Regulated vectorial secretion of cholesteryl ester transfer protein (LTP-I) by the CaCo-2 model of human enterocyte epithelium

Pathogenesis of human enterovirulent bacteria: lessons from cultured, fully differentiated human colon cancer cell lines

Hosts are protected from attack by potentially harmful enteric microorganisms, viruses, and parasites by the polarized fully differentiated epithelial cells that make up the epithelium, providing a physical and functional barrier. Enterovirulent bacteria interact with the epithelial polarized cells lining the intestinal barrier, and some invade the cells. A better understanding of the cross talk between enterovirulent bacteria and the polarized intestinal cells has resulted in the identification of essential enterovirulent bacterial structures and virulence gene products playing pivotal roles in pathogenesis. Cultured animal cell lines and cultured human nonintestinal, undifferentiated epithelial cells have been extensively used for understanding the mechanisms by which some human enterovirulent bacteria induce intestinal disorders. Human colon carcinoma cell lines which are able to express in culture the functional and structural characteristics of mature enterocytes and goblet cells have been established, mimicking structurally and functionally an intestinal epithelial barrier. Moreover, Caco-2-derived M-like cells have been established, mimicking the bacterial capture property of M cells of Peyer's patches. This review intends to analyze the cellular and molecular mechanisms of pathogenesis of human enterovirulent bacteria observed in infected cultured human colon carcinoma enterocyte-like HT-29 subpopulations, enterocyte-like Caco-2 and clone cells, the colonic T84 cell line, HT-29 mucus-secreting cell subpopulations, and Caco-2-derived M-like cells, including cell association, cell entry, intracellular lifestyle, structural lesions at the brush border, functional lesions in enterocytes and goblet cells, functional and structural lesions at the junctional domain, and host cellular defense responses.

Dyslipidaemia in the metabolic syndrome and type 2 diabetes: pathogenesis, priorities, pharmacotherapies

IMPORTANCE OF THE FIELD

Dyslipoproteinaemia is a cardinal feature of the metabolic syndrome that accelerates atherosclerosis. It is usually characterized by high plasma concentrations of triglyceride-rich and apolipoprotein B (apoB)-containing lipoproteins, with depressed concentrations of high-density lipoprotein (HDL). Drug interventions are essential for normalizing metabolic dyslipidaemia.

AREAS COVERED IN THIS REVIEW

This review discusses the mechanisms and treatment for dyslipidaemia in the metabolic syndrome and type 2 diabetes.

WHAT THE READER WILL GAIN

A comprehensive understanding of the pathophysiology and pharmacotherapy of dyslipidaemia in the metabolic syndrome and diabetes.

TAKE HOME MESSAGE

Dysregulation of lipoprotein metabolism may be due to a combination of overproduction of triglyceride-rich lipoproteins, decreased catabolism of apoB-containing particles, and increased catabolism of HDL particles. These abnormalities may be consequent on a global metabolic effect of insulin resistance and an excess of both visceral and hepatic fat. Lifestyle modifications may favourably alter lipoprotein transport in the metabolic syndrome. Patients with dyslipidaemia and established cardiovascular disease should receive a statin as first-line therapy. Combination with other lipid-regulating agents, such as ezetimibe, fibrates, niacins and fish oils may optimize the benefit of statin on atherogenic dyslipidaemia.

Lipoprotein transport in the metabolic syndrome: pathophysiological and interventional studies employing stable isotopy and modelling methods

The accompanying review in this issue of Clinical Science [Chan, Barrett and Watts (2004) Clin. Sci. 107, 221–232] presented an overview of lipoprotein physiology and the methodologies for stable isotope kinetic studies. The present review focuses on our understanding of the dysregulation and therapeutic regulation of lipoprotein transport in the metabolic syndrome based on the application of stable isotope and modelling methods. Dysregulation of lipoprotein metabolism in metabolic syndrome may be due to a combination of overproduction of VLDL [very-LDL (low-density lipoprotein)]-apo (apolipoprotein) B-100, decreased catabolism of apoB-containing particles and increased catabolism of HDL (high-density lipoprotein)-apoA-I particles. These abnormalities may be consequent on a global metabolic effect of insulin resistance, partly mediated by depressed plasma adiponectin levels, that collectively increases the flux of fatty acids from adipose tissue to the liver, the accumulation of fat in the liver and skeletal muscle, the hepatic secretion of VLDL-triacylglycerols and the remodelling of both LDL (low-density lipoprotein) and HDL particles in the circulation. These lipoprotein defects are also related to perturbations in both lipolytic enzymes and lipid transfer proteins. Our knowledge of the pathophysiology of lipoprotein metabolism in the metabolic syndrome is well complemented by extensive cell biological data. Nutritional modifications may favourably alter lipoprotein transport in the metabolic syndrome by collectively decreasing the hepatic secretion of VLDL-apoB and the catabolism of HDL-apoA-I, as well as by potentially increasing the clearance of LDL-apoB. Several pharmacological treatments, such as statins, fibrates or fish oils, can also correct the dyslipidaemia by diverse kinetic mechanisms of action, including decreased secretion and increased catabolism of apoB, as well as increased secretion and decreased catabolism of apoA-I. The complementary mechanisms of action of lifestyle and drug therapies support the use of combination regimens in treating dyslipoproteinaemia in subjects with the metabolic syndrome.

