Effect and cellular site of action of cysteine protease inhibitors on the cholesterol esterification pathway in macrophages and Chinese hamster ovary cells

Effects of high-density lipoprotein(2) on cholesterol transport and acyl-coenzyme A:cholesterol acyltransferase activity in P388D1 macrophages

High-density lipoproteins are the putative vehicles for cholesterol removal from monocyte-derived macrophages, which are an important cell type in all stages of atherosclerosis. The role of HDL(2), an HDL subclass that accounts for most variation in plasma HDL-cholesterol concentration, in cholesterol metabolism in monocyte-derived macrophages is not known. In this study, the dose-dependent effects of HDL(2) on cellular cholesterol mass, efflux, and esterification, and on cellular cholesteryl ester (CE) hydrolysis using the mouse macrophage P388D1 cell line was investigated. HDL(2) at low concentrations (40 microg protein/ml) decreased CE content without affecting cellular free cholesterol content (FC), CE hydrolysis, or cholesterol biosynthesis. In addition, HDL(2) at low concentrations reduced cellular acyl-coenzyme A:cholesterol acyltransferase (ACAT) activity and increased FC efflux from macrophages. Thus, HDL(2) has two potential roles in reverse cholesterol transport. In one, HDL(2) is an acceptor of macrophage FC. In the other, more novel role, HDL(2) increases the availability of macrophage FC through the inhibition of ACAT. Elucidation of the mechanism by which HDL(2) inhibits ACAT could identify new therapeutic targets that enhance the transfer of cholesterol from macrophages to the liver.

Multidrug resistance (MDR1) P-glycoprotein enhances esterification of plasma membrane cholesterol

Class I P-glycoproteins (Pgp) confer multidrug resistance in tumors, but the physiologic function of Pgp in normal tissues remains uncertain. In cells derived from tissues that normally express Pgp, recent data suggest a possible role for Pgp in cholesterol trafficking from the plasma membrane to the endoplasmic reticulum. We investigated the esterification of plasma membrane cholesterol under basal conditions and in response to sphingomyelinase treatment in transfected and drug-selected cell lines expressing differing amounts of functional class I Pgp. Compared with parental NIH 3T3 fibroblasts, cells transfected with human multidrug resistance (MDR1) Pgp esterified more cholesterol both without and with sphingomyelinase. Esterification also was greater in drug-selected Dox 6 myeloma cells than parental 8226 cells, which express low and non-immunodetectable amounts of Pgp, respectively. However, no differences in total plasma membrane cholesterol were detected. Transfection of fibroblasts with the multidrug resistance-associated protein (MRP) did not alter esterification, showing that cholesterol trafficking was not generally affected by ATP-binding cassette transporters. Steroidal (progesterone, dehydroepiandrosterone) and non-steroidal antagonists (verapamil, PSC 833, LY335979, and GF120918) were evaluated for effects on both cholesterol trafficking and the net content of 99mTc-Sestamibi, a reporter of drug transport activity mediated by Pgp. In Pgp-expressing cells treated with nonselective and selective inhibitors, both the kinetics and efficacy of inhibition of cholesterol esterification differed from the antagonism of drug transport mediated by Pgp. Thus, although the data show that greater expression of class I Pgp within a given cell type is associated with enhanced esterification of plasma membrane cholesterol in support of a physiologic function for Pgp in facilitating cholesterol trafficking, the molecular mechanism is dissociated from the conventional drug transport activity of Pgp.

