Regulation of low density lipoprotein receptor function in a human hepatoma cell line

Epidemiology and prognosis of paraneoplastic syndromes in hepatocellular carcinoma

Background. Paraneoplastic syndromes (PNS) such as hypercalcaemia, hypercholesterolaemia, and erythrocytosis have been described in hepatocellular carcinoma (HCC). Aims. (1) To examine the prevalence, clinical characteristics, and survival of PNS in HCC patients and (2) to evaluate the extent to which each individual PNS impacts on patient survival. Methods. We prospectively evaluated the prevalence, clinical characteristics, and survival of PNS among 457 consecutive HCC patients seen in our department over a 10-year period and compared them with HCC patients without PNS. Results. PNS were present in 127 patients (27.8%). The prevalence of paraneoplastic hypercholesterolemia, hypercalcemia, and erythrocytosis 24.5%, 5.3%, and 3.9%, respectively. Patients with PNS had significantly higher alpha-fetoprotein levels, more advanced TNM stage, and shorter survival. Among the individual PNS, hypercalcemia and hypercholesterolemia were associated with more advanced disease and reduced survival but not erythrocytosis. On multivariate analysis, the presence of PNS was not found to be an independent prognostic factor for reduced HCC survival. Conclusion. PNS are not uncommon in HCC and are associated with poor prognosis and reduced survival due to their association with increased tumor burden. However, they do not independently predict poor survival. Individual PNS impact differently on HCC outcome; paraneoplastic hypercalcemia in particular is associated with poor outcome.

Oleic, linoleic and linolenic acids enhance receptor-mediated uptake of low density lipoproteins in Hep-G2 cells

In the present study, the binding, internalization and degradation of low-density lipoprotein (LDL) was investigated in Hep-G2 cells treated with 18:0, 18:1, 18:2 and 18:3. In non-treated control cells, the surface binding (heparin-releasable) of 125I-LDL progressed in a saturable manner reaching equilibrium within 2 h, amounting 24.0 +/- 1.1, 29.5 +/- 1.3 and 31.4 +/- 2.8 (ng/mg cell protein) at 1, 2 and 4 h, respectively. The cells rapidly internalized 125I-LDL reaching a plateau at 2 h (72.4 +/- 6.3/1 h, 96.7 +/- 4.3/2 h and 100.8 +/- 4.6 ng/mg protein/4 h, respectively). The degradation of internalized LDL progressed slowly during the first hour of incubation reflecting the time required to an uptake and delivery of LDL to the cellular lysosomes. The levels of degraded LDL discharged into the medium then increased rapidly in a linear manner after the initial lag period, amounting 16.8 +/- 1.2, 51.8 +/- 7.0 and 118.2 +/- 5.7 ng/mg protein at 1, 2 and 4 h, respectively. The treatment of cells with of 1.0 mM of fatty acids for 4 h resulted in a significant increase in the surface binding of 125I-LDL compared to the control (34.9 +/- 3.0), but it was significantly lower in cells exposed to 18:0 (48.2 +/- 2.0) than to 18:1 (56.8 +/- 5.1), 18:2 (56.0 +/- 3.5) and 18:3 (57.8 +/- 6.0 ng/mg protein/4 h) (P < 0.05). The levels of degraded LDL in cells remained nearly the same regardless of fatty acid treatments, but degraded LDL levels in the medium were much higher in cells exposed to 18:1 (167.6 +/- 10.1), 18:2 (159.8 +/- 7.7) and 18:3 (165.1 +/- 14.7) than to 18:0 (142.1 +/- 8.4) and the control (121.2 +/- 3.4 ng/mg protein/4 h) (P < 0.05). The present finding that 18:1 is equally effective in enhancing the receptor-mediated LDL uptake and its degradation as those of 18:2 and 18:3 suggests that the major action of 18:1 in lowering LDL-cholesterol levels also involves an increased clearance of LDL via hepatic LDL-receptors.

