Phosphatidylcholine hydrolysis is required for pancreatic cholesterol esterase- and phospholipase A2-facilitated cholesterol uptake into intestinal Caco-2 cells

Pancreatic secretion is required for efficient cholesterol absorption by the intestine, but the factors responsible for this effect have not been clearly defined. To identify factors involved and to investigate their role in cholesterol uptake, we studied the effect of Viokase(R), a porcine pancreatic extract, on cholesterol uptake into human intestinal Caco-2 cells. Viokase is capable of facilitating cholesterol uptake into these cells such that the level of uptake is 5-fold higher in the presence of solubilized Viokase. This stimulation is time-dependent and is dependent on the presence of bile salt. However, bile salt-stimulated pancreatic cholesterol esterase, which has been proposed to mediate cholesterol uptake, is not fully responsible. The major cholesterol transport activity was purified and identified as pancreatic phospholipase A2. Anti-phospholipase A2 antibodies abolished virtually all of the phospholipase A2 and cholesterol transport activity of solubilized Viokase. We demonstrate that both phospholipase A2 and cholesterol esterase increase cholesterol uptake by hydrolyzing the phosphatidylcholine that is used to prepare the cholesterol-containing micelles. In the absence of cholesterol esterase or phospholipase A2, uptake of cholesterol from micelles containing phosphatidylcholine is not as efficient as uptake from micelles containing phospholipase A2-hydrolytic products. These results indicate that phospholipase A2 may mediate cholesterol absorption by altering the physical-chemical state of cholesterol within the intestine.

Dietary linoleic acid increases and palmitic acid decreases hepatic LDL receptor protein and mRNA abundance in young pigs

The present study was conducted to determine the effects of dietary fatty acids on hepatic LDL receptor (LDLr) protein abundance and mRNA levels. Sixty pigs were randomized into 10 groups and fed corn-soybean meal diets containing three cholesterol levels (0.25%, 0.5%, and 1.0%, w/w) with no added fat, or fats rich (30% of calories) in palmitic acid or linoleic acid. A control group was fed the base diet with no added fat. After 30 days, plasma LDL-cholesterol (LDL-C) levels increased as the dietary cholesterol increased (P < 0.05); however, there was no significant effect of either fatty acid. Dietary fatty acids, however, had distinctly different effects on hepatic LDLr protein (analyzed by ELISA) and mRNA (analyzed by Northern blot) abundance. When pigs consumed diets containing 0.25% cholesterol, linoleic acid increased hepatic LDLr protein 40% whereas palmitic acid reduced it 40% (P < 0.05). These changes in LDLr protein abundance were accompanied by parallel changes in hepatic LDLr mRNA; linoleic acid increased LDLr mRNA 2-fold (P < 0.01), whereas palmitic acid decreased it 60% (P < 0.01). The differential effects of fatty acids on LDLr expression were only observed at 0.25% cholesterol, suggesting that higher intakes of cholesterol have a dominant and repressive effect on regulation of LDLr expression. Cholesterol intake increased hepatic total cholesterol levels (P < 0.01) while dietary fatty acids had no effect on hepatic sterols. In summary, our results indicate that dietary linoleic acid and palmitic acid have markedly different effects on hepatic LDLr protein abundance that are mediated by differential effects on LDLr mRNA and protein levels. Further studies are needed to fully elucidate the molecular mechanisms by which fatty acids regulate LDLr mRNA and protein levels.

Activation of LDL receptor gene expression in HepG2 cells by hepatocyte growth factor