More than apical: Distribution of SGLT1 in Caco-2 cells

We investigated the distribution of the endogenous sodium-d-glucose cotransporter (SGLT1) in polarized Caco-2 cells, a model for enterocytes. A cellular organelle fraction was separated by free-flow electrophoresis and subjected to the analysis of endogenous and exogenous marker enzymes for various membrane vesicle components. Furthermore, the presence of SGLT1 was tested by an ELISA assay employing newly developed epitope specific antibodies. Thereby it was found that the major amount of SGLT1 resided in intracellular compartments and only a minor amount in apical plasma membranes. The distribution ratio between intracellular SGLT1 and apical membrane-associated SGLT1 was approximately 2:1. Further immunocytochemical investigation of SGLT1 distribution in fixed Caco-2 cells by epifluorescence and confocal microscopy revealed that the intracellular compartments containing SGLT1 were associated with microtubules. Elimination of SGLT1 synthesis by incubation of cells with cycloheximide did not significantly reduce the size of the intracellular SGLT1 pool. Furthermore, the half-life of SGLT1 in Caco-2 cells was determined to be 2.5 days by metabolic labeling followed by immunoprecipitation. Our data suggest that most of the intracellular SGLT1 are not transporters en route from biosynthesis to their cellular destination but represent an intracellular reserve pool. We therefore propose that intracellular compartments containing SGLT1 are involved in the regulation of SGLT1 abundance at the apical cell surface.

Markedly elevated lipid transfer inhibitor protein in hypercholesterolemic subjects is mitigated by plasma triglyceride levels

Lipid transfer inhibitor protein (LTIP, apolipoprotein F) regulates the interaction of cholesteryl ester transfer protein (CETP) with lipoproteins and is postulated to enhance the ability of CETP to stimulate reverse cholesterol transport. The factors that regulate LTIP levels and control its biosynthesis are unknown. Here, we demonstrate that plasma LTIP is dramatically increased (3-fold) in hypercholesterolemic subjects with normal to mildly elevated plasma triglyceride (TG) levels compared with control subjects. LTIP in these subjects is not correlated with the extent of hypercholesterolemia or with low density lipoprotein (LDL), high density lipoprotein, or CETP levels. However, unlike CETP, LTIP levels correlate negatively with plasma TG levels. This association does not appear to reflect decreased LTIP synthesis, inasmuch as conditions that stimulate TG synthesis and secretion (200 micromol/L oleate) do not reduce LTIP secretion by SW872 or Caco-2 cells. In contrast, native or acetyl LDL stimulates LTIP secretion 2-fold. Importantly, although plasma LTIP typically resides on LDL, up to 25% of LTIP is bound to very low density lipoprotein when this lipoprotein is enriched in cholesteryl esters, as occurs in hypercholesterolemia. In summary, LTIP levels are markedly elevated by hypercholesterolemia; however, plasma TG levels attenuate this response. We hypothesize that this arises from an increased association of LTIP with very low density lipoprotein, leading to a more rapid clearance of the inhibitor from circulation.

Cholesteryl Ester Transfer Protein Biosynthesis and Cellular Cholesterol Homeostasis Are Tightly Interconnected

