The amphipathic alpha-helical repeats of apolipoprotein A-I are responsible for binding of high density lipoproteins to HepG2 cells

A thumbwheel mechanism for APOA1 activation of LCAT activity in HDL

APOA1 is the most abundant protein in HDL. It modulates interactions that affect HDL's cardioprotective functions, in part via its activation of the enzyme, LCAT. On nascent discoidal HDL, APOA1 comprises 10 α-helical repeats arranged in an anti-parallel stacked-ring structure that encapsulates a lipid bilayer. Previous chemical cross-linking studies suggested that these APOA1 rings can adopt at least two different orientations, or registries, with respect to each other; however, the functional impact of these structural changes is unknown. Here, we placed cysteine residues at locations predicted to form disulfide bonds in each orientation and then measured APOA1's ability to adopt the two registries during HDL particle formation. We found that most APOA1 oriented with the fifth helix of one molecule across from fifth helix of the other (5/5 helical registry), but a fraction adopted a 5/2 registry. Engineered HDLs that were locked in 5/5 or 5/2 registries by disulfide bonds equally promoted cholesterol efflux from macrophages, indicating functional particles. However, unlike the 5/5 registry or the WT, the 5/2 registry impaired LCAT cholesteryl esterification activity (P < 0.001), despite LCAT binding equally to all particles. Chemical cross-linking studies suggest that full LCAT activity requires a hybrid epitope composed of helices 5-7 on one APOA1 molecule and helices 3-4 on the other. Thus, APOA1 may use a reciprocating thumbwheel-like mechanism to activate HDL-remodeling proteins.

Effect of anti-ApoA-I antibody-coating of stents on neointima formation in a rabbit balloon-injury model

Background and Aims

Since high-density lipoprotein (HDL) has pro-endothelial and anti-thrombotic effects, a HDL recruiting stent may prevent restenosis. In the present study we address the functional characteristics of an apolipoprotein A-I (ApoA-I) antibody coating in vitro. Subsequently, we tested its biological performance applied on stents in vivo in rabbits.

Materials and Methods

The impact of anti ApoA-I- versus apoB-antibody coated stainless steel discs were evaluated in vitro for endothelial cell adhesion, thrombin generation and platelet adhesion. In vivo, response to injury in the iliac artery of New Zealand white rabbits was used as read out comparing apoA-I-coated versus bare metal stents.

Results

ApoA-I antibody coated metal discs showed increased endothelial cell adhesion and proliferation and decreased thrombin generation and platelet adhesion, compared to control discs. In vivo, no difference was observed between ApoA-I and BMS stents in lumen stenosis (23.3±13.8% versus 23.3±11.3%, p=0.77) or intima surface area (0.81±0.62 mm² vs 0.84±0.55 mm², p=0.85). Immunohistochemistry also revealed no differences in cell proliferation, fibrin deposition, inflammation and endothelialization.

Conclusion

ApoA-I antibody coating has potent pro-endothelial and anti-thrombotic effects in vitro, but failed to enhance stent performance in a balloon injury rabbit model in vivo.

Fenofibrate, simvastatin and their combination in the management of dyslipidaemia in type 2 diabetic patients

OBJECTIVE

To evaluate the efficacy of fenofibrate, simvastatin or their combination in type 2 diabetic patients with combined dyslipidaemia.

RESEARCH DESIGN AND METHODS

241 patients, who had never previously taken lipid-lowering medications, received fenofibrate 145 mg/day, or simvastatin 40 mg/day, or fenofibrate 145 mg/day + simvastatin 40 mg/day combination for 12 months. We evaluated lipids, glycaemic, haemostatic, and inflammatory variables at baseline, and after 6 and 12 months.

RESULTS

After 12 months total cholesterol (TC), LDL cholesterol (LDL-C) and triglycerides (Tg) decreased while HDL cholesterol (HDL-C) increased in all groups, even if the values obtained with fenofibrate + simvastatin were the best. At the end of the study apolipoprotein A-1 (Apo A-1) increased with fenofibrate + simvastatin, while apolipoprotein B (Apo B) decreased in all groups compared to baseline. Plasminogen activator inhibitor-1 (PAI-1) and high-sensitivity C reactive protein (hs-CRP) decreased after 12 months compared to baseline with simvastatin, and with fenofibrate + simvastatin even if the value obtained with fenofibrate-simvastatin was the lowest. After 12 months, fibrinogen (Fg) decreased compared to baseline with fenofibrate + simvastatin.