The effect of recombinant human hepatocyte growth factor (HGF) on low density lipoprotein (LDL) receptor gene expression was studied in the human hepatoma cell line HepG2. HepG2 cells were incubated with serum-free media in the presence and absence of HGF for various times and 125I-labeled LDL specific binding at 4 degrees C, uptake at 37 degrees C, and the levels of LDL receptor mRNA were measured. Incubation with HGF produced time- and concentration-dependent increases in 125I-labeled LDL binding (2-fold), uptake (2.5-fold), and LDL receptor mRNA (6-fold). HGF increased the rate of LDL receptor gene transcription 4- to 5-fold relative to that of several "house-keeping" genes as measured by nuclear run-on transcription. The half-life of LDL receptor mRNA, measured with actinomycin D, was not increased in HGF-treated cells. The stimulation of LDL receptor expression occurred independently of changes in cellular cholesterol or DNA biosynthesis or total cell protein. HepG2 cells were transiently transfected with plasmids bearing either three copies of repeats 2 and 3 (pLDLR(23)3LUC) or one copy of the LDL receptor promoter from -556 to +53 (pLDLR600LUC) linked to firefly luciferase. Incubation of pLDLR(23)3LUC, or pLDLR600LUC-transfected cells with HGF for 4 or 24 h at 37 degrees C produced a concentration-dependent increase in luciferase activity. A maximal stimulation of 3 to 6-fold was achieved for each construct at an HGF concentration of 100 ng/ml. In contrast, HGF had little or no effect on reporter activity in HepG2 cells transfected with a luciferase reporter plasmid bearing the HMG-CoA reductase promoter extending from -325 to +22. Thus, when compared to the native LDL receptor promoter, multiple copies of repeats 2 and 3 of the LDL receptor promoter can fully support activation of the luciferase reporter gene by HGF, demonstrating that the effect of HGF is mediated through the SRE-1. The lack of HGF effects mediated through the HMG-CoA reductase sterol regulatory element suggests, however, that sterol depletion may not be responsible for the induction of the LDL receptor promoter by growth factors. The signalling pathways or effectors responsible for activation of the LDL receptor and HMG-CoA reductase genes thus differ in their response to HGF. These data suggest that the level of SREBP's reaching the nucleus may be determined by as yet unidentified second messengers as well as by sterols.

Tyrosine kinase inhibitors potentiate the induction of low density lipoprotein receptor gene expression by hepatocyte growth factor

The role of tyrosine kinase, protein kinase C, cyclic nucleotide- and Ca(2+)-calmodulin-dependent protein kinase second messenger pathways in the induction of LDL receptor gene expression by hepatocyte growth factor (HGF) was studied in the human hepatoma cell line Hep-G2. Incubation with media containing HGF increased the level of LDL receptor mRNA by 6.5-fold. Co-incubation with HGF and either of two tyrosine kinase inhibitors genistein (2.0-20.0 micrograms/ml) and herbimycin A (0.5-500.0 ng/ml) increased the level of LDL receptor mRNA above that observed with HGF alone by 40-60%. Incubation with HGF in the presence of the calmodulin antagonist W7 (10-30 microM) also super-induced the level of LDL receptor mRNA by nearly 230%. The protein kinase C and A inhibitors chelerythrine (0.1-10.0 microM) and H8 (0.5-5.0 microM), respectively, had no significant effects on the induction of LDL receptor mRNA by HGF. Taken together, these data suggest that tyrosine kinase, protein kinases C and A, and Ca(2+)-calmodulin dependent protein kinase activities are not essential for activation of LDL receptor gene expression in Hep-G2 cells by HGF.

Location and regulation of low-density lipoprotein receptors in intestinal epithelium

The expression, distribution, and some aspects of the regulation of low-density lipoprotein (LDL) receptors in rat intestinal epithelial cells were examined. Cells prepared by a perfusion technique provided a pure preparation of epithelial cells and could be manipulated to produce crypt-villus units or villi alone. On a total protein basis, the abundance of LDL receptors in villus cell membranes was half that in hepatic membranes. The level of receptors in both tissues was reduced by feeding an atherogenic diet but was increased only in the liver by ethinyl estradiol-induced hypocholesterolemia. The level of LDL receptor mRNA in intestinal epithelial cells was somewhat lower than in liver. Regulation of LDL receptor mRNA was similar to that of protein. Judged by the ratio of mRNA in villus cells to the villus-crypt unit and nuclear run-on assay for LDL receptor gene transcription, we conclude that LDL receptor mRNA is produced in the villus cells. The effect of fat feeding was regulated at the level of transcription. Expression in villus cells in ileum was severalfold higher than in jejunum and higher than in the liver. Together the results suggest serum cholesterol level is not the prime determinant of LDL receptor level in intestine, but LDL degradation in this organ may be regulated by factors in the lumen.