Cholesteryl ester transfer protein (CETP) mediates triglyceride and cholesteryl ester (CE) transfer between lipoproteins, and its activity is strongly modulated by dietary cholesterol. To better understand the regulation of CETP synthesis and the relationship between CETP levels and cellular lipid metabolism, we selected the SW872 adipocytic cell line as a model. These cells secrete CETP in a time-dependent manner at levels exceeding those observed for Caco-2 or HepG2 cells. The addition of LDL, 25OH-cholesterol, oleic acid, or acetylated LDL to SW872 cells increased CETP secretion (activity and mass) up to 6-fold. In contrast, CETP production was decreased by almost 60% after treatment with lipoprotein-deficient serum or beta-cyclodextrin. These effects, which were paralleled by changes in CETP mRNA, show that CETP biosynthesis in SW872 cells directly correlates with cellular lipid status. To investigate a possible, reciprocal relationship between CETP expression and cellular lipid homeostasis, CETP biosynthesis in SW872 cells was suppressed with CETP antisense oligonucleotides. Antisense oligonucleotides reduced CETP secretion (activity and mass) by 60% compared with sense-treated cells. When CETP synthesis was suppressed for 24 h, triglyceride synthesis was unchanged, but cholesterol biosynthesis was reduced by 20%, and acetate incorporation into CE increased 31%. After 3 days of suppressed CETP synthesis, acetate incorporation into the CE pool increased 3-fold over control. This mirrored a similar increase in CE mass. The efflux of free cholesterol to HDL was the same in sense and antisense-treated cells; however, HDL-induced CE hydrolysis in antisense-treated cells was diminished 2-fold even though neutral CE hydrolase activity was unchanged. Thus, CETP-compromised SW872 cells display a phenotype characterized by inefficient mobilization of CE stores leading to CE accumulation. These results strongly suggest that CETP expression levels contribute to normal cholesterol homeostasis in adipocytic cells. Overall, these studies demonstrate that lipid homeostasis and CETP expression are tightly coupled.

Regulation by long-chain fatty acids of the expression of cholesteryl ester transfer protein in HepG2 cells

Cholesteryl ester transfer protein (CETP) is an important determinant of lipoprotein function, especially high density lipoprotein (HDL) metabolism, and contributes to the regulation of plasma HDL levels. Since saturated and polyunsaturated fatty acids (FA) appear to influence the CETP activity differently, we decided to investigate the effects of FA on the expression of CETP mRNA in HepG2 cells using an RNA blot hybridization analysis. Long-chain FA (>18 carbons) at a 0.5 mM concentration were added to the medium and incubated with cells for 48 h at 37 degrees C under 5% CO2. After treatment with 0.5 mM arachidonic (AA), eicosapentaenoic (EPA), and docosahexaenoic acid (DHA), the levels of CETP mRNA were less than 50% of the control levels (AA, P = 0.0005; EPA, P < 0.01; DHA, P < 0.0001), with a corresponding significant decrease in the CETP mass. These results suggest that FA regulate the gene expression of CETP in HepG2 and this effect is dependent upon the degree of unsaturation of the acyl carbon chain in FA.

Cholesteryl ester transfer protein gene expression is not specifically regulated by CCAAT/enhancer-binding protein in HepG2-cells

Cholesteryl ester transfer protein (CETP) mediates the exchange of neutral lipids among plasma lipoproteins and is expressed predominantly in liver and intestine. In band shift assays employing nuclear extracts of HepG2 cells we identified C/EBPbeta as the predominant C/EBP isoform involved in binding to the C/EBP consensus sequence within the 5' upstream region of the CETP gene. This was demonstrated by supershift experiments using antibodies specific for C/EBPalpha, C/EBPbeta and C/EBPdelta and an oligonucleotide containing a single point mutation (CAAT-->CTAT) in this site. Expression of a CETP promoter-fragment/luciferase construct in transiently transfected HepG2 and CaCo-2 cells and enhancement of promoter activity by co-transfection with human C/EBPalpha in HepG2 cells could be influenced neither by the mutation in the consensus sequence nor by elimination of this site together with a second potential binding site for C/EBP. Furthermore, transfection of HepG2 with human C/EBPalpha did not influence the synthesis of CETP by these cells. Our results indicate that the expression of C/EBP in HepG2 cells is not able (1) to influence specifically the expression of a transfected CETP promoter dependent reporter through binding to C/EBP sites in the promoter region and (2) to significantly enhance expression of the endogenous CETP gene.

Suppression of plasma cholesteryl ester transfer protein activity in acute hyperinsulinemia and effect of plasma nonesterified fatty acid

Cholesteryl ester transfer protein (CETP) is a major determinant of the plasma high-density lipoprotein cholesterol (HDL-C) level and plays an important role in the reverse cholesterol transport system. The purpose of this study was to determine the effect of acute hyperinsulinemia on plasma CETP activity in normal subjects and patients with non-insulin-dependent diabetes mellitus (NIDDM). Hyperinsulinemia was achieved using the hyperinsulinemic-euglycemic clamp. CETP activity was determined as the transfer of radiolabeled cholesterol in HDL3 to acceptor lipoprotein. Mean plasma CETP activity during an insulin infusion in both subject groups was significantly decreased compared with the mean basal activity. Suppression of plasma CETP activity in the NIDDM patients was significantly less than in the normal subjects (-4.2% +/- 7.9% v -9.6% +/- 6.4%, P < .02). Regression analysis showed that this suppression was correlated with plasma nonesterified fatty acid (NEFA) levels after the clamp and with the magnitude of the NEFA decrease (r = .318, P < .02 and r = .292, P < .05, respectively). The data suggest that acute hyperinsulinemia reduces plasma CETP activity through a decrease in plasma NEFA.

