toxic oil syndrome (TOS) caused by ingestion of rapeseed oil adulterated with aniline is characterized by symptoms of an allergic and/or autoimmune illness associated with vessel wall lesions similar to those of atherosclerosis. Fatty acid esters of 3-(N-phenylamino)-1,2-propanediol (PAP) have been incriminated as the etiologic agents of TOS. However, the pathogenesis of TOS is yet unknown. Here, we addressed whether PAP fatty acid esters are incorporated into lipoproteins, which after transport to vascular endothelial cells are taken up to initiate TOS vasculopathy. After loading (14)C-dioleyl-ester of PAP into LDL labeled with 1,1'-dioctadecyl-3,3,3',3'-tetramethylindolcarbocyanine (DiI) we assessed receptor mediated endocytosis and intracellular localization of these lipopoproteins in vascular endothelial cells. Our data suggest that these lipoprotein-derivatives are internalized into endothelial cells by LDL receptor mediated endocytosis. Confocal microscopy revealed that DiI-LDL loaded with dioleyl-ester of PAP and incubated for 60 min with endothelial cells colocalizes with the lysosomotropic compound LysoTracker Green, indicating that internalized PAP-loaded LDL are targetted to the endolysosomal compartment for further processing. Subcellular fractionation of endothelial-like ECV-304 cells after incubation with LDL loaded with the (14)C-dioleyl-ester of PAP for 6h showed that the radioactive label accumulated in fractions containing endosomes, the Golgi apparatus and the endoplasmic reticulum.

Apolipoprotein E recycling: implications for dyslipidemia and atherosclerosis

After receptor-mediated endocytosis, the intracellular fate of triglyceride-rich lipoproteins (TRLs) is far more complex than the classical degradation pathway of low-density lipoproteins. Once internalized, TRLs disintegrate in peripheral endosomes, followed by a differential sorting of TRL components. Although core lipids and apolipoprotein B are targeted to lysosomes, the majority of TRL-derived apolipoprotein E (apoE) remains in peripheral recycling endosomes. This pool of TRL-derived apoE is then mobilized by high-density lipoproteins (HDLs) or HDL-derived apoA-I to be recycled back to the plasma membrane, followed by apoE resecretion and the subsequent formation of apoE-containing HDL. The HDL-induced recycling of apoE is accompanied by cholesterol efflux and involves the internalization and targeting of HDL-derived apoA-I to endosomes containing both apoE and cholesterol. These findings point to a yet unknown intracellular link between TRL-derived apoE, cellular cholesterol transport, and HDL metabolism. Recent studies provide first evidence that impaired recycling of TRL-derived apoE4, but not apoE3, is associated with intracellular cholesterol accumulation, which might explain some well-documented effects of apoE4 on HDL metabolism. This review summarizes the current understanding of apoE recycling and its potential role in the regulation of plasma apoE levels in the postprandial state.

Cytochrome p-450-mediated differential oxidative modification of proteins: albumin, apolipoprotein E, and CYP2E1 as targets

Although many studies established a role of cytochrome P-450s in metabolism of xenobiotics, few studies evaluating the ability of cytochrome P-450s to oxidize proteins have been reported. The ability of cytochrome P-450s to induce oxidative modification of albumin, apolipoprotein E, and CYP2E1 protein was investigated. Microsomal cytochrome P-450s induced production of reactive radical species, leading to differential modification of the proteins. Albumin remained unmodified, and CYP2E1 protein was degraded, whereas recombinant and endogenous apolipoprotein E was aggregated. The modification of apolipoprotein E was isoform independent. Cytochrome P-450 inhibitors or antioxidants inhibited the production of reactive radical species and protein modification. These results demonstrate that response of each protein to cytochrome P-450-mediated oxidative attack is different, and cytochrome P-450s can induce apolipoprotein E aggregation, a process that might be relevant to accumulation of altered protein in various abnormal conditions. In view of the ubiquitous expression of cytochrome P-450s, the present results may have important toxicological implications.