Differential effects of sphingomyelin hydrolysis and cholesterol transport on oxysterol-binding protein phosphorylation and Golgi localization

The deposition of de novo synthesized and lipoprotein-derived cholesterol at the plasma membrane and transport to the endoplasmic reticulum is dependent on sphingomyelin (SM) content. Here we show that hydrolysis of plasma membrane SM in Chinese hamster ovary cells by exogenous bacterial sphingomyelinase resulted in enhanced cholesterol esterification at the endoplasmic reticulum and rapid dephosphorylation of the oxysterol-binding protein (OSBP), a cytosolic/Golgi receptor for oxysterols such as 25-hydroxycholesterol. After sphingomyelinase treatment, restoration of OSBP phosphorylation closely paralleled resynthesis of SM and down-regulation of cholesterol ester synthesis. SM hydrolysis activated an okadaic acid-sensitive phosphatase that was not stimulated in Chinese hamster ovary cells by short chain ceramides. Agents that specifically blocked sphingomyelinase-mediated delivery of cholesterol to acyl-CoA:cholesterol acyltransferase (U18666A) or promoted cholesterol efflux to the medium (cyclodextrin) did not inhibit OSBP dephosphorylation. SM hydrolysis also promoted OSBP translocation from a vesicular compartment to the Golgi apparatus. Cyclodextrin and U18666A also caused OSBP translocation to the Golgi apparatus, suggesting that OSBP movement is coupled to changes in the cholesterol content of the plasma membrane or Golgi apparatus. These results identify OSBP as a potential target of SM turnover and cholesterol mobilization at the plasma membrane and/or Golgi apparatus.

Differential effects of lovastatin on the trafficking of endogenous and lipoprotein-derived cholesterol in human monocyte-derived macrophages

Lovastatin has been shown to reduce cholesterol esterification in cholesterol-loaded human macrophages. Surprisingly, in nonloaded macrophages, lovastatin produces the opposite effect, lowering free cholesterol and increasing cholesteryl ester levels, as measured by high-performance liquid chromatography. In cholesterol-loaded cells, lovastatin reduced the cholesteryl esters of unsaturated but not those of saturated fatty acids. In nonloaded cells, by contrast, the cholesteryl esters of unsaturated fatty acids tended to increase after lovastatin treatment. Total (free plus esterified) cellular cholesterol content in nonloaded cells fell by 18% with 12-micromol/L lovastatin treatment but did not change in cholesterol-loaded cells. Lovastatin had no effect on the binding or uptake of acetylated low density lipoprotein, acyl coenzyme A:cholesterol acyltransferase (ACAT) activity, the secretion of [3H]cholesterol into the medium, or lysosomal hydrolysis of cholesteryl esters. Apolipoprotein (apo) E mRNA levels increased but apoE secretion into the medium decreased with lovastatin treatment in both cholesterol-loaded and nonloaded cells. Cholesterol of exogenous origin has been shown to pass via the cell membrane before its esterification by ACAT. We postulate that this is not the case for endogenous cholesterol, which may have direct access to ACAT. Our findings therefore suggest that lovastatin hinders the delivery of intracellular cholesterol to the plasma membrane, resulting in increased free cholesterol and lower levels of cholesteryl ester in cholesterol-loaded cells. In nonloaded cells, virtually all cholesterol is of endogenous origin and is normally translocated to the cell membrane. Lovastatin prevents this process, thus shunting newly synthesized cholesterol toward esterification and leading to an increase in the concentration of cholesteryl esters, even in the face of a drop in total and free cholesterol levels. Intracellular apoE may play a role in this process.

Immunolocalization of acyl-coenzyme A:cholesterol O-acyltransferase in macrophages