Fatty acid composition of lipoprotein lipids in hepatobiliary diseases

Liver damage and alterations in the exocrine function of the gland lead to a profound alteration of the plasma lipoprotein profile. To determine whether hepatic disease results in changes in the lipoprotein fatty acid composition, i.e. to determine whether liver function influences the homeostasis of complex lipids in plasma, we studied the fatty acid profile of lipids from VLDL, LDL and HDL, as well as from total plasma, in thirty-one patients of both sexes with hepatobiliary pathology (compensated liver cirrhosis, uncompensated liver cirrhosis, primary biliary cirrhosis, other intrahepatic cholestasis, and acute viral hepatitis). We also studied a group of healthy adults as controls. We present the lipoprotein profile and the fatty acid composition (myristic C14, palmitic C16, palmitoleic C16: 1, stearic C18, oleic C18: 1, linoleic C18: 2, eicosatrienoic C20: 3 omega 6 and arachidonic C20: 4) of lipoprotein and total plasma triacylglycerols, cholesteryl esters and phospholipids. The main observation of this study is that, despite the profound changes in the lipoprotein profile and the lower abundance of polyunsaturated fatty acids in complex lipids, the composition of all triacylglycerols, cholesteryl esters and phospholipids is very similar for the corresponding lipoproteins of patients with hepatobiliary disease and of control subjects. This indicates that in the controls as in the studied patients, the exchange of lipids between plasmatic lipoproteins is very rapid and demonstrates the possible importance of the extrahepatic synthesis of cholesteryl ester transfer protein.

Cholesteryl ester transfer activity in liver disease and cholestasis, and its relation with fatty acid composition of lipoprotein lipids

Liver disease is accompanied by major qualitative and quantitative disturbances in plasma lipoprotein metabolism, the extent and intensity of which depend on the degree of parenchymal damage, cholestasis, or both. The main objective of this study was to determine the cholesteryl ester transfer CETP activity and its association with the lipoprotein neutral lipid composition in patients with either liver cirrhosis or cholestasis, as compared to normal controls. Lipoproteins were isolated by ultracentrifugation, lipids and apolipoproteins were measured by conventional methods, and the fatty acid composition was established by gas chromatography; CETP activity in lipoprotein-deficient plasma was measured by determining the transfer of [3H]cholesteryl esters from HDL to VLDL. Lipoprotein lipase and hepatic lipase activities were measured in post-heparin plasma by radiochemical methods. In patients with liver cirrhosis, low levels of VLDL, HDL, apo B, and Lp(a) were observed, as well as a change in the composition of HDL particles, with increases in the relative proportion of triglyceride and free cholesterol. Respectively, the last two changes could be attributed in part to the low hepatic lipase activity observed in this study, and to the low lecithin:cholesterol acyltransferase activity previously observed by others. In patients with cholestasis, a moderate hyperlipidemia due to the elevation of LDL was found. In contrast, HDL and apo A-I levels were very low reflecting a low number of HDL particles, which also had altered compositions with increases in the triglyceride and free cholesterol contents relative to apo A-I and esterified cholesterol, respectively. As regards the fatty acid composition of lipoprotein lipids, the two groups of patients showed, in general, a lower proportion of linoleic acid and a compensating higher proportion of oleic acid as compared to the controls, changes that were observed in both cholesteryl esters and triglycerides. In contrast, the proportions of oleic and palmitoleic acids in phospholipids were increased, whereas that of stearic acid was decreased in patients as compared to controls. In patients with liver cirrhosis, as well as in controls, no changes were observed in the fatty acid compositions of cholesteryl ester, triglycerides, or phospholipids among the different lipoproteins, which probably reflects the equilibration reached by the action of CETP. In patients with cholestasis, no differences were observed in fatty acid composition among the lipoprotein phospholipids but, interestingly, cholesteryl esters from VLDL had a significantly lower linoleic acid content than those from HDL, whereas triglycerides from VLDL had significantly higher oleic acid and lower linoleic acid contents than those from HDL. This distinct fatty acid composition of the neutral lipids between lipoproteins was associated with a significant decrease (25%) in the cholesteryl ester transfer activity in patients with cholestasis. We suggest that fat malabsorption due to the biliary defect may induce a decrease in cholesteryl ester transfer protein synthesis or section, which in turn would slow the equilibration of the neutral lipids among plasma lipoproteins.

