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

Modelos celulares hepáticos para el estudio del metabolismo de los lípidos. Revisión de literatura

Introducción. El hígado juega un papel importante en la homeostasis lipídica, especialmente en la síntesis de ácidos grasos y triglicéridos. Una amplia variedad de modelos celulares ha sido utilizada para investigar el metabolismo lipídico hepático y para elucidar detalles específicos de los mecanismos bioquímicos del desarrollo y progresión de enfermedades relacionadas, brindando información para tratamientos que reduzcan su impacto. Los modelos celulares hepáticos poseen un alto potencial en la investigación del metabolismo de lípidos y de agentes farmacológicos o principios activos que permiten la reducción de la acumulación de lípidos.Objetivo. Comparar algunos modelos celulares hepáticos utilizados para el estudio del metabolismo lipídico, sus características y los resultados más relevantes de investigación en ellos.Materiales y métodos. Se realizó una búsqueda sistemática en bases de datos sobre los modelos celulares hepáticos de mayor uso para el estudio del metabolismo de lípidos.Resultados. Se exponen los cinco modelos celulares más utilizados para este tipo de investigaciones, destacando su origen, aplicación, ventajas y desventajas al momento de estimular el metabolismo lipídico.Conclusión. Para seleccionar el modelo celular, el investigador debe tener en cuenta cuáles son los requerimientos y el proceso que desea evidenciar, sin olvidar que los resultados obtenidos solo serán aproximaciones de lo que en realidad podría suceder a nivel del hígado como órgano.

Plasma phospholipid transfer protein fused with green fluorescent protein is secreted by HepG2 cells and displays phosphatidylcholine transfer activity

Phospholipid transfer protein (PLTP) is a serum glycoprotein with a central role in high-density lipoprotein metabolism. We created a fusion protein in which enhanced green fluorescent protein (EGFP) was fused to the carboxyl-terminus of PLTP. Stably transfected HepG2 cells, which overexpress this fusion protein, were generated. PLTP-EGFP was translocated into the ER and fluoresced within the biosynthetic pathway, showing a marked concentration in the Golgi complex. The transfected cells secreted into the growth medium phospholipid transfer activity 7-fold higher than that of the mock-transfected controls. The medium of the PLTP-EGFP - expressing cells displayed EGFP fluorescence, demonstrating that both the PLTP and the EGFP moieties had attained a biologically active conformation. However, the specific activity of PLTP-EGFP in the medium was markedly reduced as compared with that of endogenous PLTP. This suggests that the EGFP attached to the carboxyl-terminal tail of PLTP interferes with the interaction of PLTP with its substrates or with the lipid transfer process itself. Fluorescently tagged PLTP is a useful tool for elucidating the intracellular functions of PLTP and the interaction of exogenously added PLTP with cells, and will provide a means of monitoring the distribution of exogenously added PLTP between serum lipoprotein subspecies.

PLTP secreted by HepG2 cells resembles the high-activity PLTP form in human plasma

Plasma phospholipid transfer protein (PLTP) is an important regulator of plasma HDL levels and HDL particle distribution. PLTP is present in plasma in two forms, one with high and the other with low phospholipid transfer activity. We have used the human hepatoma cell line, HepG2, as a model to study PLTP secreted from hepatic cells. PLTP activity was secreted by the cells into serum-free culture medium as a function of time. However, modification of a previously established ELISA assay to include a denaturing sample pretreatment with the anionic detergent sodium dodecyl sulphate was required for the detection of the secreted PLTP protein. The HepG2 PLTP could be enriched by Heparin-Sepharose affinity chromatography and eluted in size-exclusion chromatography at a position corresponding to the size of 160 kDa. PLTP coeluted with apolipoprotein E (apoE) but not with apoB-100 or apoA-I. A portion of PLTP was retained by an anti-apoE immunoaffinity column together with apoE, suggesting an interaction between these two proteins. Furthermore, antibodies against apoE but not those against apoB-100 or apoA-I were capable of inhibiting PLTP activity. These results show that the HepG2-derived PLTP resembles in several aspects the high-activity form of PLTP found in human plasma.

