Lipoprotein synthesis and secretion by cultured rat hepatocytes. Parallel inhibition of secretion of VLDL, HDL and albumin by monensin

Effect of Monensin on Serum Lipoproteins, Triglycerides, Cholesterol and Total Lipids of Periparturient Dairy Cows

Serum concentrations of very low-density lipoproteins (VLDL), low-density lipoproteins (LDL), high-density lipoproteins (HDL), triglycerides, cholesterol, and total lipids of a group of monensin-treated cows (n = 7) were compared with those of a control group (n = 6) from about 10 days before calving to 45 days postpartum. Monensin was fed in the diet from about 20 days before predicted calving date to 45 days after parturition. Blood samples were obtained on days 20 and 10 before predicted calving, and on days 4, 15, 25 (phase I), and days 35 and 45 (phase II) post calving. Cholesterol (p = 0.046) and total lipids (p = 0.003) were lower in the treatment group 10 days before calving. Concentrations of serum triglycerides and VLDL (p = 0.017), and that of LDL (p = 0.032) were significantly higher in the treatment group in phase I postpartum. No significant difference was detected between groups during phase II. Comparison of the entire length of the trial showed higher concentrations of triglycerides and VLDL in the treatment group (p = 0.009). It is concluded that monensin is effective in increasing secretion of triglyceride-rich lipoproteins from the liver during the postpartum transition period of dairy cows. This may be effective in reducing the intensity of hepatic lipidosis and the incidence of periparturient diseases resulting from disturbances in energy metabolism.

Intracellular Lipidation of Newly Synthesized Apolipoprotein A-I in Primary Murine Hepatocytes

Hepatocytes, which are the main site of apolipoprotein (apo)A-I and ATP-binding cassette transporter A1 (ABCA1) expression, are also the main source of circulating high density lipoprotein. Here we have characterized the intracellular lipidation of newly synthesized apoA-I, in primary hepatocytes cultured with [3H]choline to label choline-phospholipids, low density lipoprotein-[3H]cholesterol to label the cell surface, or [3H]mevalonate to label de novo synthesized cholesterol. Phospholipidation of apoA-I is significant and most evident in endoplasmic reticulum (ER) and medial Golgi, both in the lumen and on the membrane fractions of the ER and medial Golgi. In the presence of cycloheximide, endogenous apoA-I is substantially phospholipidated intracellularly but acquires some additional lipid after export out of the cell. In cells labeled with low density lipoprotein-[3H]cholesterol, intracellular cholesterol lipidation of apoA-I is entirely absent, but the secreted apoA-I rapidly accumulates cholesterol after secretion from the cell in the media. On the other hand, de novo synthesized cholesterol can lipidate apoA-I intracellularly. We also showed the interaction between apoA-I and ABCA1 in ER and Golgi fractions. In hepatocytes lacking ABCA1, lipidation by low density lipoprotein-cholesterol was significantly reduced at the plasma membrane, phospholipidation and lipidation by de novo synthesized sterols were both reduced in Golgi compartments, whereas ER lipidation remained mostly unchanged. Therefore, the early lipidation in ER is ABCA1 independent, but in contrast, the lipidation of apoA-I in Golgi and at the plasma membrane requires ABCA1. Thus, we demonstrated that apoA-I phospholipidation starts early in the ER and is partially dependent on ABCA1, with the bulk of lipidation by phospholipids and cholesterol occurring in the Golgi and at the plasma membrane, respectively. Finally, we showed that the previously reported association of newly synthesized apoA-I and apoB (Zheng, H., Kiss, R. S., Franklin, V., Wang, M. D., Haidar, B., and Marcel, Y. L. (2005) J. Biol. Chem. 280, 21612-21621) occurs after secretion at the cell surface.