Macrophages in atherosclerotic lesions accumulate large amounts of cholesteryl-fatty acyl esters ("foam cell" formation) through the intracellular esterification of cholesterol by acyl-coenzyme A:cholesterol O-acyltransferase (ACAT). In this study, we sought to determine the subcellular localization of ACAT in macrophages. Using mouse peritoneal macrophages and immunofluorescence microscopy, we found that a major portion of ACAT was in a dense reticular cytoplasmic network and in the nuclear membrane that colocalized with the luminal endoplasmic reticulum marker protein-disulfide isomerase (PDI) and that was in a similar distribution as the membrane-bound endoplasmic reticulum marker ribophorin. Remarkably, another portion of the macrophage ACAT pattern did not overlap with PDI or ribophorin, but was found in as yet unidentified cytoplasmic structures that were juxtaposed to the nucleus. Compartments containing labeled beta-very low density lipoprotein, an atherogenic lipoprotein, did not overlap with the ACAT label, but rather were embedded in the dense reticular network of ACAT. Furthermore, cell-surface biotinylation experiments revealed that freshly harvested, non-attached macrophages, but not those attached to tissue culture dishes, contained approximately 10-15% of ACAT on the cell surface. In summary, ACAT was found in several sites in macrophages: a cytoplasmic reticular/nuclear membrane site that overlaps with PDI and ribophorin and has the characteristics of the endoplasmic reticulum, a perinuclear cytoplasmic site that does not overlap with PDI or ribophorin and may be another cytoplasmic structure or possibly a unique subcompartment of the endoplasmic reticulum, and a cell-surface site in non-attached macrophages. Understanding possible physiological differences of ACAT in these locations may reveal an important component of ACAT regulation and macrophage foam cell formation.

Cholesterol-mediated changes of neutral cholesterol esterase activity in macrophages. Mechanism for mobilization of cholesteryl esters in lipid droplets by HDL

Cholesteryl esters (CE) in lipid droplets undergo a continual cycle of hydrolysis and reesterification by neutral cholesterol esterase (N-CEase) and acyl CoA:cholesterol acyltransferase (ACAT), respectively. The mechanism by which HDL mobilizes CE from lipid droplets in J774 A.1 cells was investigated, focusing on N-CEase activity. We asked whether HDL enhances the activity and, if so, what signals induce the change of the activity. An incubation of cells with HDL enhanced the decline of cholesteryl-[l-14C]-oleate in foam cells and increased N-CEase activity in the supernatant of cell homogenate in a concentration-dependent manner, whereas incubation with LDL decreased the activity. In addition, N-CEase activity was fivefold higher when cells were cultured in 10% lipoprotein-deficient serum (LPDS) medium (2 micrograms cholesterol/mL) than when cultured in 10% fetal calf serum medium (31 micrograms cholesterol/mL), suggesting that changes in N-CEase activity are mediated by cholesterol. An addition of cholesterol (0 to 30 micrograms/mL) in LPDS medium markedly inhibited N-CEase activity with a concomitant increase in cellular cholesterol concentration. This inhibitory effect of cholesterol was also observed in mouse peritoneal macrophages. In vitro addition of cholesterol did not affect N-CEase activity. Treatment of cells with HMG-CoA reductase inhibitors enhanced N-CEase activity, whereas ACAT inhibitor decreased the activity. Northern blot analysis of N-CEase mRNA showed that the expression was not altered by the presence of cholesterol in LPDS medium. These results suggest that cholesterol downregulates N-CEase activity, probably through cholesterol-dependent appearance of some factors.

Quantitation of the pool of cholesterol associated with acyl-CoA:cholesterol acyltransferase in human fibroblasts

The esterification of cholesterol in homogenates of human fibroblasts was explored as a means of estimating the size of the pool of cholesterol associated with the endoplasmic reticulum (ER) in vivo. The rationale was that the acyl-coenzyme A:cholesterol acyltransferase (ACAT) in homogenates should have access only to cholesterol associated with the (rough) ER membrane fragments in which it resides. Reacting whole homogenates to completion with an excess of [14C]oleoyl-CoA converted approximately 0.1-2% of total cell-free cholesterol to [14C]cholesteryl esters. Control studies indicated that membranes not associated with ACAT did not contribute cholesterol to this reaction. The extent of in vitro cholesterol esterification varied with pretreatment of the cells. Exposing intact cells to serum lipoproteins, oxysterols, or sphingomyelinase increased cholesterol esterification in homogenates severalfold; exposing the cells to mevinolin or cholesterol oxidase had the opposite effect. The variation in cholesterol esterification did not correlate with either the total cellular cholesterol or the intrinsic activity of ACAT, neither of which was changed significantly by the pretreatments. Rather, the total amount of cholesterol esterified in homogenates paralleled the rate of cholesterol esterification in the corresponding intact cells. The pool of cholesterol esterified in vitro therefore appears to reflect that associated with the ER in vivo. Since several of the mechanisms keeping cell cholesterol under tight feedback control are themselves located in the ER, this pool might not only be regulated physiologically, but could, in turn, help to regulate homeostatic effector pathways.