Recycling of apolipoprotein E and lipoprotein lipase through endosomal compartments in vivo

We have recently described a novel recycling pathway of triglyceride-rich lipoprotein (TRL)-associated apolipoprotein (apo) E in human hepatoma cells. We now demonstrate that not only TRL-derived apoE but also lipoprotein lipase (LPL) is efficiently recycled in vitro and in vivo. Similar recycling kinetics of apoE and LPL in normal and low density lipoprotein receptor-negative human fibroblasts also indicate that the low density lipoprotein receptor-related protein seems to be involved. Intracellular sorting mechanisms are responsible for reduced lysosomal degradation of both ligands after receptor-mediated internalization. Immediately after internalization in rat liver, TRLs are disintegrated, and apoE and LPL are found in endosomal compartments, whereas TRL-derived phospholipids accumulate in the perinuclear region of hepatocytes. Subsequently, substantial amounts of both proteins can be found in purified recycling endosomes, indicating a potential resecretion of these TRL components. Pulse-chase experiments of perfused rat livers with radiolabeled TRLs demonstrated a serum-induced release of internalized apoE and LPL into the perfusate. Analysis of the secreted proteins identified approximately 80% of the recycled TRL-derived proteins in the high density lipoprotein fractions. These results provide the first evidence that recycling of TRL-derived apoE and LPL could play an important role in the modulation of lipoproteins in vivo.

Intracellular metabolism of triglyceride-rich lipoproteins

Over the past 10 years, many advances have been made in our understanding of the intravascular metabolism of triglyceride-rich lipoproteins. It is now known that the complex extracellular interactions of triglyceride-rich lipoprotein-associated apolipoprotein E, lipoprotein lipase and hepatic lipase with heparan sulfate proteoglycans and lipoprotein receptors facilitate the hepatocellular uptake of triglyceride-rich lipoproteins. Recent studies have also revealed that the intracellular fate of internalized triglyceride-rich lipoproteins is highly complex. The dissociation of triglyceride-rich lipoprotein components within intracellular endosomal compartments involves the recycling of apolipoprotein E, whereas the remaining lipid core associated with apolipoprotein B is susceptible to lysosomal degradation. Apolipoprotein E recycling is an important newly discovered feature of lipoprotein metabolism, and will be discussed in the context of its intracellular transport mechanisms and cholesterol efflux. Current concepts concerning its potential relevance with regard to lipoprotein metabolism and atherosclerosis will also be discussed.

Apolipoprotein E is resistant to intracellular degradation in vitro and in vivo. Evidence for retroendocytosis

Apolipoprotein E (apoE) is an important determinant for the uptake of triglyceride-rich lipoproteins and emulsions by the liver, but the intracellular pathway of apoE following particle internalization is poorly defined. In the present study, we investigated whether retroendocytosis is a unique feature of apoE as compared with apoB by studying the intracellular fate of very low density lipoprotein-sized apoE-containing triglyceride-rich emulsion particles and LDL after LDLr-mediated uptake. Incubation of HepG2 cells with [(3)H]cholesteryl oleate-labeled particles at 37 degrees C led to a rapid release of [(3)H]cholesterol within 30 min for both LDL and emulsion particles. In contrast, emulsion-derived (125)I-apoE was more resistant to degradation (>/=120 min) than LDL-derived (125)I-apoB (30 min). Incubation at 18 degrees C, which allows endosomal uptake but prevents lysosomal degradation, with subsequent incubation at 37 degrees C resulted in a time-dependent release of intact apoE from the cells (up to 14% of the endocytosed apoE at 4 h). The release of apoE was accelerated by the presence of protein-free emulsion (20%) or high density lipoprotein (26%). Retroendocytosis of intact particles could be excluded since little intact [(3)H]cholesteryl oleate was released (<3%). In contrast, the degradation of LDL was complete with virtually no secretion of intact apoB into the medium. The intracellular stability of apoE was also demonstrated after hepatic uptake in C57Bl/6 mice. Intravenous injection of (125)I-apoE and [(3)H]cholesteryl oleate-labeled emulsions resulted in efficient LDLr-mediated uptake of both components by the liver (45-50% of the injected dose after 20 min). At 1 h after injection, only 15-20% of the hepatic (125)I-apoE was degraded, whereas 75% of the [(3)H]cholesteryl oleate was hydrolyzed. From these data we conclude that following LDLr-mediated internalization by liver cells, apoE can escape degradation and can be resecreted. This sequence of events may allow apoE to participate in its hypothesized intracellular functions such as mediator of the post-lysosomal trafficking of lipids and very low density lipoprotein assembly.

Apolipoprotein E participates in the regulation of very low density lipoprotein-triglyceride secretion by the liver