Protein binding to the low density lipoprotein receptor promoter in vivo is differentially affected by gene activation in primary human cells

Protein-DNA interactions within a region of the LDL receptor promoter involved in sterol-mediated feedback repression of transcription were examined using in vivo genomic footprinting with dimethylsulfate (DMS). A broad region of protein-DNA contacts spanning from repeat 1 to beyond the transcription start sites was observed in primary cultures of human skin fibroblasts and hepatocytes. Hypermethylation of guanine -59 within the sterol regulatory element-1 (SRE-1, repeat 2) occurred within a 4.0 h incubation of fibroblasts with media containing lipoprotein-deficient serum (LPDS) and cholesterol synthesis inhibitors. Methylation of this residue was reduced to control levels within 2.0 h after the addition of a mixture of 25-hydroxycholesterol and mevalonic acid. The time-dependent changes in DMS-reactivity of guanine -59 induced by the cholesterol synthesis inhibitors or oxysterols were paralleled by alterations in LDL receptor mRNA. In contrast to the results with fibroblasts, neither cholesterol synthesis inhibitors nor oxysterols produced consistent effects on the DMS-reactivity of guanine -59 in hepatocytes despite induction or repression of LDL receptor mRNA in these cells. Interestingly, no other changes in the protection pattern over repeats 1, 2, and 3 were apparent in either fibroblasts or hepatocytes. These results demonstrate that hypermethylation of guainine -59 within the SRE-1 is positively associated with activation of LDL receptor gene transcription in skin fibroblasts. Furthermore, the absence of demonstrable changes in DMS-reactivity of other purines within this region suggests that the LDL receptor promoter is poised to activate transcription with only minimal changes of protein binding to the proximal promoter in vivo.

Ketoconazole and 25-hydroxycholesterol produce reciprocal changes in the rate of transcription of the human LDL receptor gene

Sterol-dependent regulation of low-density lipoprotein (LDL) receptor gene expression was studied in the human hepatoma HepG2 cell line. Incubation of HepG2 cells with 20 microM ketoconazole increased the level of LDL receptor mRNA. After a lag of approx. 1.0 h the level rose 6.5-fold within 8.0 h and remained elevated for up to 24 h. Incubation with 10 micrograms 25-hydroxycholesterol/ml for 24 h produced a 40-50% reduction in the level of LDL receptor mRNA. Ketoconazole- and 25-hydroxycholesterol-induced changes in LDL receptor mRNA accumulation were due to alterations in the relative rate of LDL receptor gene transcription as measured by nuclear run-on transcription. Incubation with 20 microM ketoconazole for 4 h or 10 micrograms 25-hydroxycholesterol/ml for 24 h produced a 3.6-fold increase and a 40% reduction, respectively, in the transcription rate of LDL receptor gene. Removal of the Alu-like sequence elements within the LDL receptor cDNA was required to consistently measure changes in LDL receptor gene transcription. No significant changes were noted in the half-life of LDL receptor mRNA in ketoconazole or 25-hydroxycholesterol-treated cells. These data demonstrate that sterol-dependent changes in the level of LDL receptor mRNA can be completely accounted for by changes in the rate of LDL receptor gene transcription.

Evidence for sterol-independent regulation of low-density lipoprotein receptor activity in Hep-G2 cells

