Inhibition of apolipoprotein B net synthesis and secretion from cultured rat hepatocytes by the calcium-channel blocker diltiazem.

1. The effect of the Ca2+-channel blocker diltiazem on hepatic apolipoprotein B (apo B) synthesis and secretion was studied in 12-18 h cultures of collagenase-dispersed rat hepatocytes. 2. The presence of diltiazem in the medium decreased apo B secretion by hepatocytes in a concentration-dependent manner. At 25 microM, diltiazem inhibited apo B secretion by approx. 36%, but there was no evidence of intracellular accumulation of apo B. 3. The inhibition of apo B secretion by hepatocytes was significantly correlated with cell-associated diltiazem (r = 0.72, P less than 0.01). 4. The rate of apo B secretion remained linear over 16 h even in the presence of 50 microM-diltiazem. 5. At diltiazem concentrations in the medium which were inhibitory for apo B secretion, [14C]acetate incorporation into cellular lipids and [35S]methionine incorporation into protein were enhanced. 6. Diltiazem inhibited the secretion of the apo B variants with a preferential inhibition of the higher-molecular-mass form of apo B (apo BH) over the lower-molecular-mass form (apo BL) at diltiazem concentrations in the medium greater than 25 microM. 7. Together, these results suggest that Ca2+ may play an important role in the synthesis and secretion of apo B-containing lipoproteins.     

Effects of atorvastatin versus fenofibrate on apoB-100 and apoA-I kinetics in mixed hyperlipidemia

Kinetics of apo B and apo AI were assessed in 8 patients with mixed hyperlipidemia at baseline and after 8 weeks of atorvastatin 80 mg q.d. and micronised fenofibrate 200 mg q.d. in a cross-over study. Both increased hepatic production and decreased catabolism of VLDL accounted for elevated cholesterol and triglyceride concentrations at baseline. Atorvastatin significantly decreased triglyceride, total, VLDL and LDL cholesterol and apo B concentrations (-65%, -36%, -57%, -40% and -33%, respectively, P<0.05). Kinetic analysis revealed that atorvastatin stimulated the catabolism of apo B containing lipoproteins, enhanced the delipidation of VLDL1 and decreased VLDL1 production. Fenofibrate lowered triglycerides and VLDL cholesterol (-57% and -64%, respectively, P<0.05) due to enhanced delipidation of VLDL1 and VLDL2 and increased VLDL1 catabolism. Changes of HDL particle composition accounted for the increase of HDL cholesterol during atorvastatin and fenofibrate (18% and 23%, P<0.01). Only fenofibrate increased apo AI concentrations through enhanced apo AI synthesis (45%, P<0.05). We conclude that atorvastatin exerts additional beneficial effects on the metabolism of apo B containing lipoproteins unrelated to an increase in LDL receptor activity. Fenofibrate but not atorvastatin increases apo AI production and plasma turnover.     

Synthesis and secretion of lipoproteins by human hepatocytes in culture

Summary Confluent monolayers of normal human hepatocytes obtained by collagenase perfusion of liver pragments were incubated in a serum-free medium. Intracellular apolipoproteins apo AI, apo C, apo B, and apo E were detected between Day 1 and Day 6 of the culture by immunoenzymatic staining using polyclonal antibodies directed against these apoproteins and monoclonal antibodies directed against both forms of apo B (B100 and B48). Translation of mRNA isolated from these hepatocytes in an acellular system revealed that apo AI and apo E were synthesized as the precusor forms of mature plasma apo AI and apo E. Three lipoprotein fractions corresponding to the density of very low density lipoprotein (VLDL), low density lipoprotein (LDL), and high density lipoprotein (HDL) were isolated from the medium at Day 5 of culture and examined by electron microscopy after negative staining. VLDL and LDL particles are similar in size and shape to plasma lipoproteins; spherical HDL are larger than normal plasma particles isolated at the same density. Their protein represented 44, 19.5, and 36.5% respectively, of the total lipoprotein protein. The secretion rate of VLDL protein corresponded to that measured in primary cultures of rat hepatocytes. After incorporation of [³H]glycerol, more than 92% of the [³H]triglyceride secreted into the medium was recovered in the VLDL fraction. These results demonstrate that primary cultures of normal human hepatocytes are able to synthesize and secrete lipoproteins and thus could be a useful model to study lipoprotein metabolism in human liver.     