Acyl-coenzyme A:cholesterol acyltransferase

Due to its presumed role in regulating cellular cholesterol homeostasis, and in various pathophysiological conditions, acyl-coenzyme A:cholesterol acyltransferase (ACAT) has attracted much attention. Cloning the ACAT gene provides the necessary tool to advance molecular studies of this enzyme. The topics reviewed in this chapter include the pathophysiological roles of ACAT, the biochemistry and molecular biology of the ACAT protein and the ACAT gene, and the mode of regulation by sterol or nonsterol agents in mammalian cells. In addition, we present a working model linking the presumed allosteric property of ACAT with cholesterol trafficking into and out of the endoplasmic reticulum.

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.

The distal pathway of lipoprotein-induced cholesterol esterification, but not sphingomyelinase-induced cholesterol esterification, is energy-dependent

The stimulation of the intracellular cholesterol esterification pathway by atherogenic lipoproteins in macrophages is a key step in the development of atheroma foam cells. The esterification pathway can also be stimulated by hydrolysis of cell-surface sphingomyelin by the enzyme sphingomyelinase (SMase). In both cases, intracellular cholesterol transport to the cholesterol esterifying enzyme, acyl-CoA:cholesterol O-acyltransferase (ACAT), is thought to be critical, although the mechanism of cholesterol transport is not known. In this report, we explore two fundamental properties of the cholesterol esterification pathway, namely its dependence on energy and the effect of other treatments that block membrane vesicle trafficking. After the atherogenic lipoprotein, beta-very low density lipoprotein (beta-VLDL), was internalized by macrophages and hydrolyzed in lysosomes, the cells were depleted of energy by treatment with sodium azide and 2-deoxyglucose or by permeabilization. Under these conditions, which allowed equal beta-VLDL-cholesteryl ester hydrolysis, cholesterol esterification was markedly decreased in the energy-depleted cells. This effect was not due to blockage of lysosomal cholesterol export. In the permeabilized cell system, energy repletion restored beta-VLDL-induced cholesterol esterification. Remarkably, stimulation of cholesterol esterification by SMase was not inhibited by energy depletion. Energy depletion also inhibited beta-VLDL-induced, but not SMase-induced, cholesterol esterification in Chinese hamster ovary cells. Similar experiments were carried out using N-ethylmaleimide, low potassium medium, or inhibitors of phosphatidylinositol 3-kinase, each of which blocks intracellular membrane vesicle trafficking. These treatments also inhibited beta-VLDL-induced, but not SMase-induced, cholesterol esterification. Finally, we show here that SMase treatment of cells leads to an increase in plasma membrane vesiculation that is relatively resistant to energy depletion. In summary, the stimulation of cholesterol esterification by lipoproteins, but not by SMase, is energy-dependent, N-ethylmaleimide-sensitive, and blocked by both low potassium and phosphatidylinositol 3-kinase inhibitors. The affected step or steps are distal to cholesterol export from lysosomes and not due to direct inhibition of the ACAT enzyme. Thus, the mechanisms involved in lipoprotein-induced versus SMase-induced cholesterol esterification are different, perhaps due to the involvement of energy-dependent vesicular cholesterol transport in the lipoprotein pathway and a novel, energy-independent vesicular transport mechanism in the SMase pathway.