The relationship between the serum factor(s)-mediated induction of low-density lipoprotein (LDL) receptor activity and changes in cellular cholesterol metabolism was examined in the human hepatoma cell line Hep-G2. Relative to incubation with serum-free media [Eagle's minimal essential medium (MEM) control], short-term (less than 8 h) incubation with medium containing 15% of either calf serum (MEM + serum) or the d greater than 1.25 fraction of calf serum (MEM + d greater than 1.25) produced a time- and concentration-dependent increase in the uptake of 125I-LDL. Immunoblotting with anti-(LDL receptor) antibodies demonstrated that this was correlated with a 2-fold increase in the amount of the mature 136,000 Da LDL receptor protein in detergent-solubilized Hep-G2 cell membranes. Incubation with MEM + serum, but not MEM + d greater than 1.25, increased the efflux of radiolabelled cholesterol from Hep-G2 cells. However, the induction of 125I-LDL uptake by MEM + d greater than 1.25 (2.3-fold) and MEM + serum (2.2-fold) was virtually identical. Addition of the d less than 1.063 lipoproteins of calf serum to MEM + d greater than 1.25 at their original or three times their serum concentration decreased the induction of 125I-LDL uptake by MEM + d greater than 1.25 by only 20-30%. Together, these results suggest that the stimulation of 125I-LDL uptake was not due to the presence of high-density lipoprotein, the absence of LDL or the stimulation of cholesterol efflux. MEM + serum stimulated 125I-LDL uptake in cells cholesterol-loaded by incubation with rat very-low-density lipoprotein with beta electrophoretic mobility (beta-VLDL). Compared to incubation with the MEM control, either MEM + serum or MEM + d greater than 1.25 produced time-dependent increases in the activity of 3-hydroxy-3-methylglutaryl-CoA reductase which also occurred in cholesterol-loaded cells. However, cholesterol biosynthesis, whether measured from 3H2O, [14C]acetate or [3H]mevalonic acid, was not increased. Incubation with MEM + serum or MEM + d greater than 1.25 did not affect [3H]oleate incorporation into cellular cholesteryl esters, hydrolysis of intracellular [3H]cholesteryl esters or the cellular mass of unesterified or esterified cholesterol. Incubation with MEM + serum or MEM + d greater than 1.25 produced a transient increase in the level of LDL receptor mRNA, reaching a maximum of 5-10-fold by 2 h and decreasing to near baseline levels by 4 h. Actinomycin D blocked the serum-factor-mediated induction of LDL receptor mRNA.(ABSTRACT TRUNCATED AT 400 WORDS)

Differences in the processing of chylomicron remnants and beta-VLDL by macrophages

To gain a detailed understanding of those factors that govern the processing of dietary-derived lipoprotein remnants by macrophages we examined the uptake and degradation of rat triacylglycerol-rich chylomicron remnants and rat cholesterol-rich beta-very low density lipoprotein (beta-VLDL) by J774 cells and primary cultures of mouse peritoneal macrophages. The level of cell associated 125I-labeled beta-VLDL and 125I-labeled chylomicron remnants reached a similar equilibrium level within 2 h of incubation at 37 degrees C. However, the degradation of 125I-labeled beta-VLDL was two to three times greater than the degradation of 125I-labeled chylomicron remnants at each time point examined, with rates of degradation of 161.0 +/- 36.0 and 60.1 +/- 6.6 ng degraded/h per mg cell protein, respectively. At similar extracellular concentrations of protein or cholesterol, the relative rate of cholesteryl ester hydrolysis from [3H]cholesteryl oleate/cholesteryl [14C]oleate-labeled chylomicron remnants was one-third to one-half that of similarly labeled beta-VLDL. The reduction in the relative rate of chylomicron remnant degradation by macrophages occurred in the absence of chylomicron remnant-induced alterations in low density lipoprotein (LDL) receptor recycling or in retroendocytosis of either 125I-labeled lipoprotein. The rate of internalization of 125I-labeled beta-VLDL by J774 cells was greater than that of 125I-labeled chylomicron remnants, with initial rates of internalization of 0.21 ng/min per mg cell protein for 125I-labeled chylomicron remnants and 0.39 ng/min per mg cell protein for 125I-labeled beta-VLDL. The degradation of 125I-labeled chylomicron remnants and 125I-labeled beta-VLDL was dependent on lysosomal enzyme activity: preincubation of macrophages with the lysosomotropic agent monensin reduced the degradation of both lipoproteins by greater than 90%. However, the pH-dependent rate of degradation of 125I-labeled chylomicron remnants by lysosomal enzymes isolated from J774 cells was 50% that of 125I-labeled beta-VLDL. The difference in degradation rates was dependent on the ratio of lipoprotein to lysosomal protein used and was greatest at ratios greater than 50. The degradation of 125I-labeled beta-VLDL by isolated lysosomes was reduced 30-40% by preincubation of beta-VLDL with 25-50 micrograms oleic acid/ml, suggesting that released free fatty acids could cause the slower degradation of chylomicron remnants. Thus, differences in the rate of uptake and degradation of remnant lipoproteins of different compositions by macrophages are determined by at least two factors: 1) differences in the rates of lipoprotein internalization and 2) differences in the rate of lysosomal degradation.

