Regulation of gene expression and synthesis and degradation of 3-hydroxy-3-methylglutaryl coenzyme A reductase by micellar cholesterolin CaCo-2 cells

Differential Translational Effects of Myristic Acid and Eicosapentaenoic Acid on 3-Hydroxy-3-Methylglutaryl-CoA Reductase From Reuber H35 Hepatoma Cells

The mechanisms by which saturated and polyunsaturated fatty acids may exert their effects on levels of blood cholesterol and human atherosclerosis have not been fully established. In this work, we studied the translational effects of myristic (14:0) and eicosapentaenoic (20:5) acids on 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA) reductase from Reuber H35 hepatoma cells. This enzyme is an intrinsic membrane, 96-kDa protein whose proteolysis releases an enzymatically active, 52- to 56-kDa, soluble fragment. We optimized an immunoblot procedure for quantifying small amounts of both the native and the soluble forms of HMG-CoA reductase from Reuber H35 hepatoma cells. We demonstrated that the upregulation of HMG-CoA reductase by myristic acid is due to an increase of the HMG-CoA reductase protein; therefore, protein synthesis would be required for the increase of HMG-CoA reductase activity caused by this fatty acid. In contrast, the downregulation of HMG-CoA reductase caused by eicosapentaenoic acid is not due to decreased protein synthesis, since similar levels of protein were found in the presence and absence of this fatty acid. Results obtained with cycloheximide as a protein-synthesis inhibitor confirm these findings.

Novel amino-β-lactam derivatives as potent cholesterol absorption inhibitors

Two new trans-(3R,4R)-amino-β-lactam derivatives and their diastereoisomeric mixtures were synthesized as ezetimibe bioisosteres and tested in in vitro and in vivo experiments as novel β-lactam cholesterol absorption inhibitors. Both compounds exhibited low cytotoxicity in MDCKII, hNPC1L1/MDCKII, and HepG2 cell lines and potent inhibitory effect in hNPC1L1/MDCKII cells. In addition, these compounds markedly reduced cholesterol absorption in mice, resulting in reduced cholesterol concentrations in plasma, liver, and intestine. We determined the crystal structure of one amino-β-lactam derivative to establish unambiguously both the absolute and relative configuration at the new stereogenic centre C17, which was assigned to be S. The pKa values for both compounds are 9.35, implying that the amino-β-lactam derivatives and their diastereoisomeric mixtures are in form of ammonium salt in blood and the intestine. The IC50 value for the diastereoisomeric mixture is 60 μM. In vivo, it efficiently inhibited cholesterol absorption comparable to ezetimibe.

Generating ceramide from sphingomyelin by alkaline sphingomyelinase in the gut enhances sphingomyelin-induced inhibition of cholesterol uptake in Caco-2 cells

Background Sphingomyelin (SM) is present in dietary products and cell plasma membranes. We previously showed that dietary SM inhibited cholesterol absorption in rats. In the intestinal tract, SM is mainly hydrolyzed by alkaline sphingomyelinase (alk-SMase) to ceramide.Aims We investigated the influence of SM and its hydrolytic products ceramide and sphingosine on cholesterol uptake in intestinal Caco-2 cells.Methods Micelles containing bile salt, monoolein, and (14)C-cholesterol were prepared with or without SM, ceramide,or sphingosine. The micelles were incubated with Caco-2 cells, and uptake of radioactive cholesterol was quantified.Results We found that confluent monolayer Caco-2 cells expressed NPC1L1, and the uptake of cholesterol in the cells was inhibited by ezetimibe, a specific inhibitor of NPC1L1. Incorporation of SM in the cholesterol micelles inhibited cholesterol uptake dose-dependently; 38% inhibition occurred at an equal mole ratio of SM and cholesterol.The inhibition was further enhanced to 45% by pretreating the cholesterol/SM micelles with recombinant alk-SMase, which hydrolyzed SM in the micelles by 85%, indicating ceramide has stronger inhibitory effects on cholesterol uptake. To confirm this, we further replaced SM in the micelles with ceramide and sphingosine, and found that at equal mole ratio to cholesterol, ceramide exhibited stronger inhibitory effect (50% vs 38%) on cholesterol uptake than SM, whereas sphingosine only had a weak effect at high concentrations.Conclusion Both SM and ceramide inhibit cholesterol uptake, the effect of ceramide being stronger than that of SM. Alk-SMase enhances SM-induced inhibition of cholesterol uptake by generating ceramide in the intestinal lumen.