LIMITATIONS

This study has some limitations. The first one is the relatively small sample of studied patients. The second one is the lack of an advanced lipid proteins evaluation, such as lipoprotein subfraction changes in the different treatment regimen. Finally, we have not selected patients that could show the best response to fibrate (i.e.: hypertriglyceridemics) or statins (i.e.: hypercholesterolemics) monotherapy, so the effect of these drugs administered alone may have been partly attenuated.

CONCLUSIONS

Fenofibrate + simvastatin association improved lipid parameters, prothrombotic and inflammatory factors, and appeared to have a good tolerability profile over 12 months of therapy.

Effects of nateglinide and glibenclamide on prothrombotic factors in naïve type 2 diabetic patients treated with metformin: a 1-year, double-blind, randomized clinical trial

OBJECTIVE

To evaluate the effect on coagulation and fibrinolysis parameters and on non-conventional cardiovascular risk factors of metformin plus nateglinide or glibenclamide in naïve type 2 diabetes patients.

PATIENTS AND METHODS

A total of 248 type 2 diabetic patients were enrolled and randomly assigned to receive nateglinide or glibenclamide, and metformin for 12 months. We assessed body mass index (BMI), glycated hemoglobin (HbA1c), fasting plasma glucose (FPG), postprandial plasma glucose (PPG), fasting plasma insulin (FPI), postprandial plasma insulin (PPI), homeostasis model assessment index (HOMA index), lipid profile with lipoprotein (a) [Lp(a)], fibrinogen (Fg), plasminogen activator inhibitor-1 (PAI-1), tissue plasminogen activator (t-PA), homocysteine (Hcy), systolic blood pressure (SBP), diastolic blood pressure (DBP).

RESULTS

After 9 months of treatment, both tested drug combinations were similarly associated with a significant reduction in FPG (nateglinide, -17.2%; glibenclamide, -16.9%, both p<0.05) compared to the baseline, while HbA1c (-17.3%, p<0.05) and PPG (-15.2%, p<0.05) significantly decreased only in the nateglinide group. After one year of treatment, compared to the baseline the nateglinide group showed a significant reduction in HbA1c (-21%, p<0.01), FPG (-20.7%), p<0.01, PPG (-21.5%, p<0.05), HOMA index (-25.4%, p<0.05); the glibenclamide group, showed a significant reduction in HbA1c (-11%, p<0.05), FPG (-23.2%, p<0.05), PPG (-11.2%, p<0.05), and HOMA index (-23.9%, p<0.05) but to a minor extent. Moreover, the HbA1c difference value from baseline observed in the nateglinide-treated group was significantly higher than that observed in the glibenclamide group. Therefore the nateglinide-treated patients showed a significant reduction in some prothrombotic parameters (PAI-1=-19%, Lp(a)=-31%, and Hcy=-32.3%, all p<0.05), whereas the glibenclamide-treated patients did not.

CONCLUSION

Nateglinide appears to improve glycemic control as well as the levels of some prothrombotic parameters compared to glibenclamide when administered in combination with metformin.

Differential effect of glimepiride and rosiglitazone on metabolic control of type 2 diabetic patients treated with metformin: a randomized, double-blind, clinical trial

AIM

Accumulating evidence suggests that combination therapy using oral antidiabetic agents with different mechanisms of action may be highly effective in achieving and maintaining target blood glucose levels. The aim of our study is to evaluate the differential effect on glucose and lipid parameters of the association between glimepiride plus metformin and rosiglitazone plus metformin in patients affected by type 2 diabetes and metabolic syndrome.