Niacin and cholesterol: role in cardiovascular disease (review)

Niacin has been widely used as a pharmacologic agent to regulate abnormalities in plasma lipid and lipoprotein metabolism and in the treatment of atherosclerotic cardiovascular disease. Although the use of niacin in the treatment of dyslipidemia has been reported as early as 1955, only recent studies have yielded an understanding about the cellular and molecular mechanism of action of niacin on lipid and lipoprotein metabolism. In brief, the beneficial effect of niacin to reduce triglycerides and apolipoprotein-B containing lipoproteins (e.g., VLDL and LDL) are mainly through: a) decreasing fatty acid mobilization from adipose tissue triglyceride stores, and b) inhibiting hepatocyte diacylglycerol acyltransferase and triglyceride synthesis leading to increased intracellular apo B degradation and subsequent decreased secretion of VLDL and LDL particles. The mechanism of action of niacin to raise HDL is by decreasing the fractional catabolic rate of HDL-apo AI without affecting the synthetic rates. Additionally, niacin selectively increases the plasma levels of Lp-AI (HDL subfraction without apo AII), a cardioprotective subfraction of HDL in patients with low HDL. Using human hepatocytes (Hep G2 cells) as an in vitro model system, recent studies indicate that niacin selectively inhibits the uptake/removal of HDL-apo AI (but not HDL-cholesterol ester) by hepatocytes, thereby increasing the capacity of retained HDL-apo AI to augment cholesterol efflux through reverse cholesterol transport pathway. The studies discussed in this review provide evidence to extend the role of niacin as a lipid-lowering drug beyond its role as a vitamin.     

Expression of the human serum apolipoprotein AI and AII genes in Xenopus laevis oocytes. Lipid-associated secretion of gene products

The two major apolipoproteins of plasma high-density lipoproteins (HDL) are apolipoprotein AI (apo AI) and AII (apo AII). The apo AI and the correctly oriented apo CIII genes separated by 2.6 kb were obtained by fusion of two human lambda-genomic clones. The apo AII gene was isolated as a 3 kb clone. These apolipoprotein genes have been injected independently and together into Xenopus laevis oocytes and their expression studied. Both apolipoprotein genes were transcribed and translated into their preproforms and processed in Xenopus laevis oocytes to their proforms. They were secreted into the medium associated with newly synthesized phospholipids and neutral lipids as particles floating in the high-density lipoprotein range between 1.12 and 1.21 g/ml. Secreted apo AI is associated mainly with newly synthesized phosphatidylethanolamine and little triglyceride, apo AII with phosphatidylethanolamine, lysophosphatidylethanolamine and neutral lipids. Simultaneous injection of the apo AI and apo AII genes led to the secretion of both apoproteins which separated into two bands during CsCl-density gradient centrifugation. The heavier particles were associated with proapo AI and AII, phosphatidylethanolamine (greater than 90%) and traces of lysophosphatidylethanolamine as lipid components. Proapo AII was immunoprecipitated from the less dense fraction and found to be mainly associated with lysophosphatidylethanolamine. Radiolabelled newly synthesized apolipoproteins in secreted particles were characterized by immunoprecipitation after delipidation of the secreted lipoprotein particles. The oocyte-system proved very suitable for studies of the expression of serum apolipoprotein genes, the assembly of the apolipoproteins with specific lipids to lipoprotein particles and their secretion.     

The effect of HMG-CoA reductase inhibitor (CS-514) on the synthesis and secretion of apolipoproteins B and A-1 in the human hepatoblastoma Hep G2

Human hepaotoblastoma cells (Hep G2) were cultured with a competitive inhibitor of HMG-CoA reductase, CS-514. The synthesis of cholesterol was markedly inhibited after 1 h preincubation with CS-514. The synthesis and secretion of apolipoprotein (apo) B and A-1, however, were not affected. A long-term incubation (21–24 h) of cells with CS-514 did not change apo B synthesis and secretion, although a slight depression of apo A-1 synthesis was observed. Hep G2 cells were found to secrete LDL- and HDL-like lipoproteins which were poor in cholesterol when cells were incubated with the drug. These results suggest that the modulation of cholesterol synthesis affects neither the synthesis and secretion of apo B and A-1 nor the formation of lipoproteins.     

Insulin effects on apolipoprotein B production by normal, diabetic and treated-diabetic rat liver and cultured rat hepatocytes.