Curcumin inhibits cholesterol uptake in Caco-2 cells by down-regulation of NPC1L1 expression

Background

Curcumin is a polyphenol and the one of the principle curcuminoids of the spice turmeric. Its antioxidant, anti-cancer and anti-inflammatory effects have been intensively studied. Previous in vivo studies showed that administration of curcumin also decreased cholesterol levels in the blood, and the effects were considered to be related to upregulation of LDL receptor. However, since plasma cholesterol levels are also influenced by the uptake of cholesterol in the gut, which is mediated by a specific transporter Niemann-Pick Cl-like 1 (NPC1L1) protein, the present study is to investigate whether curcumin affects cholesterol uptake in the intestinal Caco-2 cells.

Methods

Caco-2 cells were cultured to confluence. The micelles composed of bile salt, monoolein, and ¹⁴C-cholesterol were prepared. We first incubated the cells with the micelles in the presence and absence of ezetimibe, the specific inhibitor of NPC1L1, to see whether the uptake of the cholesterol in the cells was mediated by NPC1L1. We then pretreated the cells with curcumin at different concentrations for 24 h followed by examination of the changes of cholesterol uptake in these curcumin-treated cells. Finally we determined whether curcumin affects the expression of NPC1L1 by both Western blot analysis and qPCR quantification.

Results

We found that the uptake of radioactive cholesterol in Caco-2 cells was inhibited by ezetimibe in a dose-dependent manner. The results indicate that the uptake of cholesterol in this study was mediated by NPC1L1. We then pretreated the cells with 25-100 μM curcumin for 24 h and found that such a treatment dose-dependently inhibited cholesterol uptake with 40% inhibition obtained by 100 μM curcumin. In addition, we found that the curcumin-induced inhibition of cholesterol uptake was associated with significant decrease in the levels of NPC1L1 protein and NPC1L1 mRNA, as analyzed by Western blot and qPCR, respectively.

Conclusion

Curcumin inhibits cholesterol uptake through suppression of NPC1L1 expression in the intestinal cells.

Regulation of the proprotein convertase subtilisin/kexin type 9 in intestinal epithelial cells

Proprotein convertase subtilisin/kexin type 9 (PCSK9) posttranslationally promotes the degradation of the low-density lipoprotein receptor (LDLr) in hepatocytes and increases plasma LDL cholesterol. It is not clear, however, whether PCSK9 plays a role in the small intestine. Here, we characterized the patterns of variations of PCSK9 and LDLr in fully differentiated Caco-2/15 cells as a function of various potential effectors. Cholesterol (100 microM) solubilized in albumin or micelles significantly downregulated PCSK9 gene (30%, P<0.05) and protein expression (50%, P<0.05), surprisingly in concert with a decrease in LDLr protein levels (45%, P<0.05). Cells treated with 25-hydroxycholesterol (50 microM) also displayed significant reduction in PCSK9 gene (37%, P<0.01) and protein (75% P<0.001) expression, whereas LDLr showed a decrease at the gene (30%, P<0.05) and protein (57%, P<0.01) levels, respectively. The amounts of PCSK9 mRNA and protein in Caco-2/15 cells were associated to the regulation of 3-hydroxy-3-methylglutaryl-CoA reductase and sterol regulatory element binding protein-2 (SREBP-2) that can transcriptionally activate PCSK9 via sterol-regulatory elements located in its proximal promoter region. On the other hand, depletion of cholesterol content by hydroxypropyl-beta-cyclodextrin upregulated PCSK9 transcripts (20%, P<0.05) and protein mass (540%, P<0.001), in parallel with SREBP-2 protein levels. The addition of bile acids (BA) taurocholate and deoxycholate to the apical culture medium lowered PCSK9 gene expression (25%, P<0.01) and raised PCSK9 protein expression (30%, P<0.01), respectively, probably via the modulation of farnesoid X receptor. Furthermore, unconjugated and conjugated BA exhibited different effects on PCSK9 and LDLr. Altogether, these data indicate that intestinal PCSK9 is highly modulated by sterols and emphasize the distinct effects of BA species.