Molecular disorders of cholesteryl ester transfer protein

Plasma cholesteryl ester transfer protein (CETP) facilitates the transfer of cholesteryl ester (CE) from HDL to apolipoprotein B-containing lipoproteins and therefore is a key protein in the reverse cholesterol transport system. The importance of plasma CETP in lipoprotein metabolism has been highlighted by the discovery of CETP-deficient subjects with a marked hyper-HDL-cholesterolemia. The deficiency of CETP causes various abnormalities in the concentration, composition, and functions of high density and low density lipoproteins. The current review will focus on some of the recent knowledge on CETP with special reference to the biochemical and molecular biological aspects of CETP. Furthermore, detailed information will be presented regarding the lipoprotein abnormalities and molecular basis of CETP deficiency.

Astroglial localization of cholesteryl ester transfer protein in normal and Alzheimer's disease brain tissues

Cholesteryl ester transfer protein (CETP) is a plasma glycoprotein that facilitates the transfer of cholesteryl esters, phospholipids and triglycerides between the lipoproteins, and regulates plasma high-density lipoprotein levels. We examined CETP-like immunolabeling in non-neurological and Alzheimer's disease (AD) liver and brain tissues. The anti-CETP antibodies showed positive staining in round cells in the liver sinusoid and in brain astrocytes. In the brains of non-neurological cases, positively stained astrocytes were preferentially distributed in the white matter. In AD tissue, many reactive astrocytes in the gray matter as well as the white matter astrocytes had CETP-like immunoreactivity. CETP-positive astrocytes may play a role for AD pathology such as tissue repair.

Evidence for nonesterified fatty acids as modulators of neutral lipid transfers in normolipidemic human plasma

The relations between the level of plasma nonesterified fatty acid (NEFA) and both the mass concentration and activity of the cholesteryl ester transfer protein (CETP) were studied in fasted normolipidemic subjects. Plasma NEFA correlated positively with both CETP mass concentration (r = .50; P < .01) and the transfer of cholesteryl ester from HDL toward plasma VLDL+LDL (CETHDL-->VLDL+LDL activity) (r = .46; P < .05) but not with the transfer of cholesteryl ester from LDL toward plasma HDL (CETLDL-->HDL activity) (r = .05; NS). The high binding capacity of albumin for NEFA was used to investigate whether lipoprotein-bound NEFAs were implicated in the modulation of the cholesteryl ester transfer reaction. As compared with nonsupplemented controls, the addition of an excess of fatty acid-free albumin (8 g/L) to total normolipidemic plasmas reduced CETHDL-->VLDL+LDL activity (18.3 +/- 5.5% versus 9.8 +/- 3.1%; P < .0001) but not CETLDL-->HDL activity (22.3 +/- 4.5% versus 23.3 +/- 5.1%; NS). Moreover, CETHDL-->VLD+LDL and CETLDL-->HDL activities correlated negatively when measured in native plasma (r = -.45; P < .05) but positively when measured in albumin-supplemented plasma (r = .40; P < .05). In long-term incubation experiments, lipoprotein-bound NEFA increased the net mass transfer of cholesteryl esters from HDL toward VLDL+LDL but reduced the net mass transfer of triglycerides in the opposite direction, from VLDL+LDL toward HDL. Taken together, data of the present study brought strong and concordant arguments in favor of a dual effect of plasma NEFA in modulating both the mass and the activity of CETP in vivo.

The expression of human plasma cholesteryl-ester-transfer protein in HepG2 cells is induced by sodium butyrate. Quantification of low mRNA levels by polymerase chain reaction

Although human plasma cholesteryl-ester-transfer protein (CETP) is primarily synthesized in the liver, its expression in a number of transformed liver cell lines is very low. However the use of the human hepatoma cell line HepG2 as a model system for the regulation of CETP on mRNA level is facilitated by a quantitative reverse-transcribed polymerase chain reaction. We demonstrate a time-dependent and concentration-dependent 3-4-fold induction of CETP mRNA by sodium butyrate. CETP mass in the medium is also augmented; however, the effect on protein level is less pronounced.

Cholesteryl ester transfer activity in lipoprotein lipase deficiency and other primary hypertriglyceridemias