On the frequency of metaphase chromosome aberrations in the small intestine of aged rats

Abnormalities in DNA synthesis and cell proliferation are characteristic of aged populations of proliferating cells. Cytogenetic analysis of jejunal crypt cells from young and senescent rats indicates that the imbalance in cell production in vivo is associated with an age-dependent increase in metaphase chromosome aberrations. Furthermore, the frequency of these chromosome aberrations is correlated with histologic evidence of cell death. These results demonstrate that the maintenance of genomic integrity is disturbed in the aged small intestine and may explain the age-related increase in the proportion of relatively undifferentiated villus epithelial cells in the small intestine of senescent rats.

A method for the preparation of metaphase chromosomes from rat small intestine

A major obstacle to successful cytogenetic analysis of small intestinal crypt cells is the acquisition of a sufficient number of high-quality metaphases. A squash procedure has been developed for analysis of metaphase chromosomes from rat small intestine that largely circumvents this difficulty. The method involves a schedule of hypotonic treatment, fixation in ethanol: acetic acid, followed by maceration of the intestinal tissue in 3.5 N HCl. The procedure resulted in large numbers of well-spread, cytoplasm-free metaphases.

Defining cellular senescence in IMR-90 cells: a flow cytometric analysis

Using multiparameter flow cytometric analysis, we find that senescent cells accumulate in a unique cell-cycle compartment characterized in cell-cycle arrest in G1 and a significantly reduced nucleocytoplasmic ratio (genome size/cell mass) relative to cycling cells. With respect to gross cellular phenotype, the quiescent state of senescent cells differs from quiescence induced by density inhibition; the former is associated with a reduction in the nucleocytoplasmic ratio, while the latter is associated with an increase in the nucleocytoplasmic ratio. Senescent cells were present at all passages examined. The frequency of senescent cells was low in early-passage cultures and increased with passage number. Senescence of populations of IMR-90 cells reflects change in the relative frequency of these cells. The frequency of cells with karyotypic changes increased with the progressive accumulation of out-of-cycle cells.

Stimulation of LDL receptor activity in Hep-G2 cells by a serum factor(s)

The regulation of low-density lipoprotein (LDL) receptor activity in the human hepatoma cell line Hep-G2 by serum components was examined. Incubation of dense monolayers of Hep-G2 cells with fresh medium containing 10% fetal calf serum (FM) produced a time-dependent increase in LDL receptor activity. Uptake and degradation of 125I-LDL was stimulated two- to four-fold, as compared with that of Hep-G2 cells cultured in the same media in which they had been grown to confluence (CM); the maximal 125I-LDL uptake plus degradation increased from 0.2 microgram/mg cell protein/4 h to 0.8 microgram/mg cell protein/4 h. In addition, a two-fold increase in cell surface binding of 125I-LDL to Hep-G2 cells was observed when binding was measured at 4 degrees C. There was no change in the "apparent" Kd. The stimulation of LDL receptor activity was suppressed in a concentration-dependent manner by the addition of cholesterol, as LDL, to the cell medium. In contrast to the stimulation of LDL receptor activity, FM did not affect the uptake or degradation of 125I-asialoorosomucoid. Addition of FM increased the protein content per dish, and DNA synthesis was stimulated approximately five-fold, as measured by [3H]thymidine incorporation into DNA; however, the cell number did not change. Cellular cholesterol biosynthesis was also stimulated by FM; [14C]acetate incorporation into unesterified and esterified cholesterol was increased approximately five-fold. Incubation of Hep-G2 cells with high-density lipoproteins (200 micrograms protein/ml) or albumin (8.0 mg/ml) in the absence of the serum factor did not significantly increase the total processed 125I-LDL. Stimulation of LDL receptor activity was dependent on a heat-stable, nondialyzable serum component that eluted in the inclusion volume of a Sephadex G-75 column. Uptake of 125I-LDL by confluent monolayers of human skin fibroblasts was not changed by incubation with FM or by incubation with Hep-G2 conditioned medium. Taken together, these data demonstrate that LDL receptor activity in Hep-G2 cells is stimulated by a serum component. Furthermore, this serum factor shows some specificity for the LDL receptor pathway in liver-derived Hep-G2 cells.