Clinical, biochemical, and immunohistochemical features of necrobiotic xanthogranulomatosis

AIMS

To describe the clinical features of two patients with paraproteinaemia and necrobiotic xanthogranulomatosis together with detailed immunohistochemistry of the lesions in one.

METHODS

The clinical history and results of biochemical investigations of the patients were retrieved from the files. Immunohistochemistry was used to investigate the expression of macrophage and mast cell markers, amyloid A and P, S-100 protein, and apolipoprotein AI and B in xanthogranulomatous skin lesions from patient 2. In addition, protein A-sepharose chromatography was used to separate serum from patient 2 and apolipoprotein B and the IgG paraprotein were measured in the fractions eluted.

RESULTS

Monocytes/macrophages comprised the major cellular component of the lesion, and unusually for xanthomata, areas of collagen necrosis were also seen. Activated mast cells were present at the margins of macrophage clusters and adjacent to areas of collagen necrosis. Serum paraprotein was bound to low density lipoproteins as judged by protein A-sepharose chromatography, and was also located within macrophagic foam cells of the lesion on immunohistochemistry.

CONCLUSIONS

These observations demonstrate many features similar to atherosclerosis including collagen necrosis and mast cell activation.

The effects of fatty acids on apolipoprotein B secretion by human hepatoma cells (HEP G2)

We have investigated the effect of fatty acids on the rate of apolipoprotein B (apo B) secretion by human hepatoma cells (Hep G2). When Hep G2 cells were maintained in tissue culture flasks oleic acid up to 0.4 mM increased apo B secretion in a dose-dependent manner, whereas increases in triacylglycerol (TG) were smaller and dose dependency was less evident. In the absence of oleic acid, apo B accumulating in the tissue culture medium was predominantly in lipoproteins of higher density than very low density lipoproteins (VLDL). However, when the rate of secretion was stimulated with oleic acid the apo B-containing lipoproteins became lower in density. We postulated that there was a high rate of lipolysis of newly secreted VLDL by Hep G2 cells, which would account both for the relatively smaller effect of oleic acid on TG as opposed to apo B accumulating in the culture medium and the predominance of apo B in lipoproteins of a higher density than VLDL, which became less evident when VLDL secretory rates were stimulated by oleic acid. To test this hypothesis, cultured Hep G2 cells were transferred to columns containing Cytodex beads, permitting their continuous perfusion with culture medium so that newly secreted VLDL did not remain in contact with the cells. Apo B recovered from the perfusate was largely in VLDL range lipoproteins and the TG measured in the perfusate indicated that the true secretory rate of TG-rich lipoproteins was substantially higher than had been reflected by TG accumulating in culture medium left in contact with cells. Apo B measured in the culture medium of Hep G2 cells may thus be a better reflection of VLDL secretion, even though it is contained in higher density lipoproteins due to removal of TG by lipolysis. The effects of saturated fatty acids (SFA), monounsaturated fatty acids (MUFA) and polyunsaturated fatty acids (PUFA) on apo B (apo B) secretion by Hep G2 cells maintained in tissue culture flasks were next investigated. SFA (0.4 mM), with the exception of stearic acid (C18:0), increased apo B secretion. Lauric acid (C12:0) increased apo B secretion by 32%, myristic acid (C14:0) by 41% (P<0.005), palmitic acid (C16:0) by 154% (P<0.025), and arachidic acid (C20:0) by 186% (P<0.005). The effect of MUFA (0.4 mM) was to increase apo B secretion, oleic acid (C18:1) by 239% ((P<0.0005) and palmitoleic acid (C16: 1) by 125% (P<0.005). Of the PUFA investigated, linolenic acid (C18:3) (0.4 mM) did not have any significant effect on apo B secretion, whereas linoleic acid (C18:2) (0.4mM) arachidonic acid (C20:4) (0.1 mM) and eicosapentaenoic acid (C20:5) (0.1 mM) caused significant increases of 164, 171 and 171%, respectively (P<0.005). The fatty acids studied increased intracellular TG and cholesteryl ester concentrations to varying extents. The increase in intracellular TG produced by the different fatty acids correlated with the rate of apo B secretion (r=0.6; P<0.05). In this human hepatoma cell line, with the exception of the saturated fatty acids, the rate of secretion of apo B-containing lipoproteins does not follow the same pattern as changes in circulating low density lipoprotein (LDL) concentrations reported with dietary manipulation in man. If our findings reflect the in vivo situation, we suggest that whilst the dietary effects of SFA on serum LDL may in part be determined by the hepatic apo B secretory rate, the effects of MUFA and PUFA must be largely mediated through a catabolic effect rather than an effect on hepatic secretion. The marked increase in apo B secretion with the more highly polyunsaturated fatty acids, such as eicosapentaenoic acid, may also explain why they do not lower circulating LDL, despite reports of their apparently favourable effect on LDL-receptor mediated clearance.