Cholesteryl ester transfer protein (CETP) activity was measured in d > 1.21 g/ml plasma from hypertriglyceridemic patients and compared with normolipidemic subjects. The assay consisted in measuring the specific transfer of [3H]cholesteryl oleate from a prelabelled, apo E-poor HDL fraction to VLDL after incubation at 37 degrees C in the presence of the d > 1.21 g/ml plasma sample: the lipoproteins were then separated by precipitation with dextran sulfate/Mg2+ solution. Increasing the volume of d > 1.21 g/ml plasma or purified human CETP in the assay produced linear responses in measured activity, whereas, either during incubation at 4 degrees C or in the presence of rat plasma instead of human plasma, the transfer of [3H]cholesteryl oleate to VLDL was not stimulated. Thus, the assay reflects changes in CETP in the sample and appears to be suitable for measuring CETP activity in d > 1.21 g/ml plasma. CETP activity was very similar in the two groups of normolipidemic subjects considered: adolescents (203 +/- 11 nmol esterified cholesterol transferred per 8 h/ml plasma) and adults (215 +/- 5). Patients were grouped into lipoprotein-lipase (LPL)-deficient and non-LPL-deficient according to their enzyme activity in postheparin plasma. CETP activity was highly increased in LPL-deficient, severe hyperchylomicronemic patients (430 +/- 42) and was directly correlated with VLDL levels in the non-LPL-deficient individuals. Marked differences were observed in the lipid composition of HDL and apolipoprotein A-I levels among patients and controls. In the control group, CETP activity was correlated only with HDL-triglyceride and HDL-triglyceride/apo A-I mass ratio, which is compatible with the physiological role of CETP in transferring triglyceride to HDL from other lipoprotein particles. When all hypertriglyceridemic patients were considered together, CETP activity was inversely correlated with apo A-I and HDL-cholesterol, whereas it was directly correlated with HDL-triglyceride/HDL-cholesterol and HDL-triglyceride/apo A-I mass ratios. The results indicate that the enhanced CETP activity associated with hypertriglyceridemia contributes to the compositional change of HDL, which in turn may be responsible for the reduction of HDL levels in this condition.

Lecithin-cholesterol acyltransferase and lipid transfer protein activities in liver disease

The activities of lecithin-cholesterol acyltransferase (LCAT) and lipid transfer protein (LTP) were assayed using sensitive radioassay methods in controls (n = 113) and in patients with various liver diseases (n = 72). Plasma LCAT activity decreased with progression of hepatocellular damage. Plasma LTP activity in controls was 216 +/- 68 nmol/mL/h, and there were no significant differences between controls and patients with chronic hepatitis ([CH], 193 +/- 70), compensated liver cirrhosis (LC) with or without hepatocellular carcinoma ([HCC], 197 +/- 48 and 193 +/- 62, respectively), or decompensated liver cirrhosis ([dLC], 182 +/- 65). In acute viral hepatitis, LTP activity decreased significantly; however, the degree of reduction was not as dramatic as that for LCAT. There was no correlation between LCAT and LTP activity both in controls and patients with various liver diseases. LCAT activity was positively correlated with serum albumin (r = .52, P < 0.1) and cholinesterase (r = .37, P < .01) levels, and inversely correlated with serum bilirubin level (r = -.38, P < 0.1); there was no correlation between plasma LTP activity and these parameters of liver function. That plasma LTP activity did not change with hepatocellular damage may indicate that the liver in humans may not be the primary site of LTP production.

Iron absorption by intestinal epithelial cells: 1. CaCo2 cells cultivated in serum-free medium, on polyethyleneterephthalate microporous membranes, as an in vitro model

Iron absorption by intestinal epithelial cells, passage onto plasmatic apotransferrin, and regulation of the process remain largely misunderstood. To investigate this problem, we have set up an in vitro model, consisting in CaCo2 cells (a human colon adenocarcinoma line, which upon cultivation displays numerous differentiation criteria of small intestine epithelial cells). Cells are cultivated in a serum-free medium, containing 1 microgram/ml insulin, 1 ng/ml epidermal growth factor, 10 micrograms/ml albumin-linoleic acid, 100 nM hydrocortisone, and 2 nM T3 on new, transparent, Cyclopore polyethyleneterephthalate microporous membranes coated with type I collagen. Cells rapidly adhere, grow, and form confluent monolayers; after 15 days, scanning electron microscopy reveals numerous uniform microvilli. Domes, which develop on nonporous substrata, are absent on high porosity membranes. Culture medium from upper and lower compartments of microplate inserts and cell lysates were immunoprecipitated after labeling with [3H]glucosamine and leucine; analysis was done by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE), followed by autoradiography. [3H]transferrin is found mainly in the lower compartment and in cells; [3H]apolipoprotein B is released in both compartments, and fibronectin almost entirely recovered in the lower compartment; [3H]transferrin receptors and ferritin are only present in cell lysates. Binding experiments also show that transferrin receptors are accessible from the lower compartment. These results suggest that CaCo2 cells, cultivated in synthetic medium on membranes of appropriate porosity, could provide an in vitro model of the intestinal barrier, with the upper compartment of the culture insert corresponding to the apical pole facing the intestinal lumen and the lower one to the basal pole in contact with blood.