METHODS

Patients were enroled, evaluated and followed at two Italian centres. We evaluated 99 type 2 diabetic patients with metabolic syndrome (48 males and 47 females; 23 males and 24 females, aged 52 +/- 5 with glimepiride; 25 males and 23 females, aged 54 +/- 4 with cglitazone). All were required to have been diagnosed as being diabetic for at least 6 months and did not have glycaemic control with diet and oral hypoglycaemic agents such as sulphonylureas or metformin, both to the maximum tolerated dose. All patients took a fixed dose of metformin, 1500 mg/day. We administered glimepiride (2 mg/day) or rosiglitazone (4 mg/day) in a randomized, controlled, double-blind clinical study. We evaluated body mass index (BMI), glycaemic control, lipid profile [total cholesterol (TC), low-density lipoprotein-cholesterol (LDL-C), high-density lipoprotein-cholesterol and triglycerides] and lipoprotein parameters [apolipoprotein A-I and apolipoprotein B (Apo B)] during 12 months of this treatment.

RESULTS

A total of 95 patients completed the study. Significant BMI decrease was observed at 12 months in glimepiride and rosiglitazone group (p < 0.05 and p < 0.01 respectively) as well as of glycated haemoglobin decrease (p < 0.05 and p < 0.01 respectively), mean fasting plasma glucose and postprandial plasma glucose levels (p < 0.05 and p < 0.01 respectively). A decrease in fasting plasma insulin and postprandial plasma insulin at 12 months (p < 0.05 and p < 0.01 respectively) compared with the baseline value in rosiglitazone group was observed. Furthermore, homeostasis model assessment index improvement was obtained only at 9 and 12 months (p < 0.05 and p < 0.01 respectively) compared with the baseline value in rosiglitazone group. Significant TC, LDL-C and Apo B improvement (p < 0.05 respectively) was present in glimepiride group after 12 months compared with the baseline values, and these variations were significant (p < 0.05) between groups. Of the 95 patients who completed the study, 8.5% of patients in glimepiride group and 12.5% of patients in rosiglitazone group had side-effects (p = not significant). Four patients had transient side-effects in glimepiride group and six patients in rosiglitazone group. Altogether, we did not have statistically significant changes in transaminases.

CONCLUSIONS

The rosiglitazone-metformin association significantly improve the long-term control of all insulin-resistance-related parameters in comparison with the glimepiride-metformin-treated group. On the other side, glimepiride treatment is associated to a slight improvement in cholesterolaemia, not observed in the rosiglitazone-treated patients.

Metabolic effects of pioglitazone and rosiglitazone in patients with diabetes and metabolic syndrome treated with glimepiride: A twelve-month, multicenter, double-blind, randomized, controlled, parallel-group trial

BACKGROUND

Glimepiride is approved as monotherapy and in combination with metformin or with insulin, whereas the combination of glimepiride with other antihyperglycemic drugs is under investigation.

OBJECTIVE

The aim of this study was to assess the differential effect on glucose and lipid variables and tolerability of the combination of glimepiride plus pioglitazone or rosiglitazone in patients with type 2 diabetes mellitus (DM) and metabolic syndrome.

METHODS

This 12-month, multicenter, double-blind, randomized, controlled, parallel-group trial was conducted at 3 study sites in Italy. We assessed patients with type 2 DM (duration, > or =6 months) and with metabolic syndrome. All patients were required to have poor glycemic control with, or to have experienced > or =1 adverse effect (AE) with, diet and oral hypoglycemic agents such as sulfonylureas or metformin, both given up to the maximum tolerated dose. All patients received a fixed oral dose of glimepiride, 4 mg/d divided into 2 doses, self-administered for 12 months. Patients also were randomized to receive oral pioglitazone (15 mg once daily) (G + P group) or oral rosiglitazone (4 mg once daily) (G + R group), self-administered for 12 months. We assessed body mass index (BMI), glycemic control (glycosylated hemoglobin [HbA(1c)], fasting and postprandial plasma glucose and insulin levels [FPG, PPG, FPI, and PPI, respectively], and homeostasis model assessment index), lipid profile (total cholesterol [TC], low-density lipoprotein cholesterol [LDL-C], high-density lipoprotein cholesterol [HDL-C], and triglycerides [TG]), and lipoprotein variables (apolipoprotein [apo] A-I and apo B) at baseline and at 3, 6, 9, and 12 months of treatment. Treatment tolerability was assessed at each study visit using a thorough interview of patients, and comparisons of clinical and laboratory values to baseline levels.