Glucose phosphorylation is essential for the turnover of neutral lipid and the second stage assembly of triacylglycerol-rich ApoB-containing lipoproteins in primary hepatocyte cultures

Primary hepatocytes cultured in a medium supplemented with amino acids and lipogenic substrates responded to increased extracellular glucose by increasing the secretion of VLDL apoB. This effect was accompanied by an increased secretion of VLDL triacylglycerol (TAG) derived from endogenous stores. Glucose also stimulated intracellular TAG mobilization via the TAG lipolysis/esterification cycle. All these effects were abolished in the presence of mannoheptulose (MH), an inhibitor of glucose phosphorylation. Glucose also gave rise to a modest (50% to 60%) increase in the incorporation of 35S methionine into newly synthesized apoB (P<0.05) and to a doubling of newly-synthesized apoB secretion as VLDL (P<0. 05). The magnitude of these effects was similar for apoB-48 and for apoB-100. MH inhibited apoB-48 and apoB-100 synthesis and VLDL secretion at all glucose concentrations. The effects of glucose and MH on the secretion of newly-synthesized apoB-48 or apoB-100 as small dense particles were less pronounced. Glucose had no effects on the posttranslational degradation of newly-synthesized apoB-100 or apoB-48. However, this process was significantly enhanced by MH. The results suggest that glucose stimulates TAG synthesis, turnover, and output as VLDL. These effects are associated with an increased VLDL output of apoB mediated mainly by an increase in the net synthesis of both apoB-48 and apoB-100. All these changes are prevented by interference with glucose phosphorylation. Output of small, dense, apoB-containing particles is relatively unaffected by the glucose and MH-induced changes in TAG synthesis and lipolysis, an observation which suggests that only the bulk lipid addition step of VLDL assembly is affected by changes in glucose metabolism.

Lysophosphatidylcholine increases apolipoprotein B secretion by enhancing lipid synthesis and decreasing its intracellular degradation in HepG2 cells

Free fatty acids and lysophosphatidylcholine (lysoPC) are the major lipids bound to human plasma albumin. The effects of fatty acids on the hepatic production of Apolipoprotein B (apo B) have been studied but those of lysoPC have not. In HepG2 cells, lysoPC increased apo B secretion in different experiments by 50-120%, but did not affect the flotation properties of secreted lipoproteins. LysoPC affected neither the cellular protein levels nor apo A-I secretion suggesting that its effect was specific to apo B. Apo B secretion was maximum after incubating cells for 6 h with 0.2 mM lysoPC as equimolar fatty acid free bovine serum albumin (BSA) complexes. LysoPC was metabolized by cells and its fatty acids were used for the synthesis of phosphatidylcholine and triglycerides (TG). Experiments were performed to understand the mechanism of lysoPC action. LysoPC increased the incorporation of 3H-glycerol into newly synthesized cellular (3-fold) and secreted (4-fold) triglycerides, and increased the synthesis (40%) and secretion (4-fold) of phospholipids. LysoPC did not affect apo B synthesis, but inhibited the intracellular degradation of apo B and increased its secretion. Triacsin C (5 microM), an inhibitor of long chain acyl-CoA synthase, completely inhibited the induction of lipid synthesis and abolished the effect of lysoPC on apo B secretion. These studies indicated that lysoPC increased apo B secretion by inducing lipid synthesis; newly synthesized lipids probably protected apo B from intracellular degradation and enhanced secretion. These studies are consistent with the hypothesis that physiologic concentrations of lysoPC can be an important modulator for hepatic apo B secretion.