ApoE-deficient mice on low fat diet show hepatic triglyceride accumulation and a reduced very low density lipoprotein (VLDL) triglyceride production rate. To establish the role of apoE in the regulation of hepatic VLDL production, the human APOE3 gene was introduced into apoE-deficient mice by cross-breeding with APOE3 transgenics (APOE3/apoe-/- mice) or by adenoviral transduction. APOE3 was expressed in the liver and, to a lesser extent, in brain, spleen, and lung of transgenic APOE3/apoe-/- mice similar to endogenous apoe. Plasma cholesterol levels in APOE/apoe-/- mice (3.4 +/- 0.5 mM) were reduced when compared with apoe-/- mice (12.6 +/- 1.4 mM) but still elevated when compared with wild type control values (1.9 +/- 0.1 mM). Hepatic triglyceride accumulation in apoE-deficient mice was completely reversed by introduction of the APOE3 transgene. The in vivo hepatic VLDL-triglyceride production rate was reduced to 36% of control values in apoE-deficient mice but normalized in APOE3/apoe-/- mice. Hepatic secretion of apoB was not affected in either of the strains. Secretion of (3)H-labeled triglycerides synthesized from [(3)H]glycerol by cultured hepatocytes from apoE-deficient mice was four times lower than by APOE3/apoe-/- or control hepatocytes. The average size of secreted VLDL particles produced by cultured apoE-deficient hepatocytes was significantly reduced when compared with those of APOE3/apoe-/- and wild type mice. Hepatic expression of human APOE3 cDNA via adenovirus-mediated gene transfer in apoE-deficient mice resulted in a reduction of plasma cholesterol depending on plasma apoE3 levels. The in vivo VLDL-triglyceride production rate in these mice was increased up to 500% compared with LacZ-injected controls and correlated with the amount of apoE3 per particle. These findings indicate a regulatory role of apoE in hepatic VLDL-triglyceride secretion, independent from its role in lipoprotein clearance.

Dissection of the complex role of apolipoprotein E in lipoprotein metabolism and atherosclerosis using mouse models

Transgenic and knockout mice have been instrumental in delineating the role of apolipoprotein (apo) E in lipoprotein metabolism and atherosclerosis. The severe hypercholesterolemia and premature atherosclerosis of the apoE knockout mouse have been the starting point from which various physiologic processes have been identified in which apoE plays a critical role. These processes include 1) very low density lipoprotein (VLDL) triglyceride production; 2) lipoprotein lipase mediated triglyceride lipolysis; 3) VLDL remnant clearance and intracellular processing; and 4) the efflux of cellular cholesterol. In this review we will discuss the recent insight in the role of apoE in these processes, which has been obtained using a variety of in vivo and in vitro approaches to modify apoE expression and function.

Intracellular processing of endocytosed triglyceride-rich lipoproteins comprises both recycling and degradation

The current study was performed to investigate the intracellular fate of triglyceride-rich lipoproteins. Triglyceride-rich lipoproteins are responsible for the delivery of lipids to various tissues, however, their intracellular pathway has not yet been elucidated. Here radiolabeled triglyceride-rich lipoproteins, associated with lipoprotein lipase, were used for the quantitative evaluation of the intracellular metabolism. Pulse chase experiments showed that after 90 minutes approximately 60% of the labeled protein, mainly apoproteins E and C, was released intact into the medium, where it re-associates with lipoproteins. Apoprotein B, in contrast, was degraded, following the same pathway as the apoprotein B from low density lipoproteins. In kinetic experiments uptake and intracellular fate of triglyceride-rich lipoproteins was compared to that of transferrin and low density lipoproteins. These experiments revealed that apoproteins were retained inside the cell much longer than transferrin, and unlike low density lipoproteins were not degraded. Using immunofluorescence it was shown that apoprotein E and lipoprotein lipase follow a distinct route from the sorting compartment to the surface, which is clearly distinguishable from the perinuclear transferrin recycling compartment. In contrast, the fluorescence labeled lipids were delivered to lysosomal compartments. The data presented here show that surface proteins of triglyceride-rich lipoproteins, such as apoproteins E and C and lipoprotein lipase follow a recycling pathway, whereas lipids and high molecular mass core proteins are degraded.

Mucolipidosis type IV: abnormal transport of lipids to lysosomes

Mucolipidosis type IV (MLIV) is a lysosomal storage disorder in which various phospholipids and gangliosides accumulate. The cause of this storage has not yet been identified. Loading experiments in cultured fibroblasts with radioactive phosphatidylcholine ([14C]PC) labelled either in the acyl groups or in the choline residue, indicated increased retention of this lipid in the lysosomes of these patients. Chase experiments in intact fibroblasts, on the other hand, indicated normal degradation and discharge of the radioactive PC in MLIV lysosomes. This was further supported by measurements of the degradation of [14C]PC by isolated lysosomes which indicated normal breakdown of PC in MLIV. Cultured fibroblasts from Hunter (MPSII) patients, which contain enlarged and numerous lysosomes, did not store [14C]PC when compared to normal controls, indicating that the storage of this lipid in MLIV is not a secondary phenomenon caused by the presence of enlarged and numerous lysosomes in these cells. Incubation of [14C]PC at 18 degrees C limits the endocytosis process only up to early endosomes. This temperature did not yield increased retention of [14C]PC in MLIV, indicating that accumulation occurs only after the PC reached late endosomes or the lysosomes. The data indicate that PC as well as other phospholipids and gangliosides accumulate in MLIV apparently owing to a defect in the endocytosis process of membranous components. This defect apparently leads to excessive transportation of these macromolecules into lysosomes rather than their recycling to the plasma membrane. The endocytosis of membrane components is different from receptor-mediated endocytosis which is not affected in MLIV. Once the membrane macromolecules reach the lysosomes in MLIV they are normally catabolized and normally discharged. This may explain the heterogeneity of the stored materials in MLIV. The normal catabolism of macromolecules in the lysosomes is reflected in the minor deterioration in the clinical manifestations of these patients.