1. The effect of insulin on apolipoprotein (apo B) secretion was studied in 24 h recirculating liver perfusions of isolated normal, diabetic and insulin-treated diabetic rats. In single perfusions from each group apo B accumulated in the media in a linear fashion. 2 In perfusions of normal rat livers, when the medium contained insulin plus cortisol, apo B production was significantly inhibited (by 35.8%), demonstrating a hormone effect on apo B secretion. 3. In perfusions of diabetic-rat livers, apo B production was decreased to 11.8% of normal when the medium contained no hormones, and was not significantly changed by the addition of insulin plus cortisol to the medium, suggesting that the hormone effect on apo B secretion is missing in long-term hypoinsulinaemic states. 4. Treatment of diabetic rats with daily insulin injection restored apo B production and restored the effect of insulin plus cortisol in the medium to inhibit apo B secretion during perfusion. 5. Parallel studies of apo B secretion with insulin alone, cortisol alone and insulin plus cortisol in the medium were performed in primary cultures of hepatocytes to compare results from liver perfusions. 6. Apo B secretion by hepatocytes from normal, diabetic and treated-diabetic rats was inhibited (by 36.8%, 57.1% and 57.9% respectively) when insulin alone was added to the medium. 7. Insulin plus cortisol inhibited apo B secretion by hepatocytes from normal and treated diabetic rats (by 30.2% and 47.2% respectively), but failed to inhibit apo B secretion by hepatocytes from diabetic rats.     

A targeted apolipoprotein B-38.9-producing mutation causes fatty livers in mice due to the reduced ability of apolipoprotein B-38.9 to transport triglycerides

Nonphysiological truncations of apolipoprotein (apo) B-100 cause familial hypobetalipoproteinemia (FHBL) in humans and mice. An elucidation of the mechanisms underlying the FHBL phenotypes may provide valuable information on the metabolism of apo B-containing lipoproteins and the structure-function relationship of apo B. To generate a faithful mouse model of human FHBL, a subtle mutation was introduced into the mouse apo B gene by targeting embryonic stem cells using homologous recombination followed by removal of the selection marker gene by Cre-loxP-mediated site-specific recombination. The engineered mice bear a premature stop codon at residue 1767 and a 42-base pair loxP inserted into intron 24 of the apo B gene, thus closely resembling the apo B-38.9-producing mutation in humans. Apo B-38.9 was the sole apo B protein in homozygote (apob(38.9/38.9)) plasma. In heterozygotes (apob(+/)(38. 9)), apo B-100 and apo B-48 were reduced by 75 and 40%, respectively, and apo B-38.9 represented 20% of total circulating apo B. Hepatic apo B-38.9 mRNA levels were reduced by 40%. In cultured apob(+/)(38. 9) hepatocytes, apo B-100 was produced in trace quantities, and the synthesis rate of apo B-38.9 relative to apo B-48 was reduced by 40%. However, almost equimolar amounts of apo B-38.9 and apo B-48 were secreted into the media. Pulse-chase studies revealed that apo B-38. 9 was secreted at a faster rate and more efficiently than apoB-48. Nevertheless, both apob(+/)(38.9) and apob(38.9/38.9) mice had reduced hepatic triglyceride secretion rates and fatty livers. Thus, low mRNA levels or defective secretion of apo B-38.9 may not be responsible for the FHBL phenotypes caused by the apo B-38.9 mutation. Rather, a reduced capacity of apo B-38.9 for triglyceride transport may account for the fatty livers in these mice.     

Apo B metabolism in the cynomolgus monkey: evidence for post-transcriptional regulation.

Previous studies have shown that hepatic apo B mRNA levels do not increase in animals fed high cholesterol diets, even though plasma apo B concentrations increase markedly. As a result, it has been suggested that the diet-induced increase in plasma apo B levels was due solely to an inhibited clearance of those lipoproteins. The present study was undertaken to test that hypothesis. Hepatic apo B mRNA levels were measured in liver biopsies taken from five male cynomolgus monkeys before and twice after, they began to consume a high cholesterol diet. The diet had no effect on hepatic apo B mRNA levels, even though it caused a 7-fold increase in the plasma apo B levels. However, measurements of the apo B secretion rate in eight separate monkeys (four chow-fed and four cholesterol-fed) by isotope dilution showed that apo B secretion by the liver was increased 4-fold in the cholesterol-fed monkeys. These data, taken together, indicate that apo B secretion is not regulated by the rate at which the apo B gene is transcribed, but at some point further along in the secretion pathway.     