Use of cyclodextrin to deliver lipids and to modulate apolipoprotein B-100 production in HepG2 cells

2-Hydroxypropyl-beta-cyclodextrin (cyclodextrin), cyclodextrin-solubilized oleate, and cyclodextrin-solubilized cholesterol were used to modulate proteolysis and secretion of newly-synthesized apolipoprotein B-100 (apoB) in HepG2 cells. Following cyclodextrin and lipid treatments, cells were pulse-labeled with [3H] leucine, and quantitative immunoprecipitation was used to measure apoB synthesis, apoB secreted into the medium, and the cellular content of undegraded apoB that was not secreted. Three-hour treatment with cyclodextrin-solubilized oleate (0.2 mM) increased secreted apoB from 4% (control cells) to 32% and cellular undegraded apoB from 15% (control cells to 64% of apoB synthesized, which is consistent with earlier studies using bovine serum albumin to complex exogenous oleate. Prolonged daily (4 d or more) administration of 0.5% (3.5 mM) cyclodextrin with medium containing 10% fetal bovine serum increased the secretion of nascent apoB from 5-10% (control) to 17-28% and cellular undegraded apoB from 15-20% (control) to 25-31% of apoB synthesized, respectively. Subsequent administration of cyclodextrin solubilized cholesterol (10-40 micrograms) for only 3 h reversed the cyclodextrin-mediated increase in apoB secretion. The application of 0.5% cyclodextrin to HepG2 cells can rapidly (within minutes) stimulate cholesterol efflux, and transiently (over a 1-2 d period) increase cholesterol synthesis. In the current studies, the cyclodextrin-mediated increase in cholesterol synthesis was not concurrent with the increase in apoB secretion. However, prolonged (15 d) administration of cyclodextrin was shown to increase the cellular free cholesterol concentration by 25-41%, reduce the cellular triglyceride concentration by 59%, and increase apoB secretion 3- to 4-fold, without affecting the cellular cholesteryl ester concentration. In comparison, 14-d treatment with cyclodextrin-solubilized cholesterol (20 micrograms/mL) followed by 1-d equilibration without cholesterol was shown to increase the cellular free cholesterol and cholesteryl ester concentrations by 76% and 10-fold, respectively, although apoB secretion was not affected. It is hypothesized that chronic daily administration of 0.5% cyclodextrin increased the cellular cholesterol concentration and flux in discrete putative regulatory compartments, which "shielded" nascent apoB from rapid proteolysis and facilitated apoB secretion. In conclusion, cyclodextrin was used independently and in combination with cholesterol or oleate to modulate apoB proteolysis and secretion. We speculate that subcellular changes in cholesterol concentration and flux may modulate apoB production in HepG2 cells.

Increased immunolocalization of paraoxonase, clusterin, and apolipoprotein A-I in the human artery wall with the progression of atherosclerosis

Using immunolocalization techniques, we have shown that paraoxonase (Pon), clusterin, and apolipoprotein (apo) A-I accumulate in the artery wall during the development of atherosclerosis. In normal aortas (n = 6) there were low levels of extracellular Pon, clusterin, and apoA-I, immunoreactivity. The cytoplasm of smooth muscle cells in the media showed granular positivity for both Pon and apoA-I, indicating that these proteins were undergoing lysosomal degradation. This activity was also indicated by the presence of both intact and degradation products of Pon in smooth muscle cells as shown by Western blotting. With the progression of disease from fatty streaks (n = 3) to advanced atherosclerosis (n = 8) there was an increase in Pon, apoA-I, and clusterin immunoreactivity, indicating the increasing presence of these proteins with disease progression. These proteins are the components of a specific HDL subspecies that has been implicated in the prevention of peroxidative damage to phospholipids in LDL and membranes. The increase in Pon, clusterin, and apoA-I during the development of atherosclerosis may therefore represent a protective response to the oxidative stress associated with the development of atherosclerosis.