Effect of chylomicron remnants on cholesterol metabolism in cultured rabbit hepatocytes: very low density lipoprotein and bile acid production

The interrelationship between very low density lipoprotein (VLDL) secretion and bile acid production was studied in primary culture of rabbit hepatocytes. Chylomicron remnants (CR) were added to the cultures to study their effect on VLDL secretion and bile acid production. After 24 hr preincubation of cells with CR (10-50 micrograms protein/mL), intercellular neutral lipid content was increased 1.5-4-fold in a dose-dependent manner. Neutral lipid accumulation was accompanied by a 70-90% reduction of [14C]acetate incorporation into cholesterol, while no stimulation of [14C]oleate incorporation into cholesteryl esters was observed. Incubation of cells with CR increased secretion of free cholesterol, triacylglycerol and apoproteins B and E in VLDL. Stimulation of VLDL cholesterol secretion was accompanied by a reduction of taurocholic acid synthesis. These data demonstrate the existence of an inverse relationship between secretion of VLDL cholesterol and bile acid production under conditions of effective uptake of triacylglycerol-rich CR by hepatocytes.

Secretion of lipid-poor nascent human apolipoprotein apoAI, apoCIII, and apoE by cell clones expressing the corresponding genes

The human apolipoprotein apoAI, apoCIII, and apoE genes were placed under the control of the mouse metallothionein 1 promoter in a bovine papilloma virus vector that also contained the human metallothionein 1A gene. Following transfection of mouse C127 cells with the expression vector, cell clones resistant to Cd2+ were selected and found to express in high abundance specific apolipoprotein genes. Individual cell clones expressing apoAI, apoCIII, or apoE genes were used further to study the isoprotein composition and the flotation properties of the corresponding nascent apolipoproteins. It was found that the lipoproteins secreted by cell clones expressing the apoAI, apoCIII, and apoE genes consisted of the proapoAI disialylated form of apoCIII (apoCIIIS2) and mainly sialylated forms of apoE. Separation of the secreted apolipoproteins by density gradient ultracentrifugation resulted in limited flotation of nascent apoAI, apoE and apoCIII in the high density lipoprotein (HDL) fraction. Similar analysis in the presence of human serum increased the flotation of apoAI, apoE, and apoCIII to 6.5-, 4.5-, and 5.5-fold, respectively, and resulted in their redistribution to various lipoprotein fractions. HDL increased the flotation of apoAI to 12-fold and very low density lipoprotein (VLDL) increased the flotation of apoCIII and apoE to 6.5- and 5.5-fold, respectively. These findings suggest that in the cell system used, the majority of nascent apoAI, apoCIII and apoE is secreted in the lipid-poor form, which then associates extracellularly with preexisting lipoproteins.

Dual effect of metformin in cultured rat hepatocytes: potentiation of insulin action and prevention of insulin-induced resistance

The ability of the biguanide hypoglycemic agent metformin to improve the acute effects of insulin on glucose and/or lipid metabolism was investigated in both insulin-responsive and insulin-resistant cultured rat hepatocytes: (1) metformin (20 micrograms/mL, 16 hours) increased the insulin-dependent stimulation of glycogen and lipid synthesis through an exclusive enhancement of the responsiveness without modification of the cell sensitivity to the hormone; (2) metformin neither altered basal glycogenesis from [U-14C]glucose and basal lipogenesis from [1-14C]acetate nor insulin binding. These results indicate the ability of this drug to selectively potentiate the acute action of insulin at a postreceptor step in normal liver cells. A prolonged incubation with insulin (16 hours, 5 x 10(-7) mol/L) led the hepatocytes to a state of resistance evidenced by a 50% decrease in their maximal responsiveness and sensitivity to a subsequent acute stimulation by the hormone, as assessed on lipogenesis. Addition of metformin (20 micrograms/mL) during the overnight incubation of hepatocytes with insulin prevented the decrease in cell responsiveness and sensitivity to the hormone for the stimulation of lipogenesis, thus showing that metformin was able to hamper the development of the resistant state to the hormone in this pathway. These results strongly suggest that metformin improves type 2 diabetes through an effect at the hepatic level on both insulin action and insulin-induced resistance.