Intestinal cholesterol transport proteins: an update and beyond

PURPOSE OF REVIEW

Various studies have delineated the causal role of dietary cholesterol in atherogenesis. Strategies have thus been developed to minimize cholesterol absorption, and cholesterol transport proteins found at the apical membrane of enterocytes have been extensively investigated. This review focuses on recent progress related to various brush-border proteins that are potentially involved in alimentary cholesterol transport.

RECENT FINDINGS

Molecular mechanisms responsible for dietary cholesterol and plant sterol uptake have not been completely defined. Growing evidence, however, supports the concept that several proteins are involved in mediating intestinal cholesterol transport, including SR-BI, NPC1L1, CD36, aminopeptidase N, P-glycoprotein, and the caveolin-1/annexin-2 heterocomplex. Other ABC family members (ABCA1 and ABCG5/ABCG8) act as efflux pumps favoring cholesterol export out of absorptive cells into the lumen or basolateral compartment. Several of these cholesterol carriers influence intracellular cholesterol homeostasis and are controlled by transcription factors, including RXR, LXR, SREBP-2 and PPARalpha. The lack of responsiveness of NPC1L1-deficient mice to ezetimibe suggests that NPC1L1 is likely to be the principal target of this cholesterol-lowering drug.

SUMMARY

The understanding of the role, genetic regulation and coordinated function of proteins mediating intestinal cholesterol transport may lead to novel ways of treating cardiovascular disease.

Functional characterization of three clones of the human intestinal Caco-2 cell line for dietary lipid processing

We aimed to improve the use of the human intestinal Caco-2 cell line for studying dietary lipid and cholesterol processing by using isolated pure clones (). Three clones (TC7, PD7 and PF11) were grown as monolayers on semi-permeable filters and compared for cell viability, fatty acid and cholesterol apical uptake or basolateral secretion, apolipoprotein B-48 basolateral secretion and 3-hydroxy-3-methylglutaryl (HMG)-CoA reductase activity. The TC7 clone showed the best viability upon apical incubation with mixed micelles and should be preferred for routine work. Short-term (3·0 h) rates of apical uptake of cholesterol were not different with the three clones, whereas the rate of apical uptake of oleic acid (18 : 1) was lower (P<0·05) with PF11 (250·6 nmol/mg) and the basolateral secretion of cholesterol and oleic acid was lower with the TC7 clone (0·40 and 29·1 nmol/mg respectively). The secretion of apolipoprotein B-48 basolaterally was about 2-fold lower than from PD7 clone. The basal levels of HMG-CoA reductase activity were significantly different (P<0·05; TC7>PF11>PD7). The down-regulation of the enzyme activity was moderate (range 13·8–21·0 %) and comparable in the presence of apical micellar cholesterol, but was much marked upon basolateral incubation with LDL (range 34·0–53·6 %), especially for the PD7 clone. In conclusion, the Caco-2 clones characterized here proved to be particularly suitable for studying lipid nutrients processing. Because these three clones exhibit some different metabolic capabilities, they provide a new tool to study intestinal response to lipid nutrients.