Human plasma lipid transfer protein catalyzes the speciation of high density lipoproteins

The role of purified plasma lipid transfer protein complexes in determining the particle size distribution of human plasma high density lipoproteins (HDL) was examined in vitro. Incubation of HDL2 or HDL3, isolated from normolipemic subjects with very low density lipoproteins (VLDL) or VLDL-remnants and lipid transfer protein complex had little or no effect on HDL particle size. In contrast, HDL isolated from patients with hypertriglyceridemia, designated HDL3D, showed speciation of particle size distribution when incubated with VLDL-remnants and the transfer protein. Incubation of HDL3D with VLDL-remnants and lipid transfer complex resulted in the production of two particles of radius 4.3 and 3.7 nm; incubation with VLDL or in the absence of the transfer protein did not result in a redistribution of particle size. We suggest that the action of lipid transfer protein complex on triacylglycerol-rich lipoprotein remnants and HDL accounts for the low levels of HDL-cholesterol observed in subjects with severe hypertriglyceridemia.

Transport of beta-very low density lipoproteins and chylomicron remnants by macrophages is mediated by the low density lipoprotein receptor pathway

The receptor-mediated uptake of rat hypercholesterolemic very low density lipoproteins (beta VLDL) and rat chylomicron remnants was studied in monolayer cultures of the J774 and P388D1 macrophage cell lines and in primary cultures of mouse peritoneal macrophages. Uptake of 125I-beta VLDL and 125I-chylomicron remnants was reduced 80-90% in the presence of high concentrations of unlabeled human low density lipoproteins (LDL). Human acetyl-LDL did not significantly compete at any concentration tested. Uptake of 125I-beta VLDL and 125I-chylomicron remnants was also competitively inhibited by specific polyclonal antibodies directed against the estrogen-induced LDL receptor of rat liver. Incubation in the presence of anti-LDL receptor IgG, but not nonimmune IgG, reduced specific uptake greater than 80%. Anti-LDL receptor IgG, 125I-beta VLDL, and 125I-chylomicron remnants bound to two protein components of apparent molecular weights 125,000 and 111,000 on nitrocellulose blots of detergent-solubilized macrophage membranes. Between 70-90% of 125I-lipoprotein binding was confined to the 125,000-Da peptide. Binding of 125I-beta VLDL and 125I-chylomicron remnants to these proteins was competitively inhibited by anti-LDL receptor antibodies. Comparison of anti-LDL receptor IgG immunoblot profiles of detergent-solubilized membranes from mouse macrophages, fibroblasts, and liver, and normal and estrogen-induced rat liver demonstrated that the immunoreactive LDL receptor of mouse cells is of a lower molecular weight than that of rat liver. Incubation of J774 cells with 1.0 micrograms of 25-hydroxycholesterol/ml plus 20 micrograms of cholesterol/ml for 48 h decreased 125I-beta VLDL uptake and immuno- and ligand blotting to the 125,000- and 111,000-Da peptides by only 25%. Taken together, these data demonstrate that uptake of beta VLDL and chylomicron remnants by macrophages is mediated by an LDL receptor that is immunologically related to the LDL receptor of rat liver.