Sandwich enzyme-linked immunosorbent assay of rat apolipoprotein E

A sandwich ELISA for rat apo E was developed. Blood was drawn from rats that had been administered with triamcinolone diacetate to increase apo E-rich HDL in serum. Apo E was purified from the d less than 1.225 g/ml lipoproteins, and antiserum was raised in rabbits. Diluted samples and standards were added into the wells of polystyrene microtiter plates precoated with immunoaffinity-purified IgG and incubated for 90 min. After washing, immunoaffinity-purified Fab'-horseradish peroxidase conjugate was added to each well and incubated for 90 min. The bound enzyme was assayed by a colorimetric method. Samples and standards were pretreated with 6 M guanidine. HCl to maximize the antigenic response of apo E. The sensitivity lies around 1 pg apo E, and the working range was 0.1 to 1.0 ng. All assay procedures were completed within 4-5 hr. The mean intra- and interassay coefficients of variation were 1.8 and 4.1%, respectively. Serum apo E concentrations were 21.2 +/- 2.1 and 61.3 +/- 17.0 mg/dl (mean +/- SD) for young (8-12 weeks old, n = 9) and old (36-40 weeks old, n = 16) rats, respectively. As determined by gel filtrations, most of the apo E in fasted rat serum was associated with larger HDL particles (or HDL1) and a small portion of apo E was present in a free form.

The effects of xenobiotics on hepatic lipid and lipo-protein metabolism

The liver occupies a central position in lipid and lipo-protein metabolism. Its function includes lipid and lipoprotein biosynthesis, assembly, packaging, transport, secretion, uptake and degradation of lipoproteins. In addition, enzymes synthesized and secreted by the liver into the blood stream or remaining bound to the endothelial cells in the capillaries, affect lipoprotein metabolism in the circulation. Xenobiotics may influence each of these steps. The mechanisms include more specific actions such as hormone or transmitter agonism and antagonism, membrane effects (stabilization or changes in trans-membrane gradients), influence on protein synthesis, influence on lipid metabolism by induction or inhibition of involved enzymes, or more general actions such as disturbances or damage of cellular membranes and cellular function. Some of these effects can easily be described as pharmacological actions, more or less independent of specific requirements in the chemical structure of the xenobiotics. Others are linked to specific chemical substituents such as carboxyl or alcoholic hydroxyl groups allowing the formation of lipid-xenobiotic-conjugates and/or the channeling of xenobiotics into lipid metabolism. This review will give a short overview of the mechanisms of xenobiotic-influenced hepatic lipid and lipoprotein metabolism.

Expression and secretion of chicken apolipoprotein AI in transfected COS cells

A full-length chicken apolipoprotein A-I (apoAI) cDNA has been cloned into an expression vector, pRSVapoAI. This plasmid was transfected into a monkey kidney (COS-1) cell line in order to study apolipoprotein-lipid assembly. Chicken apoAI is the major apolipoprotein of chicken high-density lipoprotein (HDL), which is less complex in apolipoprotein content than the HDL of human plasma. The transient transfected COS-1 cells synthesized and secreted authentic plasma apoAI. Under serum-free medium conditions, COS cells secreted only proapoAI. A small portion (15%) of the secreted apoAI floated at a density 1.07-1.20 g/ml. Upon incubation with fetal bovine serum at 10 degrees C, a majority of the apoAI was recovered in the HDL density (1.06-1.20 g/ml) region. Secreted apoAI was labeled when transfected COS cells were incubated with [U-14C]palmitate, but the incorporation of radioactivity was not the result of fatty acid acylation through ester bond formation. These results indicate that heterologous COS-1 cells are capable of synthesizing and secreting apoAI, and that intracellular association of apoAI with lipids is not necessary for secretion.