Effect of coffee lipids (cafestol and kahweol) on regulation of cholesterol metabolism in HepG2 cells

We studied the effect of the coffee diterpene alcohols, cafestol and kahweol, on cholesterol metabolism in HepG2 cells. Uptake of 125I-tyramine cellobiose-labeled LDL was decreased by 15% to 20% (P < .05) after 18 hours of preincubation with cafestol (20 micrograms/mL), whereas 25-hydroxycholesterol reduced uptake by 55% to 65% (P < .05). Degradation of LDL in the presence of cafestol was decreased by 20% to 30% (P < .05) under the same conditions. The effect of cafestol (20 micrograms/mL) on uptake and degradation of LDL was greatest (35% to 40%, P < .05) after 6 and 10 hours of preincubation, respectively. Furthermore, the effect of cafestol was also dependent on its concentration, and a significant decrease in the LDL uptake (19%) was observed at 10 micrograms/mL (P < .05). Specific binding of LDL was reduced by 17% (P < .05) and 60% (P < .05) after preincubation with cafestol (20 micrograms/mL) and 25-hydroxycholesterol (5 micrograms/mL) for 6 hours, respectively, compared with control cells. Analysis of LDL binding showed that cafestol reduced the number of binding sites for LDL on the cell surface (capacity) by 35% (P < .05). In contrast, no significant effect on the level of mRNA for the LDL receptor was observed after incubation with cafestol, whereas 25-hydroxycholesterol reduced the mRNA level for the LDL receptor by 40% to 50% (P < .05). A fusion gene construct consisting of a synthetic sterol regulatory element-1 (SRE-1) promoter for the human LDL receptor coupled to the reporter gene for chloramphenicol acetyltransferase (CAT) was transfected into HepG2 cells. No change was observed in CAT activity in SRE-1-transfected cells after incubation with cafestol, whereas 25-hydroxycholesterol reduced CAT activity by 30% to 40% (P < .05). Incorporation of [14C]acetate into unesterified cholesterol and 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase activity were unaffected in cells incubated with cafestol as well as the cafestol-kahweol mixture compared with control cells. Moreover, cafestol and the cafestol-kahweol mixture did not promote increased incorporation of radiolabeled [14C]oleic acid into cholesteryl esters after short-term incubation compared with control cells. On the other hand, 25-hydroxycholesterol caused a 70% to 90% reduction of cholesterol synthesis (P < .05) and HMG-CoA reductase activity (P < .05), decreased HMG-CoA reductase mRNA level by 70% to 80% (P < .05), and promoted a twofold increase in cholesterol esterification (P < .05). Finally, no effect of the coffee diterpenes on bile acid formation was observed. These results suggest that cafestol (and kahweol) may reduce the activity of hepatic LDL receptors and thereby cause extracellular accumulation of LDL.

Large versus small unilamellar vesicles mediate reverse cholesterol transport in vivo into two distinct hepatic metabolic pools. Implications for the treatment of atherosclerosis

Phospholipid liposomes are synthetic mediators of "reverse" cholesterol transport from peripheral tissue to liver in vivo and can shrink atherosclerotic lesions in animals. Hepatic disposal of this cholesterol, however, has not been examined. We compared hepatic effects of large (approximately equal to 120-nm) and small (approximately equal to 35-nm) unilamellar vesicles (LUVs and SUVs), both of which mediate reverse cholesterol transport in vivo but were previously shown to be targeted to different cell types within the liver. On days 1, 3, and 5, rabbits were intravenously injected with 300 mg phosphatidylcholine (LUVs or SUVs) per kilogram body weight or with the equivalent volume of saline. After each injection, LUV- and SUV-injected animals showed large increases in plasma concentrations of unesterified cholesterol, indicating mobilization of tissue stores. After hepatic uptake of this cholesterol, however, SUV-treated animals developed persistently elevated plasma LDL concentrations, which by day 6 had increased to more than four times the values in saline-treated controls. In contrast, LUV-treated animals showed normal LDL levels. By RNase protection assay, SUVs suppressed hepatic LDL receptor mRNA at day 6 (to 61 +/- 4% of control, mean +/- SEM), whereas LUVs caused a statistically insignificant stimulation. Hepatic HMG-CoA reductase message was also significantly suppressed with SUV, but not LUV treatment, and hepatic 7 alpha-hydroxylase message showed a similar trend. These data on hepatic mRNA levels indicate that SUVs, but not LUVs, substantially perturbed liver cholesterol homeostasis. We conclude that LUVs and SUVs mobilize peripheral tissue cholesterol and deliver it to the liver, but to distinct metabolic pools that exert different regulatory effects. The effects of one of these artificial particles, SUVs, suggest that reverse cholesterol transport may not always be benign. In contrast, LUVs may be a suitable therapeutic agent, because they mobilize peripheral cholesterol to the liver without suppressing hepatic LDL receptor mRNA and without provoking a subsequent rise in plasma LDL levels.