RESULTS

A total of 91 patients were enrolled in the study; 87 patients completed it (G + P group: 24 women, 21 men; mean [SD] age, 53 [6] years; G + R group: 20 women, 22 men; mean [SD] age, 54 [5] years). Patients in the G + P and G + R groups experienced significant increases in mean BMI at 12 months compared with baseline (4.92% and 6.17%, respectively; both, P < 0.05). The combination of glimepiride with pioglitazone or rosiglitazone significantly improved glycemic control in the study patients. At 12 months, we observed a 1.3% improvement in mean values for plasma HbA(1c) concentration (P < 0.01) 19.3% in FPG (P < 0.01), 16.3% in PPG (P < 0.01), 42.4% in FPI ), and 23.3% in PPI (P <0.05); no significant differences were found between treatment groups. Although the G + P group experienced a significant improvement at 12 months in almost all variables of lipid metabolism from baseline (TC, - 11%; LDL-C, -12%; HDL-C, 15%; and apo B, - 10.6% [all, P , 0.05]), the G + R group experienced a significant increase in mostly the lipid risk factors for cardiovascular disease (TC, 14.9%; LDL-C, 16.5%; TG, 17.9%; and apo B, 10.3% [all, P , 0.05]). Overall, no statistically significant changes in plasma aminotransferase activities were observed. Of the 87 patients who completed the study, 6.7% (3/45) of patients in the G + P group and 11.9% (5/42) of patients in the G + R group had transient, mild to moderate AEs that did not cause withdrawal from the trial.

CONCLUSION

In this study of patients with type 2 DM and metabolic syndrome who did not respond adequately to, or experienced AEs with, diet and either a sulfonylurea or metformin previously, the combination of glimepiride plus pioglitazone was associated with a significant improvement in lipid and lipoprotein variables, whereas the combination of glimepiride plus rosiglitazone appears to not have had any clinically significant effect on lipid metabolism.

A randomized, double-blind, controlled, parallel-group comparison of perindopril and candesartan in hypertensive patients with type 2 diabetes mellitus

BACKGROUND

When choosing an antihypertensive drug for patients with hypertension and diabetes mellitus (DM), the metabolic side effects, possibility of improving some metabolic parameters, and need for adequate blood pressure control must all be considered.

OBJECTIVE

The goal of this study was to compare the impacts of perindopril and candesartan on blood pressure, glucose metabolism, serum lipid profile, and metabolic parameters in patients with mild hypertension and type 2 DM during therapy and after a 1-month washout period.

METHODS

Type 2 DM patients with mild hypertension and good glucose control who were not taking hypercholesterolemic drugs were enrolled. Perindopril 4 mg QD or candesartan 16 mg QD was administered for 12 months in this randomized, double-blind, controlled, parallel-group clinical trial. Fasting plasma glucose (FPG), fasting plasma insulin (FPI), glycosylated hemoglobin, homeostasis model assessment (HOMA) index, systolic blood pressure (SBP), diastolic blood pressure (DBP), total cholesterol (TC), low-density lipoprotein cholesterol (LDL-C), high-density lipoprotein cholesterol, triglycerides, lipoprotein(a) (Lp[a]), plasminogen activator inhibitor 1 (PAI-1), homocysteine, body mass index (BMI), and albumin excretion rate (AER) were assessed.