Evidence that chylomicron remnants and beta-VLDL are transported by the same receptor pathway in J774 murine macrophage-derived cells

To characterize lipoprotein uptake by macrophages, we studied J774 murine macrophage-derived cells. Uptake of 125I-labeled beta-VLDL and 125I-labeled chylomicron remnants was saturable, specific, and of high affinity. Maximal specific uptake and the concentration at which half-maximal uptake occurred were similar for both beta-VLDL and chylomicron remnants. Specific uptake of 125I-labeled chylomicrons was only 1/5 that of the other two lipoproteins. Cholesterol loading decreased 125I-labeled chylomicron remnant and 125I-labeled beta-VLDL uptake by 25%. Chylomicron remnants and beta-VLDL were equipotent in cross-competition studies; acetyl-LDL did not compete, and human LDL was a poor competitor. Although the amounts of cell-associated lipoproteins were similar, beta-VLDL and chylomicron remnants had different effects on cellular lipid metabolism. beta-VLDL produced a threefold stimulation while chylomicron remnants caused a decrease in [3H]oleate incorporation into cholesteryl ester. beta-VLDL had no effect while chylomicron remnants caused a threefold increase in [3H]oleate incorporation into triacylglycerol. beta-VLDL produced a 44% suppression and chylomicron remnants produced a 78% increase in HMG-CoA reductase activity. In summary, J774 macrophages express a receptor site that recognizes both beta-VLDL and chylomicron remnants; however, these lipoproteins exhibit strikingly different effects on intracellular lipid metabolism.

Very low and low density lipoprotein synthesis and secretion by the human hepatoma cell line Hep-G2: effects of free fatty acid

The liver is a major source of the plasma lipoproteins; however, direct studies of the regulation of lipoprotein synthesis and secretion by human liver are lacking. Dense monolayers of Hep-G2 cells incorporated radiolabeled precursors into protein ([35S]methionine), cholesterol ([3H]mevalonate and [14C]acetate), triacylglycerol, and phospholipid ([3H]glycerol), and secreted them as lipoproteins. In the absence of free fatty acid in the media, the principal lipoprotein secretory product that accumulated had a density maximum of 1.039 g/ml, similar to serum low density lipoprotein (LDL). ApoB-100 represented greater than 95% of the radiolabeled apoprotein of these particles, with only traces of apoproteins A and E present. Inclusion of 0.8 mM oleic acid in the media resulted in a 54% reduction in radiolabeled triacylglycerol in the LDL fraction and a 324% increase in triacylglycerol in the very low density lipoprotein (VLDL) fraction. Similar changes occurred in the secretion of newly synthesized apoB-100. The VLDL contained apoB-100 as well as apoE. In the absence of exogenous free fatty acid, the radiolabeled cholesterol was recovered in both the LDL and the high density lipoprotein (HDL) regions. Oleic acid caused a 50% decrease in HDL radiolabeled cholesterol and increases of radiolabeled cholesterol in VLDL and LDL. In general, less than 15% of the radiolabeled cholesterol was esterified, despite the presence of cholesteryl ester in the cell. Incubation with oleic acid did not cause an increase in the total amount of radiolabeled lipid or protein secreted. We conclude that human liver-derived cells can secrete distinct VLDL and LDL-like particles, and the relative amounts of these lipoproteins are determined, at least in part, by the availability of free fatty acid.

Immunoaffinity purification of the lipid transfer protein complex directly from human plasma

The human cholesteryl ester (CE) and triglyceride (TG) exchange protein (denoted LTC or lipid transfer complex) was isolated in a single step from plasma using immunoaffinity batch extraction. Antibodies were raised against two preparations of conventionally purified LTC. LTC-I and LTC-II (purified 20,000-fold and 3500-fold, respectively) were used as immunogens. The antiLTC antibodies were isolated by anion-exchange chromatography and coupled to Affi-Gel 10. Chromatography of plasma on antiLTC Affi-Gel removed all of the CE and TG transfer activity. Moreover, LTC prepared from both antiLTC-I and antiLTC-II-Affi-Gel matrices were identical when analyzed by sodium dodecyl sulfate-polyacrylamide gel LTC electrophoresis. LTC exhibited two protein bands of Mr (apparent) 67,000 and 58,000 and a broad, faintly staining region at greater than 150,000. Analysis of LTC by immunoblotting indicated that both antiLTC-I and antiLTC-II antibodies recognized the same LTC proteins. Isoelectric focussing of LTC gave two pI values, 5.2 and 8.7. These data suggest that LTC is a complex of specific proteins and perhaps lipid. Specific CE and TG exchange activities of immunoaffinity-purified LTC were comparable, although the activities were low with respect to that of the antigen used to generate antiLTC-I. This is not due to contamination of LTC by albumin, lecithin:cholesterol acyltransferase, or apolipoproteins AI, AII, B, CIII, D, or E.