Regulation of apoB secretion from HepG2 cells: evidence for a critical role for cholesteryl ester synthesis in the response to a fatty acid challenge

Secretion of hepatic apoB lipoproteins removes excess triglyceride from the liver. However, the mechanism by which synthesis of apoB, which occurs on the rough endoplasmic reticulum, is coordinated with synthesis of triglyceride, which takes place in the smooth endoplasmic reticulum, is not known. To examine this question, we have manipulated intracellular synthesis of triglyceride and cholesteryl ester in HepG2 cells and determined the impact of these maneuvers on apoB secretion. Since cholesteryl ester is the only major lipid class synthesized in the rough endoplasmic reticulum, our hypothesis was that, in response to a fatty acid challenge, synthesis of cholesteryl ester rather than synthesis of triglyceride would be the immediate trigger to apoB secretion. Oleate complexed to bovine serum albumin caused intracellular triglyceride synthesis to increase 6-fold and cholesteryl ester synthesis to increase almost 3-fold, while apoB secretion into the medium increased by 2.5-fold (P less than 0.0125) at all time points between 4 and 24 h. Addition of acylation stimulating protein to the medium further stimulated both triglyceride and cholesteryl ester synthesis (58% and 108%, respectively) above oleate alone and this resulted in a 50% increase in apoB secretion (P less than 0.0025). By contrast, both progesterone and 2-bromooctanoate inhibited triglyceride and cholesteryl ester synthesis and these effects were associated with reduced apoB secretion. Lovastatin inhibited cholesteryl ester synthesis (45%, P less than 0.0025); however, at the doses used, triglyceride formation was unaffected. Under these circumstances, apoB secretion was reduced by 25% (P less than 0.05). Similarly, 58-035 (an inhibitor of acyl CoA:cholesterol acyltransferase) on the one hand reduced cholesteryl ester synthesis markedly (59%, P less than 0.005), but on the other increased triglyceride synthesis though not statistically significantly (65%, P NS), and again this resulted in decreased apoB secretion (25%, P less than 0.005). Control experiments established that changes in low density lipoprotein catabolism did not contribute importantly to the quantity of apoB in the medium. Taken together, the data indicate that, at least in HepG2 cells, there are parallel changes in cholesteryl ester synthesis and apoB secretion and suggest that it is cholesteryl ester synthesis, not triglyceride synthesis, that is the immediate regulator of apoB secretion when these cells are exposed to an increased influx of fatty acids. However, alternative or additional regulatory mechanisms, such as, for example, a role for acylation of apoB, are not excluded by these studies.     

Lipoprotein secretion by the human hepatoma cell line Hep G2: differential rates of accumulation of apolipoprotein B and lipoprotein lipids in tissue culture media in response to albumin, glucose and oleate

Serum low-density lipoprotein (LDL) concentration is a major determinant of susceptibility to the development of atherosclerosis. A major component of the protein moiety of LDL and its precursor very-low-density lipoprotein is apolipoprotein B (apo B). The human hepatoma cell line, Hep G2, was used as a model for the investigation of mechanisms which control hepatic secretion of the apo B and lipid components of lipoproteins. Using a sensitive immunoradiometric assay for apo B developed in this laboratory, we showed that bovine serum albumin inhibited and glucose, and fatty acids enhanced the rate of accumulation of apo B in the culture medium of Hep G2 cells. However, these substances did not necessarily affect LDL lipids in the same way as apo B. This finding appeared to be due to Hep G2 cells expressing lipase activities which led to triacylglycerol and phospholipid hydrolysis and lipid reuptake. Reuptake of apo B also occurred, but its rate of accumulation in the culture medium suggested it was a closer reflection of its true secretory rate.     