ApoB-100 secretion by HepG2 cells is regulated by the rate of triglyceride biosynthesis but not by intracellular lipid pools

Triglycerides (TGs), cholesteryl esters (CEs), cholesterol, and phosphatidylcholine have been independently proposed as playing regulatory roles in apoB-100 secretion; the results depended on the cellular model used. In this study, we reinvestigate the role of lipids in apoB-100 production in HepG2 cells and in particular, we clarify the respective roles of intracellular mass and the biosynthesis of lipids in the regulation of apoB-100 production. In a first set of experiments, the pool size of cholesterol, CEs, and TGs was modulated by a 3-day treatment with either lipid precursors or inhibitors of enzymes involved in lipid synthesis. We used simvastatin (a hydroxymethylglutaryl coenzyme A reductase inhibitor), 58-035 (an acyl coenzyme A cholesterol acyltransferase inhibitor), 5-tetradecyloxy-2-furancarboxylic acid (TOFA, an inhibitor of fatty acid synthesis), and oleic acid. The secretion rate of apoB-100 was not affected by the large modulation of lipid mass induced by these various pre-treatments. In a second set of experiments, the same lipid modulators were added during a 4-hour labeling period. Simvastatin and 58-035 inhibited cholesterol and CE synthesis without affecting apoB-100 secretion. By contrast, treatment of HepG2 cells with TOFA resulted in the inhibition of TG synthesis and apoB-100 secretion. This effect was highly specific for apoB-100 and was reversed by adding oleic acid, which stimulated both TG synthesis and apoB-100 secretion. Moreover, a combination of oleic acid and 58-035 inhibited CE biosynthesis and increased both TG synthesis and apoB-100 secretion. These results show that in HepG2 cells TG biosynthesis regulates apoB-100 secretion, whereas the rate of cholesterol or CE biosynthesis has no effect.

Intracellular apoprotein B degradation is suppressed by decreased albumin concentration in Hep G2 cells

It is generally accepted that hepatic secretion of apoprotein (apo) B-containing lipoproteins is substantially increased in nephrosis. To elucidate the mechanisms for the oversecretion of apo B, we investigated the effect of a various concentration of albumin on apo B kinetics in the absence or presence of oleate in Hep G2 cells. Hep G2 cells were labeled with [3H]-leucine in leucine-free medium containing 0, 1.5, 3.0 or 4.5% BSA for 180 minutes, and the secreted radiolabeled apo B, apo A1 and albumin were isolated by immunoprecipitation and counted. The secretions of apo B and albumin were suppressed by BSA (bovine serum albumin) in a dose-dependent manner, but the secretion of apo A1 was not suppressed significantly. Oleate (0.4 mM) increased the rate of apo B secretion by 2.5-fold when oleate was bound to 1.5% BSA, but at higher concentrations of BSA (3.0 or 4.5%), apo B secretion was less responsive to oleate. A pulse-chase study indicated that early apo B degradation was significantly suppressed in cells incubated with lower concentrations of BSA (0 or 1.5% BSA), thereby rapidly stimulating apo B secretion. Oleate (0.4 mM) potently inhibited apo B degradation when oleate was bound to 1.5% BSA, whereas the inhibition was not observed when oleate was bound to 4.5% BSA. Intracellular albumin synthesis was stimulated in BSA-free medium, but intracellular decay of albumin was essentially unaffected by concentration of BSA. Similar to BSA, a higher concentration of dextran (3.0 or 4.5%) reduced apo B secretion, and this was the result of increased early apo B degradation in the cells. These results indicate that reduced albumin suppresses intracellular apo B degradation, and the inhibition of apo B degradation by oleate is manifested only at a low concentration of albumin. Therefore, the present study suggests that free fatty acids bound to low concentration of albumin in the circulating plasma play an important role on hepatic oversecretion of apo B-containing lipoprotein in hypoalbuminemic state, such as nephrotic syndrome.