Re-uptake of nascent low-density lipoproteins by HepG2 cells

The kinetics and specificity of the interaction of nascent HepG2 LDL with the HepG2 LDL receptor were examined. 125I-Labeled HepG2 LDL and plasma LDL were bound by HepG2 cells in a specific and saturable manner at 4 degrees C. Competition studies with HepG2 LDL and plasma LDL indicated that both ligands bound to the same receptor. Scatchard analyses of the specific 4 degrees C-binding data revealed a Kd of 75 nM for HepG2 LDL and a Kd of 30 nM for plasma LDL suggesting that HepG2 LDL bind less efficiently to the HepG2 LDL receptor than plasma LDL. Binding, internalization and degradation studies carried out at 37 degrees C indicated that HepG2 cells are capable of catabolizing their own nascent LDL; however, under normal experimental conditions re-uptake of nascent LDL is quantitatively insignificant.

Apolipoprotein B genetic polymorphisms in several human hepatoma derived liver cell lines

The genetic polymorphism of apoB EcoRI and XbaI restriction sites and the 3' VNTR hypervariable region was examined in nine human hepatoma derived liver cell lines and related to the cells' ability to secrete lipids and apoB. EcoRI and XbaI genotypes appeared to be unrelated to triglyceride, cholesterol and apoB accumulating in the medium. The VNTR consisted of alleles with 47 to 67 repeats; however, these repeats were not associated with elevated concentrations of lipid or apoB. Data suggest that in the hepatoma cell lines, apoB polymorphisms in EcoRI, XbaI and the VNTR hypervariable region are not sufficient in themselves to account for triglyceride, cholesterol and apoB in the medium. It is possible that intracellular apoB synthesis and/or degradation as well as postsecretory apoB binding and uptake are responsible for the variability of apoB and lipid accumulation in the culture medium.

The human asialoglycoprotein receptor is a possible binding site for low-density lipoproteins and chylomicron remnants

Binding and internalization of chylomicron remnants from rat mesenteric lymph by HepG2 cells was inhibited by both excess remnants and low-density lipoprotein (LDL) to the same extent. Ligand blots revealed binding of remnants and LDL to the LDL receptor. Measures regulating LDL receptor activity greatly influenced the binding of remnants: ethinyloestradiol, the hydroxymethylglutaryl-CoA reductase inhibitor pravastatin and the absence of LDL all increased binding, whereas high cell density or the presence of LDL decreased binding. Also, asialofetuin, asialomucin, the neoglycoprotein galactosyl-albumin and an antibody against the asialoglycoprotein receptor all decreased substantially the binding of remnants. At high cell density, binding internalization and degradation of chylomicron remnants was inhibited by up to 70-80%, yet binding of LDL was inhibited by no more than 20-30%. In cross-competition studies, the binding of 125I-asialofetuin was efficiently competed for by asialofetuin itself or by the antibody, and also by LDL and remnants, yet remnants displayed an approx. 100-fold higher affinity than LDL. Likewise, remnants of human triacylglycerol-rich lipoproteins and asialofetuin interfered with each others' binding to HepG2 cells or human liver membranes. It is concluded that the LDL receptor mediates the internalization of chylomicron remnants into hepatocytes depending on its activity, according to demand for cholesterol. Additionally, the asialoglycoprotein receptor may contribute to the endocytosis of LDL, but predominantly of chylomicron remnants.