Polymorphisms in the apolipoprotein (apo) AI-CIII-AIV gene cluster: detection of genetic variation determining plasma apo AI,apo CIII and apo AIV concentrations

We have examined the associations between levels of plasma apolipoprotein (apo) AI, apo CIII and apo AIV and genetic variation in the apo AI-CIII-AIV gene cluster in 162 boys and young men from Belgium aged from 7 to 23 years. Genotypes were determined for six restriction enzymes XmnI, PstI, SstI, PvuIIA-CIII, PvuIIB-AIV and XbaI, and for the G to A substitution at -75 bp in the 5' region of the apo AI gene. The polymorphism most strongly associated with apo AI levels was the G to A substitution (P = 0.025, R2 x 100 = 3.6%) confirming previous observations. The polymorphism most strongly associated with apo CIII levels was that of PvuIIA-CIII (P = 0.023, R2 x 100 = 2.9%) in the apo CIII gene. This novel association must be interpreted with caution until it has been confirmed in an independent sample. The polymorphism associated with the largest effect on apo AIV levels was that detected with XbaI in the apo AIV gene, but this association was not statistically significant. Previously reported associations between the SstI polymorphism and triglyceride levels, and between the PstI polymorphism and apo AI levels, were weakly detected in the present sample. Our results show that variation associated with some of the polymorphisms in the apo AI-CIII-AIV cluster makes a small, but statistically significant, contribution to the determination of apo AI and apo CIII levels in this sample of young men and boys. These observations may, in part, explain reported associations between polymorphisms in this gene cluster, differences in plasma lipid and lipoprotein levels, and prevalence of coronary artery disease.     

Effects of hyperthyroidism on synthesis, secretion and metabolism of the VLDL apoproteins by the perfused rat liver

Livers from fed male Sprague-Dawley rats, made hyperthyroid by treatment with triiodothyronine (T3), were isolated and perfused in vitro. T3 (9.6 micrograms/day) was administered by osmotic minipump implanted intraperitoneally. Treatment with T3 for either 7 or 28 days reduced hepatic output of very-low-density lipoprotein (VLDL) and net synthesis of total associated apoproteins. After 7 days treatment, incorporation of [4,5-3H]leucine by livers from hyperthyroid rats into VLDL apo E was reduced while incorporation into apo B100, apo B48, and apo C's did not differ from euthyroid controls. The depressed incorporation of radioactivity into total VLDL protein was accounted for almost entirely on the basis of apo E. Incorporation of leucine into the total lipoprotein apo E isolated in the d less than 1.210 was also diminished by the hyperthyroid state, while that into apo B100, apo B48, and apo C in the total perfusate lipoprotein was similar to that of the euthyroid, as was found for the VLDL. Increased amounts of radioactive apo B100 and apo B48, however, were detected in the HDL fraction isolated from the medium perfusing livers from hyperthyroid rats. Hepatic uptake of VLDL protein and lipid was similar in euthyroid and hyperthyroid rats. Reduction of VLDL lipid and protein in the medium perfusing livers from T3-treated rats, therefore reflects hormonal action on synthesis and secretion, rather than uptake. Since the availability of apo B is thought to be required for secretion of VLDL, our observation suggests that synthesis of apo B is not depressed by treatment with T3 and that apoprotein synthesis is not a significant factor in the decreased output of VLDL by the liver, but that, as reported earlier, the lower output is a consequence of decreased synthesis of TG, the result of a diminished supply of hepatic glycero-3-phosphate in the hyperthyroid. The diminished amount of VLDL protein appears to be accounted for by the decreased quantity of apo E associated with a smaller VLDL particle secreted by livers from T3-treated rats.     

B48 is preferentially translated over B100 in cells with increased endogenous apo B mRNA

We recently demonstrated that expression of BHMT in McArdle RH-7777 (McA-BHMT) cells increases apo B mRNA abundance, leading to parallel increases in apo B secretion. The ratio of unedited to edited apo B mRNA was unchanged by BHMT expression. Based on the observation that secretion of B48 is increased relative to B100 in McA-BHMT cells, current studies now include comparison of B48 and B100 synthesis and intracellular degradation. Minor differences in co- and posttranslational degradation were unable to account for relative increase in B48 secretion, and the disappearance kinetics of B48 were similar in McA-BHMT and control cells. Consistent with the increase in endogenous apo B mRNA in McA-BHMT cells, B48 synthesis is increased significantly. In contrast, synthesis of B100 was not significantly increased. We conclude that B48 is preferentially translated compared to B100 when endogenous apo B mRNA is increased.     