RESULTS

Ninety-six patients (49 women and 47 men; mean [SD] ages, 53 [10] years [perindopril] and 55 [9] years [candesartan]) were enrolled. Mean (SD) body weight, height, and BMI were 78.2 (9.4) kg, 1.69 (0.05) m, and 27.2 (2.0) kg/m(2) in the perindopril group and 77.5 (8.6) kg, 1.70 (0.06) m, and 26.8 (2.5) kg/m(2) in the candesartan group. A significant change occurred from baseline to month 12 during treatment with perindopril in SBP and DBP (both P < 0.01), FPG (P < 0.05), FPI (P < 0.05), TC (P < 0.05), LDL-C (P < 0.05), Lp(a) (P < 0.05), PAM (P < 0.05), and AER (P < 0.05). Significant changes from baseline to month 12 occurred with candesartan in SBP and DBP (both P < 0.01) and AER (P < 0.05). The HOMA index was significantly lower at month 12 in the perindopril group than in the candesartan group (P < 0.05). When we interrupted perindopril and candesartan therapy for a 1-month washout period, changes in SBP and DBP values were significant compared with month 12 in both groups (all P < 0.05). Changes in TC and LDL-C from month 12 to the end of washout were significant only in the perindopril group (both P < 0.05).

CONCLUSIONS

Perindopril and candesartan both effectively lowered blood pressure in this group of patients with mild hypertension and type 2 DM. Perindopril showed an improvement on some metabolic parameters compared with candesartan. However, the inclusion/exclusion criteria could limit the ability to extrapolate the results to a general population.

The effect of L-carnitine on plasma lipoprotein(a) levels in hypercholesterolemic patients with type 2 diabetes mellitus

BACKGROUND

A previous study has demonstrated that L-carnitine reduces plasma lipoprotein(a) (Lp[a]) levels in patients with hypercholesterolemia.

OBJECTIVE

To test a tolerable Lp(a)-reducing agent in diabetic patients, we assessed the effect of a dietary supplementation of L-carnitine on plasma lipid levels, particularly Lp(a), of patients with type 2 diabetes mellitus (DM) and hypercholesterolemia.

METHODS

In this 6-month, randomized, double-masked, placebo-controlled clinical trial, patients were enrolled, assessed, and followed up at the Diabetic and Metabolic Diseases Center of the Department of Internal Medicine and Therapeutics at the University of Pavia, Pavia, Italy. All study patients had newly diagnosed type 2 DM that was managed through dietary restriction alone throughout the study, as well as hypercholesterolemia. Patients were randomized to 1 of 2 groups. One group received L-carnitine, one 1-g tablet BID. The other group received a corresponding placebo. We assessed body mass index, fasting plasma glucose, postprandial plasma glucose, glycosylated hemoglobin, fasting plasma insulin, total cholesterol, low-density lipoprotein cholesterol, high-density lipoprotein cholesterol, triglycerides, apolipoprotein (apo) A-I, apo B, and Lp(a) at baseline and at 1, 3, and 6 months of treatment.

RESULTS

This study included 94 patients. The treatment group included 24 men and 22 women (mean [SD] age, 52 [6] years). The placebo group included 23 men and 25 women (mean [SD] age, 50 [7] years). The baseline characteristics of the groups did not differ significantly. The mean (SD) body weight, height, and body mass index were 78.2 (5.8) kg, 1.70 (0.04) m, and 27.3 (2.5) kg/m(2), respectively, in the L-carnitine group and 77.6 (6.4) kg, 1.71 (0.05) m, and 26.8 (2.2) kg/m(2), respectively, in the placebo group. In the treatment group, Lp(a) was significantly reduced at 3 and 6 months compared with baseline (P < 0.05) and P < 0.01, respectively). We observed a significant improvement after 6 months (P < 0.05) in the Lp(a) value in patients taking L-carnitine compared with those taking placebo. Between-group differences in other variables did not reach a level of significance at months 3 and 6. No drug-related adverse events were reported or observed.

CONCLUSION

In this preliminary study, after 3 and 6 months, L-carnitine significantly lowered the plasma Lp(a) level compared with placebo in selected hypercholesterolemic patients with newly diagnosed type 2 DM.

Comparison between repaglinide and glimepiride in patients with type 2 diabetes mellitus: a one-year, randomized, double-blind assessment of metabolic parameters and cardiovascular risk factors

BACKGROUND

Repaglinide and glimepiride are relatively new oral hypoglycemic agents. Few data are available concerning their effects on metabolic parameters other than measures of glycemic control.