Very low density lipoprotein secretion by cultured hepatocytes of rabbits fed purified or autoxidized cholesterol

The main objectives of this study were to compare the effects of dietary commercial cholesterol (containing 5% of oxidized cholesterol derivatives) and purified cholesterol on the secretion rate of very low density lipoprotein apolipoproteins and lipids by cultured rabbit hepatocytes and to verify the hypothesis that products of cholesterol autoxidation stimulate the rapid development of hypercholesterolemia. Rabbits fed dietary (old) commercial cholesterol for six weeks showed a fivefold increase in the serum concentration of cholesterol compared with that in purified cholesterol-fed rabbits. The secretion rates of very low density lipoprotein total protein and very low density lipoprotein [3H]apolipoproteins were similar for the hepatocytes of these two cholesterol-fed groups of animals and were two- and threefold greater, respectively, than for cells from control rabbits. Cholesteryl ester content of the hepatocytes from dietary (old) commercial cholesterol-fed rabbits was dramatically increased in comparison with hepatocytes from control and purified cholesterol-fed rabbits. The elevated intracellular cholesteryl ester content is assumed to account for such an increase of very low density lipoprotein-cholesteryl ester secretion by cells prepared from dietary (old) commercial cholesterol-fed rabbits. These effects appear to be caused by activation of cholesterol esterification by oxidized cholesterol derivatives. The rapid development of hypercholesterolemia induced by dietary (old) commercial cholesterol is associated, at least in part, with the stimulated production of hepatic very low density lipoprotein apolipoproteins and cholesteryl esters.

Serum albumin

The liver manufactures albumin at a massive rate and decreases production in times of environmental, nutritional, toxic and trauma stress. Osmotic pressure is a basic evolutionary regulatory factor, and hormonal control over albumin production has been demonstrated. Where and why new or old albumin is degraded are questions which have not been clarified, although the vascular endothelium may well be the degradative site. Albumin is important as a transport protein, as a measure of evolution and as a model to study secretion following synthesis without the intervening steps of glycosylation. Investigations as to how this protein enters the endoplasmic membrane may well answer some of the questions concerning signal peptide insertion (288). The role of the urea cycle intermediate ornithine and its participation in polyamine synthesis, which has a positive effect on albumin synthesis, is under study. Likewise, the inverse relation between acute-phase protein synthesis and albumin synthesis regulated by interleukin 1 and other cytokines will merit further study. These are a few of the concepts which will be tested in the future.

Effect of bezafibrate on lipoprotein secretion by cultured human hepatocytes

The secretion of newly synthesized very low density lipoprotein and low density lipoprotein (VLDL + LDL) and high density lipoprotein (HDL) in cultured human hepatocytes in the presence or absence of bezafibrate added to the culture medium has been evaluated. The content of triacylglycerol, apolipoprotein B, 3H-labeled proteins and 14C-labeled lipids accumulated in a culture medium increased linearly for periods up to 18 h. During incubation, cellular triacylglycerol content was unchanged. This hypolipidemic agent, at a concentration of 10 microM, inhibited secretion of the several VLDL + LDL [3H]apolipoproteins and the VLDL + LDL [14C]lipids, suggesting it can affect the processes of biosynthesis and secretion. The secretion of total [3H]apolipoproteins in the HDL fraction and [3H]protein (d greater than 1.21 g/ml) was unchanged after an exposure to bezafibrate (10 microM). Incubation in the presence of 10 microM bezafibrate resulted also in decreases in both total VLDL neutral lipids and apolipoprotein B secreted into the medium. These results indicate that bezafibrate might exert its plasma lipid-lowering effect by suppressing the VLDL production in liver and reducing the secretion of VLDL into the circulation.