Insulin regulation of triacylglycerol-rich lipoprotein synthesis and secretion

This review has considered a number of observations obtained from studies of insulin in perfused liver, hepatocytes, transformed liver cells and in vivo and each of the experimental systems offers advantages. The evaluation of insulin effects on component lipid synthesis suggests that overall, lipid synthesis is positively influenced by insulin. Short-term high levels of insulin through stimulation of intracellular degradation of freshly translated apo B and effects on synthesis limit the ability of hepatocytes to form and secrete TRL. The intracellular site of apo B degradation may involve membrane-bound apo B, cytoplasmic apo B and apo B which has entered the ER lumen. How insulin favors intracellular apo B degradation is not known. An area of recent investigation is in insulin-stimulated phosphorylation of intracellular substrates such as IRS-1 which activates insulin specific cellular signaling molecules [245]. Candidate molecules to study insulin action on apo B include IRS-1 and SH2-containing signaling molecules. Insulin dysregulation in carbohydrate metabolism occurs in non-insulin-dependent diabetes mellitus due to an imbalance between insulin sensitivity of tissue and pancreatic insulin secretion (reviewed in Refs. [307,308]). Insulin resistance in the liver results in the inability to suppress hepatic glucose production; in muscle, in impaired glucose uptake and oxidation and in adipose tissue, in the inability to suppress release of free FA. This lack of appropriate sensitivity towards insulin action leads to hyperglycemia which in turn stimulates compensatory insulin secretion by the pancreas leading to hyperinsulinemia. Ultimately, there may be failure of the pancreas to fully compensate, hyperglycemia worsens and diabetes develops. The etiology of insulin resistance is being intensively studied for the primary defect may be over secretion of insulin by the pancreas or tissue insulin resistance and both of these defects may be genetically predetermined. We suggest that, in addition to effects in carbohydrate metabolism, insulin resistance in liver results in the inability of first phase insulin to suppress hepatic TRL production which results in hypertriglyceridemia leading to high levels of plasma FA which accentuate insulin resistance in other target organs. As recently reviewed [17,254] the role of insulin as a stimulator of hepatic lipogenesis and TRL production has been long established. Several lines of evidence support that insulin is stimulatory to the production of hepatic TRL in vivo. First, population based studies support a positive relationship between plasma insulin and total TG and VLDL [253]. Second, there is a strong association between chronic hyperinsulinemia and VLDL overproduction [309].(ABSTRACT TRUNCATED AT 400 WORDS)

Effects of albumin on lipid synthesis, apo B-100 secretion, and LDL catabolism in HepG2 cells

This study examines the effects of extracellular albumin on hepatic apo B-100 metabolism. To do so, a transformed human liver cell line, HepG2, was used as a hepatocyte model and the concentration of albumin in the medium was varied between 0 and 5 g%. Apo B-100 and apo A1 concentrations in the medium were determined by specific enzyme-linked immunoassay (ELISA) and intracellular synthesis of cholesterol ester and triglyceride were determined by addition of appropriate radiolabels to the medium. The data demonstrate that the reduction of extracellular albumin concentration resulted in increased apo B-100 concentration in the medium. Apo A1 secretion, however, was unaffected. While the differences in apo B-100 concentration in the medium were statistically significant (33% +/- 7%, P < 0.0025, 0 g% albumin compared to 5 g% albumin in the medium), the absolute magnitude of the effect under these conditions was relatively modest. Nevertheless, the changes were consistent and evident over incubation periods as long as 8 days. Of interest, although triglyceride synthesis was unaffected, cholesterol ester synthesis changed such that as albumin concentration decreased, synthesis of cholesterol ester increased paralleling the changes in apo B-100 (170% +/- 9%, P < 0.005). These findings were extended by studying interventions which altered cholesterol ester synthesis. Addition of the compound 58-035 (5 micrograms/ml, a specific inhibitor of acylcholesterol acyltransferase activity) resulted in substantial inhibition of cholesterol ester synthesis (39% to 66%, P < 0.025 and P < 0.005, respectively) and apo B-100 concentrations in the medium which decreased by 20% to 28%, P < 0.025. Triglyceride synthesis, in contrast, increased significantly by 32% P < 0.025. Therefore, addition of 58-035 confirmed the previous findings of a parallel relation between cholesterol ester synthesis and apo B-100 concentration in the medium. Nonetheless, albumin still had an additional inhibitory effect on cholesterol ester and apo B-100 secretion. Of interest, when chylomicron remnants (25 micrograms/ml cholesterol), which cause apo B-100 secretion to increase by more than threefold, were added to the medium, albumin now had a more pronounced absolute effect on apo B-100 secretion with a 48% inhibition observed as albumin was increased from 0 to 5 g% in the medium (P < 0.0125). The effect of extracellular albumin on the low density lipoprotein (LDL) pathway was also examined. No differences in non-specific cell association component were detected.(ABSTRACT TRUNCATED AT 400 WORDS)