OBJECTIVES

In addition to assessing the effects of repaglinide and glimepiride on glycemic control in patients with type 2 diabetes mellitus, this study also examined the effects of these agents on 3 metabolic parameters known to be cardiovascular risk factors--lipoprotein(a) (Lp[a]), plasminogen activator inhibitor-1 (PAI-1), and homocysteine (Hcy).

METHODS

This randomized, placebo-controlled, double-blind trial was conducted at a single center in Italy. Eligible patients were nonsmokers; had no hypertension or coronary heart disease; were taking no hypolipidemic drugs, diuretics, beta-blockers, or thyroxin; and had normal renal function. After an initial 4-week placebo washout period, patients were randomized to receive repaglinide 1 mg/d or glimepiride 1 mg/d. The dose of study drug was optimized over an 8-week titration period, which was followed by a 12-month treatment period. Measures of glycemic control (glycated hemoglobin [HbA1c], fasting plasma glucose [FPG], postprandial plasma glucose [PPG], fasting plasma insulin [FPI], postprandial plasma insulin [PPI]) and the other metabolic parameters of interest were assessed after 6 and 12 months of treatment.

RESULTS

One hundred twenty-four patients (63 women, 61 men) completed the study, 62 in each treatment group. There were no significant differences in demographic characteristics between groups. After 6 and 12 months of treatment, FPG levels and HbA1c values were significantly reduced from baseline in both groups (6 months, P < 0.05; 12 months, P < 0.01). After 6 months, PPG levels were significantly decreased only in the repaglinide group (P < 0.05 vs baseline); at 12 months, however, PPG levels were significantly reduced from baseline in both groups (P < 0.01 repaglinide, P < 0.05 glimepiride). No significant changes from baseline in FPI or PPI levels were seen in either group at 6 months, although FPI levels were significantly increased in the repaglinide group at 12 months (P < 0.05). Repaglinide significantly lowered levels of Lp(a), PAI-1, and Hcy after 12 months (all, P < 0.05 vs baseline). Glimepiride significantly lowered levels of Lp(a) and Hcy after 6 months (both, P < 0.05 vs baseline) and levels of Lp(a) (P < 0.01 vs baseline), Hcy (P < 0.01 vs baseline), and PAI-1 (P < 0.05 vs baseline) after 12 months.

CONCLUSIONS

Repaglinide and glimepiride improved glycemic control and reduced levels of other metabolic parameters of interest in this population of patients with type 2 diabetes. It is possible that the reductions in Lp(a), PAI-1, and Hcy were the result of improved glucose metabolism; however, the possibility that repaglinide and glimepiride may have a direct effect on these parameters should not be excluded.

Identification of an ApoA-I ligand domain that interacts with high-affinity binding sites on HepG2 cells

We have previously described the presence of two (high- and low-affinity) HDL binding sites on the hepatoma cell line (HepG2) (R. Barbaras, X. Collet, H. Chap, and B. Perret (1994) Biochemistry 33, 2335-2340]. Moreover, apoA-I, the major HDL apolipoprotein, interacts with these two binding sites, while lipid-free apoA-I binds only to the high-affinity sites. Using tryptic HDL fragments and HepG2 cell monolayers as an "affinity matrix," we identified an apoA-I peptide of 16 amino acids, spanning between residues 62 and 77, as a ligand domain. The corresponding synthetic peptide displays high-affinity (K(d) approximately 10(-7) M) and low-capacity (B(max) 8 pmol/mg of cell protein) binding components. Competition experiments with this peptide, using (125)I-labeled free apoA-I as a ligand, show that this binding corresponds to the high-affinity binding sites already described. In conclusion, we identified the apoA-I 62-77 region as a specific high-affinity ligand domain of HDL on HepG2 cells.