Cholesterol modulates human intestinal sodium-dependent bile acid transporter

Bile acids are efficiently absorbed from the intestinal lumen via the ileal apical sodium-dependent bile acid transporter (ASBT). ASBT function is essential for maintenance of cholesterol homeostasis in the body. The molecular mechanisms of the direct effect of cholesterol on human ASBT function and expression are not entirely understood. The present studies were undertaken to establish a suitable in vitro experimental model to study human ASBT function and its regulation by cholesterol. Luminal membrane bile acid transport was evaluated by the measurement of sodium-dependent 3H-labeled taurocholic acid (3H-TC) uptake in human intestinal Caco-2 cell monolayers. The relative abundance of human ASBT (hASBT) mRNA was determined by real-time PCR. Transient transfection and luciferase assay techniques were employed to assess hASBT promoter activity. Caco-2 cell line was found to represent a suitable model to study hASBT function and regulation. 25-Hydroxycholesterol (25-HCH; 2.5 microg/ml for 24 h) significantly inhibited Na(+)-dependent 3H-TC uptake in Caco-2 cells. This inhibition was associated with a 50% decrease in the V(max) of the transporter with no significant changes in the apparent K(m). The inhibition in hASBT activity was associated with reduction in both the level of hASBT mRNA and its promoter activity. Our data show the inhibition of hASBT function and expression by 25-HCH in Caco-2 cells. These data provide novel evidence for the direct regulation of human ASBT function by cholesterol and suggest that this phenomenon may play a central role in cholesterol homeostasis.

Osteoclast formation, survival and morphology are highly dependent on exogenous cholesterol/lipoproteins

Osteoporosis is associated with both atherosclerosis and vascular calcification. No mechanism yet explains the parallel progression of these diseases. Here, we demonstrate that osteoclasts (OCL) depend on lipoproteins to modulate cellular cholesterol levels and that this controls OCL formation and survival. Removal of cholesterol in OCL via high-density lipoprotein or cyclodextrin treatment dose-dependently induced apoptosis, with actin disruption, nuclear condensation and caspase-3 activation. One mechanism linked to the induction of OCL apoptosis was the cell-type-specific failure to induce HMG-CoA reductase mRNA expression, suggesting an absence of feedback regulation of de novo cholesterol biosynthesis. Furthermore, cyclodextrin treatment substantially suppressed essential M-CSF and RANKL-induced survival signaling pathways via Akt, mTOR and S6K. Consistent with these findings, cholesterol delivery via low-density lipoprotein (LDL) significantly increased OCL viability. Interestingly, OCLs from the LDL receptor (LDLR)-/- mouse exhibited reduced size and lifespan in vitro. Remarkably, LDLR+/+ OCL in lipoprotein-deficient medium phenocopied LDLR-/- OCL, while fusion and spreading of LDLR-/- OCL was rescued when cholesterol was chemically delivered during differentiation. With hyperlipidemia being associated with disease of the vascular system and bone, these findings provide novel insights into the selective lipoprotein and cholesterol dependency of the bone resorbing cell. Cell Death and Differentiation (2004) 11, S108-S118. doi:10.1038/sj.cdd.4401399 Published online 12 March 2004

Acyl-coenzymeA (CoA):cholesterol acyltransferase inhibition in rat and human aortic smooth muscle cells is nontoxic and retards foam cell formation

OBJECTIVE

Studies in vitro and in vivo of macrophage foam cells have shown evidence of cytotoxicity after acyl-CoA:cholesterol acyltransferase (ACAT) inhibition. Foam cells of smooth muscle origin are also found in human and animal atherosclerotic lesions.