Diet-induced alteration in the activity of plasma lipid transfer protein in normolipidemic human subjects

Studies were performed to investigate the effect of diets rich in oleic or linoleic acids on the activity of plasma cholesteryl ester transfer protein (CETP) in normolipidemic subjects. Previous to the test diets, all subjects consumed a baseline diet rich in saturated fatty acids ("sat-diet") for 17 days. The test diets, rich in either monounsaturated fatty acids ("mono-diet") or rich in polyunsaturated fatty acids ("poly-diet"), were given for 5 weeks to 52 normolipidemic healthy volunteers. The activity of CETP was measured, using a method independent of endogenous plasma lipoproteins, as the rate of exchange of radioactive cholesteryl oleate between labelled LDL and unlabelled HDL. The "mono-diet" induced a statistically significant decrease in CETP activity (from 115 +/- 20 to 102 +/- 19 units/ml plasma, P less than 0.01), while the small decrease on the "poly-diet" (from 111 +/- 23 to 107 +/- 22 units/ml plasma) did not reach significancy. The percentual decrease in CETP activity induced by the "mono-diet" was higher than that induced by the "poly-diet" as was also found for the decrease in LDL cholesterol. In both diet groups a positive correlation was found between changes in CETP activity and changes in plasma total or (VLDL + LDL) cholesterol. The results suggest that high levels of dietary monounsaturated fatty acids may result in decreased plasma CETP activity, as well as LDL cholesterol levels. The mechanisms of these effects, and their possible interrelations, remain to be established.

Uptake of the cephalosporin, cephalexin, by a dipeptide transport carrier in the human intestinal cell line, Caco-2

The transport of the orally absorbed cephalosporin, cephalexin, was examined in the human epithelial cell line, Caco-2 that possesses intestinal enterocyte-like properties when cultured. In sodium-free buffer, the cells accumulated 1 mM D-[9-14C]cephalexin against a concentration gradient and obtained a distribution ratio of 3.5 within 180 min. Drug uptake was maximal when the extracellular pH was 6.0. Uptake was reduced by metabolic inhibitors and by protonophores indicating that uptake was energy- and proton-dependent. Kinetic analysis of the concentration dependence of the rate of cephalexin uptake showed that a non-saturable component (Kd of 0.18 +/- 0.01 nmol/min per mg protein per mM) and a transport system with a Km of 7.5 +/- 2.8 mM and a Vmax of 6.5 +/- 0.9 nmol/min per mg protein were responsible for drug uptake. Uptake was competitively inhibited by dipeptides. The transport carrier exhibited stereospecificity for the L-isomer of cephalexin. Drug uptake was not affected by the presence of amino acids, organic anions, 4-acetamido-4'-isothiocyanostilbene-2,2'-disulfonic acid or 4,4'-diisothiocyano-2,2'-disulfonic stilbene. Therefore, Caco-2 cells take up cephalexin by a proton-dependent dipeptide transport carrier that closely resembles the transporter present in the intestine. Caco-2 cells represent a cellular model for future studies of the dipeptide transporter.

Interaction of lipid transfer protein with plasma lipoproteins and cell membranes

The hydrophobic lipid components of lipoproteins, cholesteryl ester and triglyceride, are transferred between all lipoproteins by a specific plasma glycoprotein, termed lipid transfer protein (LTP). LTP facilitates lipid transfer by an exchange process in which cholesteryl ester and triglyceride compete for transfer. Thus, LTP promotes remodeling of the lipoprotein structure, and plays an important role in the intravascular metabolism of these particles and in the lipoprotein-dependent pathways of cholesterol clearance from cells. The properties of LTP, its mechanisms of action, its roles in lipoprotein metabolism, and its modes of regulation are reviewed along with recent data that suggest a possible role for this protein in directly modifying cellular lipid composition.

Effect of a high fat/cholesterol diet with or without eicosapentaenoic acid on plasma lipids, lipoproteins and lipid transfer protein activity in the marmoset