Differences in metabolism of three low density lipoprotein subfractions in Hep G2 cells

The metabolism of low density lipoprotein (LDL) subfractions was investigated in the human hepatoma cell line Hep G2. By means of a density gradient ultracentrifugation method three LDL subfractions were isolated from pooled serum of normolipidemic subjects: very light LDL-1A, light LDL-1B and heavy LDL-2, differing in size, relative lipid and protein content. Cell specific association, stimulation of cholesterol esterification and inhibition of sterol synthesis were determined in parallel after incubation of Hep G2 cells with increasing amounts of LDL-protein of the three LDL subfractions. These parallel experiments were repeated four times with freshly prepared LDL subfractions. Response curves were parametrized with the function y = a square root of x, depicting the relation between the cellular metabolic event (y) and the LDL-protein (x) or LDL-cholesterol (x) levels. An analysis of covariance model was used to test differences between parameters of the three LDL subfractions. When the results of all four experiments were taken into account, the cell specific association increased more with increasing LDL-protein concentration for LDL-1A than for LDL-2 (P less than 0.05). At the LDL-protein level of 80 micrograms/ml the cell specific association for LDL-2 amounted to 85.5% of that for LDL-1A. Results for LDL-1B were intermediate between those for LDL-1A and LDL-2. The corresponding cholesteryl ester formation increased more with increasing LDL-protein concentration for LDL-1A than for LDL-1B (P less than 0.001), and for LDL-1B more than for LDL-2 (P less than 0.001). At the LDL-protein level of 70 micrograms/ml the cholesterol ester accumulation for LDL-2 and LDL-1B was 48.4% and 70.3%, respectively, of that for LDL-1A. These differences between LDL subfractions in cholesteryl esterification were independent of the cholesterol content of the subfractions. Consistent with these findings, the [14C]acetate incorporation to sterols decreased more with increasing LDL-protein concentration for LDL-1A and LDL-1B than for LDL-2 (P less than 0.05). At the LDL-protein concentration of 70 micrograms/ml the decrease in sterol synthesis after incubation with LDL-2 and LDL-1B was 79.2% and 98.0%, respectively, relative to that for LDL-1A. When adjusted for differences in cholesterol content of the LDL subfractions these differences with regard to the 14C-acetate incorporation were not significant. The metabolic differences between LDL subfractions in vitro may have implications for the metabolism and atherogenic potential of the distinct LDL subfractions in vivo.(ABSTRACT TRUNCATED AT 400 WORDS)

The effect of (-)-hydroxycitrate on the activity of the low-density-lipoprotein receptor and 3-hydroxy-3-methylglutaryl-CoA reductase levels in the human hepatoma cell line Hep G2

(-)-Hydroxycitrate, a potent inhibitor of ATP citrate-lyase, was tested in Hep G2 cells for effects on cholesterol homoeostasis. After 2.5 h and 18 h incubations with (-)-hydroxycitrate at concentrations of 0.5 mM or higher, incorporation of [1,5-14C]citrate into fatty acids and cholesterol was strongly inhibited. This most likely reflects an effective inhibition of ATP citrate-lyase. Cholesterol biosynthesis was decreased to 27% of the control value as measured by incorporations from 3H2O, indicating a decreased flux of carbon units through the cholesterol-synthetic pathway. After 18 h preincubation with 2 mM-(-)-hydroxycitrate, the cellular low-density-lipoprotein (LDL) receptor activity was increased by 50%, as determined by the receptor-mediated association and degradation. Measurements of receptor-mediated binding versus LDL concentration suggests that this increase was due to an increase in the numbers of LDL receptors. Simultaneously, enzyme levels of 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA) reductase as determined by activity measurements increased 30-fold. Our results suggest that the increases in HMG-CoA reductase and the LDL receptor are initiated by the decreased flux of carbon units in the cholesterol-synthetic pathway, owing to inhibition of ATP citratelyase. A similar induction of HMG-CoA reductase and LDL receptor was also found after preincubations of cells with 0.3 microM-mevinolin, suggesting that the underlying mechanism for this induction is identical for both drugs.