Relation of plasma triglyceride and apoB levels to insulin-mediated suppression of nonesterified fatty acids. Possible explanation for sex differences in lipoprotein pattern

To test whether a sex difference in insulin-mediated suppression of nonesterified fatty acids (NEFAs) could account for sex differences in plasma triglyceride levels, we studied 632 normoglycemic men and women of European and South Asian descent aged 40 to 69 years. Mean fasting NEFA levels were 19% higher in women than in men. Between fasting and 2 hours after a 75-g oral glucose load, NEFA levels fell by 69% in women and 55% in men, so that mean NEFA levels at 2 hours after loading were 19% lower in women than in men. Plasma triglyceride and apolipoprotein B levels were correlated with 2-hour NEFA levels in each sex and ethnic group, and these associations were independent of glucose, insulin, and central obesity. These results are consistent with experimental studies of the effects of insulin and NEFAs on hepatic production of triglycerides and apolipoprotein B. Suppression of NEFA levels in response to insulin is greater in women than in men, and this may account for some of the sex differences in lipoprotein pattern and coronary heart disease risk.

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.

Substrate delivery as a determinant of hepatic apoB secretion

The evidence that apoB particles secreted by the liver can differ in number and composition has been reviewed. No evidence has yet emerged that changes in apoB100 itself affect the rate of its secretion from the liver. The metabolic role of apoB appears to be the prevention of lipid accumulation within the liver cell: when delivery of lipid to the liver increases, apoB secretion will increase pari passu. This reality in no way detracts from the critical role played by the LDL receptor in determining the number of LDL particles in plasma, nor does it diminish the potential importance of intracellular processes such as 7 alpha-hydroxylase activity to also mediate LDL receptor activity. However, it should be obvious that variation in catabolism by itself cannot explain all that has been observed in physiological and pathological studies. On the contrary, the whole process must be taken into account--the rate at which apoB particles are added to the circulation, the rate at which they are converted to LDL, and the rate at which they are irreversibly removed from plasma--if we are to understand and appreciate this most peculiar and most important of transport systems.

Non-enzymatic glycation of apolipoprotein B in the sera of diabetic and non-diabetic subjects

Utilising a combination of m-aminophenyl-borate affinity chromatography and an immunoradiometric assay for apolipoprotein B (apo B), we have developed a specific and highly sensitive (6 ng/ml) procedure for the assay of glycated apo B. We studied 52 diabetic patients, 50 non-diabetic control subjects and 12 patients heterozygous for familial hypercholesterolaemia (FH). Both insulin-dependent and non-insulin dependent diabetics were included in our study. Total apo B in the diabetics (108 +/- 5 mg/dl; mean +/- S.E.M) was increased (controls: 95 +/- 4 mg/dl; P less than 0.05). In the FH group the serum apo B concentration (216 +/- 24 mg/dl) was significantly higher (P less than 0.001) than both the other groups studied. Both the serum glycated apo B concentration (9.3 +/- 0.8 mg/dl versus 4.8 +/- 0.7 mg/dl) and the percentage glycated apo B (7.9 +/- 0.4% compared to 3.9 +/- 0.2%) were significantly higher in the diabetics than in non-diabetic controls (P less than 0.001). A positive correlation was found between the percentage of glycated apo B and glycated haemoglobin (r = 0.65; P less than 0.001) and fasting glucose concentration (r = 0.52; P less than 0.001) in diabetics. The percentage of glycated apo B in FH patients was not significantly different from controls, but the serum concentration of glycated apo B, because of the greatly increased total level of apo B was raised (8.2 +/- 1.4 mg/dl) to a similar extent to that of the diabetics.