Acute suppression of apo B secretion by insulin occurs independently of MTP

Secretion of apolipoprotein (apo) B-containing lipoproteins by the liver depends mainly upon apo B availability and microsomal triglyceride transfer protein (MTP) activity and is subject to insulin regulation. Hepatic MTP mRNA expression is negatively regulated by insulin which correlates with inhibition of apo B secretion suggesting that insulin might suppress apo B secretion through an MTP-dependent mechanism. To investigate this possibility, we examined the acute effect of insulin on hepatic MTP expression and activity levels in vivo utilizing apobec-1^−/− mice. Insulin did not significantly alter hepatic MTP mRNA levels or lipid transfer activity 2 h following injection, but suppressed expression of genes important in gluconeogenesis. To study the specific role of MTP, we expressed human MTP (hMTP) in primary rat hepatocytes using adenoviral gene transfer. Increased expression of hMTP resulted in a 47.6 ± 17.9% increase in total apo B secreted. Incubation of hepatocytes with insulin suppressed apo B secretion by 50.1 ± 10.8% in cells over-expressing hMTP and by 53.0 ± 12.4% in control transfected hepatocytes. Results indicate that even under conditions of increased hepatic apo B secretion mediated by MTP, responsiveness of hepatocytes to insulin to suppress apo B secretion is maintained.     

Variation at the apolipoprotein (apo) AI-CIII-AIV gene cluster and apo B gene loci is associated with lipoprotein and apolipoprotein levels in Italian children.

We have used RFLPs of the apolipoprotein (apo) B gene and apo AI-CIII-AIV gene cluster to estimate the genetic contribution of variation at these loci to the variability of plasmid lipid, lipoprotein, and apolipoprotein levels in 209 children from Sezze in central Italy. The sample was randomly divided into group I (107 children) and group II (102 children). Four site polymorphisms (PvuII, XbaI, MspI, and EcoRI) of the apo B gene and five site polymorphisms (XmnI, PstI, SstI, PvuII-CIII, and PvuII-AIV) of the apo AI-CIII-AIV gene cluster were examined in group I children. After adjustment for gender, age, and body-mass index, polymorphisms at both gene loci (PvuII-B, PvuII-CIII, and PvuII-AIV) were associated with significant effects on the levels of plasma apo AI, apo B, or high-density lipoprotein-cholesterol. RFLPs that showed significant effects in group I were genotyped in group II. All three polymorphisms were associated with similar effects on apolipoprotein levels, though for all RFLPs the magnitude of the effects was smaller in the group II children and only statistically significant for the effect of the PvuII-B genotype on apo AI levels. In the total sample of 209 children 7.4% of the sample variance in apo AI levels was explained by variation associated with the apo B PvuII-B RFLP. In addition, the PvuII-B RFLP was associated with significant effects on plasma apo B levels and explained 5.7% of the sample variance. The PvuII-CIII and PvuII-AIV polymorphisms were both associated with differences in apo AI levels, explaining 3.7%-5.7% of the sample variance. Taken together, the three PvuII polymorphisms explained 17.7% of the phenotypic variance in apo AI levels. There was significant evidence for an effect of nonlinearity of the PvuII-CIII genotypes on apo AI levels, with the individuals heterozygous for the polymorphism having the highest apo AI levels. No evidence of interaction between genotype and gender, age, and body-mass index was shown by covariance analysis. The molecular explanation of this effect is unclear. Our data show that variation at both the apo AI-CIII-AIV and apo B loci are associated with lipoprotein and apolipoprotein levels in this sample of Italian children.     

Apolipoprotein B production and very low density lipoprotein secretion by calf liver slices.

Secretion of triglycerides by the liver in ruminants as components of very low density lipoproteins particles is low as compared with that in primates or rodents. The rate-limiting steps for the hepatic export of very low density lipoproteins have been studied in liver slices to determine the origin of the low lipotropic capacity of calf liver compared to that of rat liver. The rates of production of apolipoprotein B (apo B) and albumin as well as the rate of secretion of VLDL-apolipoproteins were measured during 12-h incubation of liver slices in organo-culture using [35S]methionine-cysteine labeling. Hepatic apo B production was similar in the two animal species but the VLDL-apolipoprotein secretion rate for calf liver slices amounted to only 20% of that observed for rat liver slices. Although calf and rat liver slices synthesized similar amounts of total protein, the hepatic production of albumin, measured in cells and media, was much higher in calf than rat liver slices (around 2.7-fold), whereas the rate secretion of albumin was similar in the two species. Our results showed that the slow rate of secretion of VLDL by calf liver cells was not consecutive to a low rate of synthesis of apo B but rather to a defect in VLDL assembly and/or secretion.