Hep-G2 cells and primary rat hepatocytes differ in their response to inhibitors of HMG-CoA reductase

CI-981, a novel synthetic inhibitor of HMG-CoA reductase, was previously reported to be highly liver-selective using an ex vivo approach. In order to determine liver-selectivity at the cellular level, CI-981 was evaluated in cell culture and compared to lovastatin, pravastatin, fluvastatin and BMY-21950. Using human cell lines, none of the compounds tested showed liver-selectivity, i.e. strong inhibition of cholesterol synthesis in Hep-G2 cells (liver model) but weak inhibition in human fibroblasts (peripheral cell model). In contrast, all drugs tested produced equal and potent inhibition of sterol synthesis in primary cultures of rat hepatocytes, and CI-981, pravastatin and BMY-21950 were more than 100-fold more potent in rat hepatocytes compared to human fibroblasts. Since all compounds were also equally potent at inhibiting sterol synthesis in a rat subcellular system and in vivo, the data suggest that the use of Hep-G2 cells may not be the cell system of choice in which to study inhibition of hepatic cholesterogenesis or to demonstrate liver selectivity of inhibitors of HMG-CoA reductase.

Characterization of the Ah receptor mediating aryl hydrocarbon hydroxylase induction in the human liver cell line Hep G2

The Ah receptor, a soluble cytoplasmic receptor that regulates induction of cytochrome P450IA1 and mediates toxic effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), was detected and characterized in the continuous human liver cell line Hep G2. The mean concentration of specific binding sites for TCDD was 112 +/- 26 (SEM) fmol/mg cytosol protein as determined in eight separate cytosol preparations in the presence of sodium molybdate. This is equivalent to 14,000 binding sites per cell, approximately 40% of the sites per cell found in the mouse hepatoma line Hepa-1. The cytosolic Ah receptor from Hep G2 cells sedimented at 9 S and was specific for those halogenated and nonhalogenated aromatic compounds known to be agonists for the Ah receptor in rodent tissues and cells. Specific binding in the 9 S region was detected with both [3H]TCDD and 3-[3H]methylcholanthrene. 3-[3H]Methylcholanthrene did not bind to any component besides that at approximately 9 S. Phenobarbital, dexamethasone, and estradiol did not compete with [3H]TCDD for binding to the Hep G2 Ah receptor. Specific binding of [3H]triamcinolone acetonide to glucocorticoid receptor could also be demonstrated in Hep G2 cytosol. The apparent equilibrium dissociation constant (Kd) for binding of [3H]TCDD to Hep G2 Ah receptor was 9 nM by Woolf plot analysis, about an order of magnitude weaker than the affinity of [3H]TCDD for the mouse Hepa-1 Ah receptor or for the C57BL/6 murine hepatic Ah receptor. [3H]TCDD.Ah receptor complex, which was extracted from nuclei of Hep G2 cells incubated with [3H]TCDD at 37 degrees C in culture, sedimented at approximately 6 S under conditions of high ionic strength. Aryl hydrocarbon hydroxylase (AHH) activity was significantly induced after 24 h of incubation with polycyclic aromatic hydrocarbons: the EC50 for AHH induction was 5.3 microM for benz(a)anthracene and 1.3 microM for 3-methylcholanthrene. Modification of the preparative technique for cell cytosol, especially inclusion of 20 mM sodium molybdate in homogenizing and other buffers, was necessary to detect cytosolic Hep G2 Ah receptor. Hep G2 cells appear to conserve drug-metabolizing activity associated with cytochrome P450IA1 as well as the receptor mechanism which regulates its induction.