Marmosets fed a diet supplemented with 0.2% cholesterol and 10% sheep fat (by weight) developed hypercholesterolemia with a 4-fold increase in plasma cholesterol (4.28 +/- 0.57-16.38 +/- 4.22 mmol/l, mean +/- SD, P less than 0.001). This was due mainly to a 5-fold increase in the intermediate density lipoprotein (IDL) and low density lipoprotein (LDL) fraction (d = 1.006-1.063 g/ml). The proportion of plasma cholesterol in high density lipoproteins (HDL) decreased from 56% to 25% although HDL cholesterol increased from 2.40 +/- 0.42 to 4.09 +/- 0.92 mmol/l (P less than 0.001), and HDL particle radius increased from 5.10 +/- 0.18 nm to 6.06 +/- 0.73 nm (P less than 0.05). Plasma lipid transfer protein (LTP) activity increased 2.5-fold in whole plasma and 2-fold in lipoprotein-deficient plasma. The atherogenic lipoprotein profile was attenuated by adding 0.8% eicosapentaenoic acid (EPA, 20:5 n - 3, as the ethyl ester) to the atherogenic diet. Plasma cholesterol increased only 55% to 6.64 +/- 2.55 mmol/l with only an 80% increase in lipoproteins in the d = 1.006-1.063 g/ml fraction and a more favourable proportion of plasma cholesterol in HDL (44%) than without EPA. LTP activity was reduced to 1.7-fold above control in whole plasma by addition of EPA to the atherogenic diet. There was a positive correlation between plasma cholesterol and LTP activity in whole plasma (r = 0.89, P less than 0.001) and in lipoprotein-deficient plasma (r = 0.67, P less than 0.001). EPA therefore attenuated some of the adverse effects of a 0.2% cholesterol, 10% sheep fat diet on plasma lipids and lipoproteins and induced a less atherogenic profile.

Regulation of LTP-I secretion from human monocyte-derived macrophages by differentiation and cholesterol accumulation in vitro

Human macrophages in vitro synthesize and secrete the cholesteryl ester (CE) transfer protein, LTP-I. The effect of differentiation of monocyte-to-macrophage on the synthesis and secretion of LTP-I cholesteryl ester transfer activity was investigated. One marker of macrophage differentiation is expression of the 'scavenger' receptor, which mediates macrophage uptake and degradation of acetylated low-density lipoprotein. Monocytes secreted very little detectable CE transfer activity in the first 24 h following cell isolation. Both CE transfer activity and scavenger receptor activity increased with time in culture. Thus, although circulating monocytes probably do not secrete CE transfer activity, tissue macrophages such as hepatic Kupffer cells may contribute to plasma CE transfer activity. Resident macrophages of the arterial wall are derived from circulating monocytes which enter the vessel wall where they differentiate into macrophages. Such macrophages are the principal source of lipid-laden foam cells of the atherosclerotic plaque. Cholesterol accumulation results when uptake of lipoprotein cholesterol overwhelms the capacity of macrophages to excrete cholesterol. Since LTP-I is postulated to function in reverse cholesterol transport, the effect on LTP-I secretion of loading macrophages with cholesterol was determined after exposure of macrophages to acetylated-LDL or free cholesterol (FC). Cholesterol loading by both these maneuvers resulted in dose-dependent increases in macrophage secretion of CE transfer activity, and there was a significant positive correlation between CE transfer activity secreted and accumulation of CE. Thus, LTP-I may function at the cellular level in maintenance of lipid homeostasis: macrophage LTP-I secretion may be a protective mechanism in response to excess cholesterol accumulation in resident macrophages of the arterial wall.

Cholesteryl ester transfer activity in hamster plasma: increase by fat and cholesterol rich diets

We investigated the presence of cholesteryl ester transfer activity (CETA) in plasma of hamsters kept on various dietary regimens. In hamsters kept on a regular diet, CETA activity was about 5 units/4 mg protein of d greater than 1.21 g/ml fraction of plasma, as compared to about 35 units present in human d greater than 1.21 g/ml fraction. Addition of 15% margarine or butter alone or together with 2% cholesterol resulted in a 2-3-fold increase in plasma CETA. The increase in plasma CETA was correlated with plasma cholesterol levels (r = 0.78; P less than 0.001) and plasma triacylglycerol levels (r = 0.56, P less than 0.001). Hamsters consuming the cholesterol + butter-supplemented diets had the highest plasma CETA, cholesterol and triacylglycerol levels, while CETA in plasma of rats and mice remained nondetectable even after 4 weeks on the diet. The causal relation between hypercholesterolemia, hypertriglyceridemia and evaluation in CETA in hamsters remains to be elucidated.

Secretion of cholesteryl ester transfer protein-lipoprotein complexes by human HepG2 hepatocytes

We have employed immunoaffinity chromatography to characterize the distribution of cholesteryl ester transfer activity in particles secreted by HepG2 hepatocytes. HepG2-secreted cholesteryl ester transfer activity is associated with apoprotein (apo) A-I (58%) as well as apo A-II (55%), and is not associated with apo B or E. In contrast, our previous studies have shown that most (88%) cholesteryl ester transfer activity in human plasma is associated with apo A-I whereas very little (7%) is associated with apo A-II. Thus, the distribution of cholesteryl ester transfer activity in plasma particles likely reflects active remodeling of nascent particles in the plasma compartment. Further data suggested that HepG2 cells secrete a lipid transfer inhibitor activity which is associated with apo E-containing lipoprotein particles. This inhibitory activity is heat labile.