Efficient adenovirus-mediated ectopic gene expression of human lipoprotein lipase in human hepatic (HepG2) cells

Gene therapy to deliver and express a corrective lipoprotein lipase (LPL) gene may improve the lipid profile and reduce the morbidity and potential atherogenic risk from hypertriglyceridemia and dyslipoproteinemia in patients with complete or partial LPL deficiency. We have used an E1-/E3- adenoviral vector, with an RSV-driven human LPL cDNA expression cassette (Ad-RSV-LPL), to achieve high ectopic LPL gene expression in the human hepatoma cell line HepG2, an accepted hepatocellular model of lipoprotein metabolism. Ad-RSV-LPL transduction of HepG2 cells with a multiplicity of infection (moi) between 12.5 and 100 yielded dose-dependent increments in LPL mass and activity. Peak levels of LPL protein of 2,032.1 +/- 274.5 ng/10(5) cells per ml (mol 100) correlated with increased activity of 92.7 +/- 22.6 mU/10(5) cells per ml relative to negligible LPL levels in Ad-RSV-LacZ (beta-galactosidase) controls. Exogenous LPL expression over a 5-day period peaked at day 3. Susceptibility to inhibition by 1 M NaCl and an anti-LPL monoclonal antibody confirmed that lipase activity was indeed derived from human LPL. Hydrolysis, by LPL-overexpressing HepG2 cells, of TG carried in very-low-density lipoprotein (VLDL) showed that greater than 50% of the triglycerides (TG) disappeared after 4 hr of incubation. These results were compatible with FPLC evidence of a marked reduction in VLDL-TG. These results provide strong in vitro evidence that adenoviral-mediated ectopic expression of the human LPL gene could render hepatic cells capable of VLDL catabolism and thus support the possibility for in vivo adenoviral vector-mediated liver-targeted LPL gene therapy.

Lack of correlation between ACAT mRNA expression and cholesterol esterification in primary liver cells

A partial rabbit cDNA clone (14b) for ACAT has been characterized and used to demonstrate that hepatic and aortic ACAT mRNA14b abundance increased 2-3-fold in rabbits receiving a high fat/high cholesterol-diet compared to chow fed animals (Pape et al. (1995) J. Lipid Res. 36, 823-838). Because of those data we hypothesized that increased hepatic cholesteryl ester mass and synthesis rates in rabbit liver cells are associated with an increase in ACAT mRNA14b levels. To test this hypothesis we altered cellular cholesteryl ester mass and synthesis rates in primary parenchymal and nonparenchymal cells using various extracellular agents and measured the accumulated mass of ACAT mRNA14b. Parenchymal cells incubated with rabbit beta VLDL or mevalonolactone displayed a 6-10-fold increase in cellular cholesteryl ester mass over a three day treatment with no significant changes in cellular free cholesterol, triacylglycerols, or ACAT mRNA14b levels; HMG CoA reductase and LDL receptor mRNA mass decreased initially as a result of cholesteryl ester loading. Treatment of parenchymal cells with CI-976, an ACAT inhibitor, showed a marked reduction in cholesteryl ester synthetic rate compared to beta VLDL controls but displayed no change in ACAT mRNA14b levels. A mixed population of rabbit hepatic nonparenchymal cells was incubated with beta VLDL for 24 h in culture which resulted in a 6-fold increase in cellular cholesteryl ester mass; there was no change in ACAT mRNA14b levels. In an in vivo study, rabbits consuming a high fat/high cholesterol-diet for three weeks showed a 10-fold increase in hepatic cholesteryl ester with no significant changes in ACAT mRNA14b levels. Together these data indicate that rabbit liver cellular cholesteryl ester mass increases of up to 10-fold are not correlated with ACAT mRNA14b changes. Thus, hepatic ACAT mRNA14b expression and cellular cholesterol esterification do not appear to be coordinately regulated at this level of cholesteryl ester loading.