Studies on the regulation of cholesterol metabolism by low- and high-density lipoproteins in HepG2 cells

The uptake and degradation of low-density lipoproteins and the esterification and synthesis of cholesterol were poorly down-regulated by low-density lipoproteins in HepG2 cells. Addition of low-density lipoproteins to the cells increased the free and esterified cholesterol in the cells. The heavier fraction of high-density lipoproteins enhanced the degradation of low-density lipoproteins and cholesterol synthesis and decreased acyl CoA:cholesterol acyltransferase activity. Addition of the heavier fraction of high-density lipoproteins also caused a net efflux of cholesterol from HepG2 cells. The lighter fraction did not have any significant effect on cholesterol metabolism or cellular cholesterol level. Neither the lighter nor the heavier fractions of high-density lipoproteins were found to have any specific binding properties to HepG2 cells.

Some metabolic characteristics of low-density lipoprotein subfractions, LDL-1 and LDL-2: in vitro and in vivo studies

Two low-density lipoprotein subfractions, LDL-1 and LDL-2, with density ranges of respectively 1.023-1.034 and 1.036-1.041 g/ml, were isolated by aspiration after density gradient ultracentrifugation of human pooled serum. In vitro interactions of both LDL subfractions with the LDL receptor of human cultured fibroblasts, human hepatoma cell line Hep G2 and human hepatocytes were compared. No difference in association (binding and internalization) nor in degradation between LDL-1 and LDL-2 by these cells was found. However, kinetic studies in guinea pigs showed that LDL-2 disappeared faster from the circulation and accumulated to a greater extent in the liver, compared to LDL-1. Thus, we were unable to show a difference in the LDL receptor-mediated uptake of both LDL subfractions by various cells in vitro. The results obtained in vivo suggest that LDL-1 is more atherogenic than LDL-2, because its longer half-life renders the particle more susceptible to uptake by the scavenger LDL receptor on macrophages.

Cellular free cholesterol in Hep G2 cells is only partially available for down-regulation of low-density-lipoprotein receptor activity

We have previously shown that in Hep G2 cells and human hepatocytes, as compared with fibroblasts, the low-density lipoprotein (LDL) receptor activity is only weakly down-regulated after incubation of the cells with LDL, whereas incubation with high-density lipoproteins (HDL) of density 1.16-1.20 g/ml (heavy HDL) strongly increased the LDL-receptor activity. To elucidate this difference between hepatocytes and fibroblasts, we studied the cellular cholesterol homoeostasis in relation to the LDL-receptor activity in Hep G2 cells. (1) Interrupting the cholesteryl ester cycle by inhibiting acyl-CoA: cholesterol acyltransferase (ACAT) activity with compound 58-035 (Sandoz) resulted in an enhanced LDL-mediated down-regulation of the receptor activity. (2) The stimulation of the receptor activity by incubation of the cells with cholesterol acceptors such as heavy HDL was not affected by ACAT inhibition. (3) Incubation of the Hep G2 cells with LDL, heavy HDL or a combination of both grossly affected LDL-receptor activity, but did not significantly change the intracellular content of free cholesterol, suggesting that in Hep G2 cells the regulatory free cholesterol pool is small as compared with the total free cholesterol mass. (4) We used changes in ACAT activity as a sensitive (indirect) measure for changes in the regulatory free cholesterol pool. (5) Incubation of the cells with compactin (2 microM) without lipoproteins resulted in a 4-fold decrease in ACAT activity, indicating that endogenously synthesized cholesterol is directed to the ACAT-substrate pool. (6) Incubation of the cells with LDL or a combination of LDL and heavy HDL stimulated ACAT activity 3-5 fold, whereas incubation with heavy HDL alone decreased ACAT activity more than 20-fold. Our results suggest that in Hep G2 cells exogenously delivered (LDL)-cholesterol and endogenously synthesized cholesterol are primarily directed to the cholesteryl ester (ACAT-substrate) pool or, if present, to extracellular cholesterol acceptors (heavy HDL) rather than to the free cholesterol pool involved in LDL-receptor regulation.