METHODS AND RESULTS

To study whether cytotoxicity from ACAT inhibition is independent of cell type, we first established a protocol to conveniently induce aortic smooth muscle foam cell formation using cholesterol-cyclodextrin complexes (CCC). Rat aortic smooth muscle cells (ASMCs) treated for 48 hours with CCC (20 microg/mL) became foam cells by morphological (oil-red-O staining) and biochemical (approximately 1200% and approximately 180% increase in cellular esterified and free cholesterol, respectively) criteria. ACAT activity increased 500% (P<0.01 versus untreated). Similar results were obtained in human ASMC, but ACAT activity increased to an even greater extent (3200%; P<0.01 versus untreated). Western blots indicated that CCC treatment increased human (to 380+/-20% of untreated, P<0.001), but not rat, ACAT protein expression. ACAT inhibition by Fujirebio compound F1394 suppressed CCC-induced foam cell formation in rat and human ASMC, but, notably, did not induce significant cytotoxicity.

CONCLUSIONS

ASMC might be more resistant to FC-induced adverse effects than are macrophages.

Inhibition of cholesterol biosynthesis in cultured fibroblasts by D003, a mixture of very long chain saturated fatty acids

The present study was undertaken to investigate the effects of D003, a mixture of very long chain saturated fatty acids isolated and purified from sugar cane wax, on cholesterol biosynthesis in cultured fibroblasts. Cholesterol biosynthesis is regulated through feedback regulation of at least two sequentially acting enzymes, 3-hydroxy-3-methyl coenzyme A (HMG-CoA) synthase and reductase. They are up-regulated when sterol levels fall and down-regulated when sterol levels rise. The exposure of cultured fibroblasts to a lipid-depleted medium (LDM) and D003 (0.05-50 microg ml(-1)) for 12 h inhibited, in a dose-dependent manner, cholesterol biosynthesis from 14C-labelled acetate (33-68%). The addition of D003 at concentrations inhibiting cholesterol biosynthesis from labelled acetate significantly decreased incorporation of radioactivity from 3H2O into sterols, but not from 14C-mevalonate. These data indicate that D003 inhibits cholesterol biosynthesis by interfering with early steps of cholesterol biosynthetic pathway. We reasoned that D003 acts directly on HMG-CoA reductase, the main regulatory enzyme of cholesterol biosynthetic pathway. However, when enzyme activity was measured in cell extracts in the presence of various concentrations of D003 (0.5-50 microg ml(-1)), reductase activity was not inhibited. Thus, there was no evidence for a competitive or non-competitive inhibition of enzyme activity by D003. Treatment with D003 significantly suppressed (68%) the enzyme up-regulation when cells were cultured in LDM, which suggests a depression of de novo synthesis of HMG-CoA reductase and/or a stimulation of its degradation. However, since the suppressive action of D003 on cholesterol biosynthesis was observed in metabolic conditions under which synthase up-regulation was also enhanced, we cannot rule out a possible effect of D003 on HMG-CoA synthase. Thus, further studies are needed to clarify the precise mechanism of the inhibitory effect of D003 on cholesterol biosynthesis.

Regulation of sterol regulatory element-binding proteins in hamster intestine by changes in cholesterol flux