Lipid-poor apolipoprotein A-I in Hep G2 cells: formation of lipid-rich particles by incubation with dimyristoylphosphatidylcholine

Apolipoprotein A-I is a major secretory product of the human hepatoma cell line, Hep G2; approx. 70% of apolipoprotein A-I was separated from the medium as lipid-poor apolipoprotein A-I in the d greater than 1.21 g/ml fraction while 30% was associated with high-density lipoproteins (HDL) of d 1.063-1.21 g/ml. The lipid-poor apolipoprotein A-I contains 50% proapolipoprotein A-I which is similar to the isoform distribution in Hep G2 preformed HDL. We tested the ability of lipid-poor apolipoprotein A-I from Hep G2 to form complexes with dimyristoylphosphatidylcholine (DMPC) vesicles at DMPC/apolipoprotein A-I molar ratios of 100:1 and 300:1. Lipid-poor apolipoprotein A-I was recovered in complex form while at a 300:1 ratio, 68.8 +/- 6.3% was recovered. On electron microscopy, the former complexes were small discs 16.9 nm +/- 4.5 S.D. in diameter while the latter were larger discs 21.4 +/- 4.4 nm diameter. Non-denaturing gradient gel electrophoresis of complexes formed at a 100:1 ratio had a peak in the region corresponding to 9.64 +/- 0.08 nm; these particles possessed two apolipoprotein A-I molecules. At the higher ratio, 300:1, two distinct complexes were identifiable, one which banded in the 9.7 nm region and the other in the 16.9-18.7 nm region. The former particles contained two molecules of apolipoprotein A-I and the latter, three molecules. This study demonstrates that lipid-poor apolipoprotein A-I which is rich in more basic isoforms forms discrete lipoprotein complexes similar to those formed by mature apolipoprotein A-I. It is further suggested that, under the appropriate conditions, precursor or nascent HDL may be assembled extracellularly.

Quantification of apolipoproteins in rat serum and in cultured rat hepatocytes by enzyme-linked immunosorbent assay

An enzyme-linked immunosorbent assay (ELISA) has been developed to measure apolipoproteins in rat serum. Nondelipidated whole serum was heat-treated at 52 degrees C for 3 h in phosphate-buffered saline containing 0.1% Tween-20 before assay. Monospecific rabbit anti-rat apolipoprotein antibodies were added to 96-well polystyrene microtiter plates which had been coated with purified rat serum apolipoproteins or unknown samples. After incubation and washing, goat anti-rabbit serum antibodies conjugated with horseradish peroxidase were added to the plates and incubated. The bound peroxidase activity was assayed after further washing. Serum apolipoprotein concentrations were calculated by comparison against purified standards that were assayed simultaneously with the unknown samples. The intraassay coefficients of variation for apolipoprotein AI, E, and AIV (Apo AI, E, and AIV) were 2.3, 4.4, and 5.3%, and interassay coefficients of variation were 6.1, 5.5, and 7.9%, respectively. The ELISA assay is sensitive to nanogram quantities of rat serum apolipoproteins and the results agree well with those measured by densitometry. The serum concentrations of Apo AI, E, and AIV of a normal fed rat were found to be 504 +/- 8, 413 +/- 20, and 262 +/- 20 micrograms/ml, respectively. When cultured as monolayers in Waymouth's medium for 1 day, rat hepatocytes secreted Apo AI, E, and AIV at rates of 2.51, 61.8, and 48.9 ng protein/mg cell protein/h.

Effect of monensin and nocodazole on the intestinal lipid esterification in mouse jejunal organ culture

The ability of mouse jejunal explants to esterify a lipid emulsion containing oleic acid, palmitic acid and monopalmitin has been studied in different in vitro experimental conditions. The incubating lipid solution must have a minimum volume for obtaining optimal triglyceride esterification by the cultured intestinal mucosa. In our incubating conditions the exchange of oleic for palmitic acid does not significantly modify the amount of lipids esterified by the explants in 15 min. Monensin or nocodazole, added to the culture medium of intestinal explants for 3 hr, significantly change the amount of lipids esterified and secreted. The inhibition observed after nocodazole treatment disappears, however, when the explants are rinsed and the culture is allowed to continue for an additional 3 hr in a drug-free medium. These results suggest that the regulation of lipid metabolism can be studied in organ culture.