Cell cholesterol efflux to reconstituted high-density lipoproteins containing the apolipoprotein A-IMilano dimer

The apolipoprotein A-IMilano (apoA-IM) is a molecular variant of apoA-I characterized by the Arg(173)-->Cys substitution, resulting in the formation of homodimers A-IM/A-IM. The introduction of the interchain disulfide bridge in the A-IM dimer limits the apolipoprotein conformational flexibility and restricts HDL particle size heterogeneity, thus possibly affecting HDL function in lipid metabolism and atherosclerosis protection. To investigate whether the structural changes in A-IM/A-IM affect apoA-I capacity for cell cholesterol uptake, we tested the ability of four reconstituted HDL (rHDL), that contained either apoA-I or A-IM/A-IM, to remove cholesterol from Fu5AH hepatoma cells and cholesterol-loaded murine primary macrophages (MPM). As the HDL particle size is known to affect the rHDL capacity for cell cholesterol uptake, the reconstitution conditions were carefully selected to produce two sets of rHDL particles of small and large size (7.8 and 12.5 nm in diameter). The small A-IM/A-IM rHDL were more efficient than the corresponding apoA-I particles as acceptors of membrane cholesterol from Fu5AH cells and MPM, and as inhibitors of cholesterol esterification in MPM. The large rHDL and the lipid-free apolipoproteins displayed instead similar capacities for cell cholesterol efflux. These results suggest that cell cholesterol efflux to rHDL particles of different size occurs through different mechanisms. Large HDL accommodate and retain the cholesterol molecules that have desorbed from the cell membrane into the extracellular fluid, in a process that is less sensitive to protein conformation. Small HDL accelerate the desorption of cholesterol from the cell membrane, in a process that is influenced by the conformation of the proteins on the surface of the acceptor particle. The enhanced efficiency of small A-IM/A-IM rHDL seems related to the peculiar structure of the protein on the rHDL surface, with a hydrophobic C-terminal domain extending out of the rHDL particle, available for anchoring the acceptor to the plasma membrane.

Low density lipoprotein-receptor plays a major role in the binding of very low density lipoproteins and their remnants on HepG2 cells

The binding to HepG2 cells of very low density lipoproteins (VLDL) and their remnants (IDL) was alternatively, in the past, attributed to the low density lipoprotein receptor (LDLr) or to an apoE-specific receptor. In order to resolve this issue, we have compared the binding of those lipoproteins labelled with iodine-125 to normal and LDLr deficient HepG2 cells. Those deficient cells were obtained by a constitutive antisense strategy and their LDLr level is 14% the level of normal HepG2 cells. By saturation curve analysis, we show that VLDL and IDL bind to high and low affinity sites on cells. The low affinity binding was eliminated by conducting the assay in presence of a 200-fold excess of HDL3 respective to the concentrations of 125I-labelled VLDL and IDL. For 125I-VLDL high affinity binding to normal HepG2 cells, we found a dissociation constant (Kd) of 21.2 +/- 3.7 micrograms prot./ml (S.E., N = 5) and a maximal binding capacity (Bmax) of 0.0312 +/- 0.0063 microgram prot./mg cell prot, while we have measured a Kd of 5.3 +/- 0.8 and a Bmax of 0.0081 +/- 0.0014 with LDLr deficient cells. This indicates that LDLr is responsible for 74% of VLDL binding to HepG2 cells and that the non-LDLr high affinity receptor has a higher affinity for VLDL than LDLr. A 53% loss of 125I-IDL binding capacity was measured with LDLr deficient cells compared with normal cells (Bmax: 0.028 +/- 0.005 versus 0.059 +/- 0.006), while no significant statistical difference was found between affinities. The study shows that the LDLr is almost the only contributor in VLDL binding, while it shares IDL binding capacity with another high affinity receptor. The physiological importance of LDLr is confirmed by an almost equivalent loss of IDL and VLDL degradation in LDLr deficient cells.