A control chow diet or diets containing 1% cholesterol (cholesterol-enriched) or 4% cholestyramine and 0.15% lovastatin (cholesterol-depletion) were fed to hamsters for 2 weeks. Sterol regulatory element-binding protein (SREBP)-1a, SREBP-1c, SREBP-2, 3-hydroxy-3-methylglutaryl-coenzyme A reductase, 3-hydroxy-3-methylglutaryl-coenzyme A synthase, and LDL receptor mRNA levels and SREBP-1 and -2 protein expression were estimated in villus cell populations from duodenum, jejunum, and ileum. SREBP-1a was a minor transcript in hamster intestine, and its gene expression was not altered by changes in dietary cholesterol flux. In contrast, SREBP-1c gene expression was increased by dietary cholesterol and decreased by cholesterol depletion. mRNA levels for SREBP-2 and the other sterol-responsive genes were increased in intestines of animals on the cholesterol depletion diet but minimally suppressed if at all, by the diet enriched in cholesterol. In general, the amount of the precursor form of SREBP-1 was higher in cells of the upper villus and lower in cells of the lower villus. SREBP-2 precursor was higher in cells of the lower villus and lower in cells of the upper villus. Protein expression of precursor correlated with the location of gene expression for SREBPs. The amount of precursor mass of SREBP-2 was not altered by cholesterol feeding but was increased by cholesterol depletion. The mature form of SREBP-2 was in very low abundance and difficult to detect in intestines of animals fed control chow or cholesterol. It was readily detectable and increased in intestines of animals on the cholesterol-depletion diet. The diets did not significantly alter the amount of precursor or mature forms of SREBP-1. Cholesterol feeding had no effect on cholesterol or fatty acid synthesis, whereas synthesis of these lipids was increased in intestines of hamsters on the cholesterol-depleted diet. These results suggest that SREBP-1a has little or no role in regulating intestinal cholesterol synthesis. It is postulated that under basal conditions, SREBP-1c regulates intestinal fatty acid synthesis and SREBP-2 regulates cholesterol synthesis. Following marked changes in cholesterol flux across the intestine, SREBP-2 assumes the role of SREBP-1 and regulates both cholesterol and fatty acid synthesis in intestine.

Overexpression of MDR1 in an intestinal cell line results in increased cholesterol uptake from micelles

The multiple drug resistance protein, MDR1, is highly expressed on the apical surface of intestinal epithelial cells. The physiologic substrate of this protein remains unclear. Several studies using compounds known to act as MDR1 inhibitors have suggested that MDR1 may be involved in the transport of cholesterol from the plasma membrane to the endoplasmic reticulum where it is esterified. To examine the role of MDR1 in cholesterol uptake by intestinal cells, the rat intestinal epithelial cell line IEC-18, was stably transfected with human MDR1. MDR1-transfected cells exhibited increased expression of MDR1 protein, reduced accumulation of vinblastine and increased uptake of [(3)H]cholesterol from cholesterol/monolein/taurocholate micelles. These studies provide the first direct evidence that the level of MDR1 expression in intestinal cells can influence the amount of cholesterol taken up by those cells. This is also the first demonstration that a multiple drug resistance protein can function in the net uptake, rather than efflux, of a substrate.

Oxysterols: modulators of cholesterol metabolism and other processes

Oxygenated derivatives of cholesterol (oxysterols) present a remarkably diverse profile of biological activities, including effects on sphingolipid metabolism, platelet aggregation, apoptosis, and protein prenylation. The most notable oxysterol activities center around the regulation of cholesterol homeostasis, which appears to be controlled in part by a complex series of interactions of oxysterol ligands with various receptors, such as the oxysterol binding protein, the cellular nucleic acid binding protein, the sterol regulatory element binding protein, the LXR nuclear orphan receptors, and the low-density lipoprotein receptor. Identification of the endogenous oxysterol ligands and elucidation of their enzymatic origins are topics of active investigation. Except for 24, 25-epoxysterols, most oxysterols arise from cholesterol by autoxidation or by specific microsomal or mitochondrial oxidations, usually involving cytochrome P-450 species. Oxysterols are variously metabolized to esters, bile acids, steroid hormones, cholesterol, or other sterols through pathways that may differ according to the type of cell and mode of experimentation (in vitro, in vivo, cell culture). Reliable measurements of oxysterol levels and activities are hampered by low physiological concentrations (approximately 0.01-0.1 microM plasma) relative to cholesterol (approximately 5,000 microM) and by the susceptibility of cholesterol to autoxidation, which produces artifactual oxysterols that may also have potent activities. Reports describing the occurrence and levels of oxysterols in plasma, low-density lipoproteins, various tissues, and food products include many unrealistic data resulting from inattention to autoxidation and to limitations of the analytical methodology. Because of the widespread lack of appreciation for the technical difficulties involved in oxysterol research, a rigorous evaluation of the chromatographic and spectroscopic methods used in the isolation, characterization, and quantitation of oxysterols has been included. This review comprises a detailed and critical assessment of current knowledge regarding the formation, occurrence, metabolism, regulatory properties, and other activities of oxysterols in mammalian systems.