Immunoblotting and ligand blotting of the low-density lipoprotein receptor of human liver, HepG2 cells and HeLa cells

Low-density lipoprotein receptors from adult human liver and the human hepatoblastoma cell line HepG2 were analyzed by polyacrylamide electrophoresis in SDS followed by immuno- and ligand blotting. In both liver and HepG2 we detected a protein band with apparent relative molecular mass of 130 kDa, which is similar to that of the LDL receptor in fibroblasts. In addition we showed that HeLa cells also possess this LDL-receptor protein.

Regulation of low density lipoprotein receptor activity in primary cultures of human hepatocytes by serum lipoproteins

The low density lipoprotein receptor activity was measured in primary cultures of human hepatocytes. The receptor-mediated association and degradation of low density lipoprotein increased gradually up to 140 and 190%, respectively, upon incubation of the cells with increasing amounts of whole serum (up to 100%). Preincubation of the cells with low density lipoprotein resulted in a weak downregulation of the receptor-mediated association of low density lipoprotein (only 35% reduction at 100 micrograms low density lipoprotein per ml). However, preincubation with high density lipoproteins with density between 1.16 and 1.20 gm per ml (heavy high density lipoprotein) resulted in a more than 2-fold stimulation of the receptor-mediated association of low density lipoprotein. This heavy high density lipoprotein-mediated stimulation could not be antagonized by a simultaneous addition of low density lipoprotein during that preincubation. We conclude that, in primary cultures of human hepatocytes, the downregulation of the low density lipoprotein receptor activity by low density lipoprotein is weak and completely overruled by heavy high density lipoprotein. If these results for human hepatocytes in vitro hold true for hepatocytes in vivo, our results might explain why in vivo liver cells still display low density lipoprotein receptor activity notwithstanding the exposure of these cells to physiological concentrations of low density lipoprotein.

Stimulation of the LDL receptor activity in the human hepatoma cell line Hep G2 by high-density serum fractions

The regulation of the LDL receptor activity in the human hepatoma cell line Hep G2 was studied. In Hep G2 cells, in contrast with fibroblasts, the LDL receptor activity was increased 2.5-fold upon increasing the concentration of normal whole serum in the culture medium from 20 to 100% by volume. Incubation of the Hep G2 cells with physiological concentrations of LDL (up to 700 micrograms/ml) instead of incubation under serum-free conditions resulted in a maximum 2-fold decrease in LDL receptor activity (10-fold decrease in fibroblasts). Incubation with physiological concentrations of HDL with a density of between 1.16 and 1.20 g/ml (heavy HDL) resulted in an approximately 7-fold increase in LDL receptor activity (1.5-fold increase in fibroblasts). This increased LDL receptor activity is due to an increase in the number of LDL receptors. Furthermore, simultaneous incubation of Hep G2 cells with LDL and heavy HDL (both 200 micrograms/ml) resulted in a 3-fold stimulation of the LDL receptor activity as compared with incubation in serum-free medium. 3-Hydroxy-3-methylglutaryl-CoA reductase activity was also stimulated after incubation of Hep G2 with heavy HDL (up to 3-fold). The increased LDL receptor activity in Hep G2 cells after incubation with heavy HDL was independent of the action of lecithin:cholesterol acyltransferase during that incubation. However, previous modification of heavy HDL by lecithin:cholesterol acyltransferase resulted in an enhanced ability of heavy HDL to stimulate the LDL receptor activity. Our results indicate that in Hep G2 cells the heavy HDL-mediated stimulation of the LDL receptor activity overrules the LDL-mediated down-regulation and raises the suggestion that in man the presence of heavy HDL and the action of lecithin:cholesterol acyltransferase in plasma may be of importance in receptor-mediated catabolism of LDL by the liver.