Interaction of a human plasma lipid transfer protein complex with lipid monolayers

The interaction of a purified human plasma lipid transfer complex with cholesteryl ester, triacylglycerol and phosphatidylcholine in binary and ternary lipid monolayers was investigated. The lipid transfer complex, designated LTC, catalyzes the removal of cholesteryl oleate and triacylglycerol from phosphatidylcholine monolayers. Preincubation of LTC with p-chloromercuriphenyl sulfonate inhibits LTC-catalyzed removal of triacylglycerol; cholesteryl ester removal is not affected. The rate of LTC-facilitated removal of cholesteryl oleate from a phosphatidylcholine monolayer depends on the amount of LTC added to the subphase up to 100 micrograms protein. In addition, the rate of the LTC-catalyzed transfer of cholesteryl oleate to the subphase increases linearly as the amount of cholesteryl oleate in the monolayer increases to 6 mol%. LTC also removes cholesterol from phosphatidylcholine-cholesterol monolayers, albeit at a rate which is 15% of that for removal of cholesteryl oleate. The ability of LTC to facilitate triacylglycerol and cholesteryl ester removal depends on the composition of the monolayer. Phosphatidylcholine supports cholesteryl ester transfer whereas sphingomyelin-cholesteryl ester monolayers are almost refractory to LTC. In contrast, LTC removes triacylglycerol from either a phosphatidylcholine or a sphingomyelin monolayer. The results suggest the existence of at least two lipid transfer proteins, one of which catalyzes the removal of cholesteryl ester and the other triacylglycerol. The role of these proteins as they relate to lipoprotein metabolism is discussed.

Plasma protein-facilitated coupled exchange of phosphatidylcholine and cholesteryl ester in the absence of cholesterol esterification

A protein(s) which catalyzes the exchange of phosphatidylcholine and cholesteryl ester between plasma lipoproteins has been purified 10,000-fold from lipoprotein-free human plasma. The apparent molecular weight of the protein of the active fraction, designated lipid transfer complex (LTC), is approximately 61,000; when electrophoresed in 6 M urea, 0.1% sodium dodecyl sulfate on a 3-20% polyacrylamide gradient, the protein appears as a doublet of molecular weights 58,000 and 63,000. The active material is a glycoprotein which binds to concanavalin A. Human LTC is a lipid-protein complex with phospholipid, cholesterol, cholesteryl ester, and glyceride comprising 7% of the total mass. A similar glycoprotein (or glycoproteins) exists in rat plasma, although the fold-purification thus far achieved is low: about 500-fold. Moreover, the rat preparation enhances exchange of phosphatidylcholine, but does not appreciably enhance exchange of cholesteryl ester. Partially purified LTC (less than or equal to 3500-fold) exists in a complex with lecithin: cholesterol acyltransferase. Active lecithin: cholesterol acyltransferase is not, however, required for exchange of phosphatidylcholine or cholesteryl ester facilitated by human LTC. The rates of exchange of phosphatidylcholine and cholesteryl ester facilitated by human LTC are equal. Coupled lipid exchange occurs at all stages of LTC purification, at values of pH between 5 and 10, and at ionic strengths as great as 0.9. Moreover, phosphatidylcholine and cholesteryl ester are exchanged with 1:1 stoichiometry in the presence of thiol group reagents such as 5,5'-dithiobis-(2-nitrobenzoic acid). Both lipid exchange activities are relatively resistant to elevated temperatures. Coupled exchange of phospholipid and neutral lipid is not dictated by the nature of the lipoprotein donor and acceptor substrates: bovine liver phospholipid exchange protein catalyzes exchange of phosphatidylcholine but not cholesteryl ester between low and high density lipoproteins under conditions identical with those in which human LTC facilitates exchange of both lipids.