Review of progress in sterol oxidations: 1987-1995

Material dealing with the chemistry, biochemistry, and biological activities of oxysterols is reviewed for the period 1987-1995. Particular attention is paid to the presence of oxysterols in tissues and foods and to their physiological relevance.

Mevalonate regulates polysome distribution and blocks translation-dependent suppression of 3-hydroxy-3-methylglutaryl coenzyme A reductase mRNA: relationship to translational control

We reported previously that 3-hydroxy-3-methylglutaryl coenzyme A reductase synthesis is regulated at the translational level by mevalonate. To determine at what stage mevalonate affects reductase synthesis, we examined the distribution of reductase mRNA in polysomes from cells treated with lovastatin alone; lovastatin and 25-hydroxycholesterol; or lovastatin, 25-hydroxycholesterol, and mevalonate. In lovastatin-treated cells, reductase mRNA was primarily associated with heavy polysome fractions. When 25-hydroxycholesterol was added to lovastatin-treated cells, reductase mRNA levels were reduced approximately fourfold in all polysome fractions, with no accompanying redistribution of reductase mRNA into lighter polysome fractions. However, addition of both 25-hydroxycholesterol and mevalonate to lovastatin-treated cells shifted reductase mRNA from heavier to lighter polysome fractions. No change in the distribution of control beta-actin or ribosomal protein S17 mRNA occurred with any of the treatments. These results suggest that mevalonate suppresses reductase synthesis at the level of initiation. When the translation inhibitor cycloheximide was added to all three regimens, reductase mRNA shifted into heavy polysome fractions. Treatment with either lovastatin alone or lovastatin plus 25-hydroxycholesterol resulted in a 50% greater loss of reductase mRNA from the heavy polysome fractions compared to the same fractions from noncycloheximide-treated cells. No loss of reductase mRNA occurred when cycloheximide was added to cells treated with both 25-hydroxycholesterol and mevalonate. beta-Actin mRNA levels and polysome distribution were not significantly changed by cycloheximide under any of these conditions. Translationally mediated suppression of reductase mRNA did not occur when protein synthesis was inhibited with puromycin. Our results indicate that regulation of reductase mRNA levels is translation-dependent and is linked to the rate of elongation.

Regulation of LDL receptor expression by luminal sterol flux in CaCo-2 cells

The regulation of expression of the intestinal low density lipoprotein (LDL) receptor by luminal (apical) sterol flux was investigated in the human intestinal cell line CaCo-2. Cells were cultured on semipermeable micropore filters, which separated an upper and lower well. To the apical media were added solutions containing either taurocholate micelles alone or micelles containing sterols. Because of an efflux of cholesterol, which occurred from cells incubated with micelles alone, LDL receptor mRNA levels increased threefold. With an influx of micellar sterols, receptor mRNA levels decreased in a dose-dependent manner. Synthesis and degradation of the LDL receptor were addressed by pulse-chase experiments. In cells incubated with micelles containing 25-hydroxycholesterol, the rate of receptor synthesis was significantly decreased, whereas the rate of receptor turnover remained unchanged. As assessed by immunoblots and steady-state labeling of proteins followed by immunoprecipitation of the LDL receptor, cells incubated with micellar 25-hydroxycholesterol contained substantially less receptor protein. These cells also bound and degraded less LDL. In contrast, in cells incubated with micelles alone, the rate of receptor synthesis was increased and cells contained more LDL receptor protein, although this was not reflected in an increased in LDL binding. The results suggest that LDL receptor expression in CaCo-2 cells is regulated by luminal sterol flux and that this regulation occurs at the level of transcription.