Uptake and fate of class B scavenger receptor ligands in HepG2 cells

Class B scavenger receptors (SR-Bs) interact with native, acetylated and oxidized low-density lipoprotein (LDL, AcLDL and OxLDL), high-density lipoprotein (HDL3) and maleylated BSA (M-BSA). The aim of this study was to analyze the catabolism of CD36- and LIMPII-analogous-1 (CLA-1), the human orthologue for the scavenger receptor class B type I (SR-BI), and CD36 ligands in HepG2 (human hepatoma) cells. Saturation binding experiments revealed moderate-affinity binding sites for all the SR-B ligands tested with dissociation constants ranging from 20 to 30 microg.mL-1. Competition binding studies at 4 degrees C showed that HDL and modified and native LDL share common binding site(s), as OxLDL competed for the binding of 125I-LDL and 125I-HDL3 and vice versa, and that only M-BSA and LDL may have distinct binding sites. Degradation/association ratios for SR-B ligands show that LDL is very efficiently degraded, while M-BSA and HDL3 are poorly degraded. The modified LDL degradation/association ratio is equivalent to 60% of the LDL degradation ratio, but is three times higher than that of HDL3. All lipoproteins were good cholesteryl ester (CE) donors to HepG2 cells, as a 3.6-4.7-fold CE-selective uptake ([3H]CE association/125I-protein association) was measured. M-BSA efficiently competed for the CE-selective uptake of LDL-, OxLDL-, AcLDL- and HDL3-CE. All other lipoproteins tested were also good competitors with some minor variations. Hydrolysis of [3H]CE-lipoproteins in the presence of chloroquine demonstrated that modified and native LDL-CE were mainly hydrolyzed in lysosomes, whereas HDL3-CE was hydrolyzed in both lysosomal and extralysosomal compartments. Inhibition of the selective uptake of CE from HDL and native modified LDL by SR-B ligands clearly suggests that CLA-1 and/or CD36 are involved at least partially in this process in HepG2 cells.

Endothelial and serum factors which include apolipoprotein A1 tether elastin to smooth muscle cells inducing serine elastase activity via tyrosine kinase-mediated transcription and translation

We previously reported that serine elastase activity is induced in cultured porcine pulmonary artery (PA) smooth muscle cells (SMC) following serum stimulation by a mechanism involving adhesion of elastin to an elastin binding protein and tyrosine kinase activity. The present study demonstrates that a PA endothelial cell factor also promotes a fourfold increase in elastin adhesion to PA SMC and a twofold increase in serine elastase activity. The mechanism involves tethering of the factor to SMC, since [3H]-elastin pre-incubated with serum or endothelial cell (EC)-conditioned medium or SMC pre-treated with serum accelerates binding of elastin and tyrosine-kinase related elastase activity. The serum factor appears to interact with integrins as elastase induction is partially inhibited by RGD peptides. The elastase-inducing properties of serum could not, however, be attributed to several RGD-containing proteins. While a 120 kD fibronectin fragment partially reproduced the effect, it was not found in the serum fraction containing elastase-inducing activity. Instead, a 27 kD serum protein was enriched by elastin affinity chromatography, identified as apolipoprotein (Apo) A1 by microsequence analysis, and found to have about 50% of the elastase-inducing activity of serum. Elastase induction is inhibited by actinomycin and cycloheximide, suggesting a requirement for mRNA transcription and protein synthesis. Our results suggest a novel cell-extracellular matrix interaction whereby a soluble factor, in this case a lipoprotein, binds and tethers a matrix component to the cell surface and induces tyrosine kinase-dependent transcription of mRNA culminating in substrate proteolysis.

In vivo glucosylated LpA-I subfraction. Evidence for structural and functional alterations

This study compared the structural and functional properties of glucosylated and non-glucosylated LpA-I particle subfractions (GLpA-I and NGLpA-I, respectively) isolated from patients with poorly controlled type 1 (insulin-dependent) diabetes. Compared with NGLpA-I, GLpA-I showed an enrichment in triglycerides (P < .05) and a depletion in phospholipid (P < .05) content. Moreover, the triglycerides-to-cholesteryl esters ratio was increased (P < .05), suggesting an increased cholesteryl ester transfer protein activity and a possible transport defect that accelerates atherogenesis. The surface-to-core constituents ratio, an indirect estimate of particles size, is lower in GLpA-I (P < .01) than in NGLpA-I, correlating well with a larger median size (P < .05) as seen by electron microscopy. The apolipoprotein (apo) A-I conformation was evaluated through determination of the immunological accessibility of three different domains defining specific epitopes for anti-apo A-I monoclonal antibodies. We observed a marked decreased accessibility for two of these regions, which interestingly have already been implicated in the interaction with cells. Cell culture data suggest that nonenzymatic glycosylation occurring on apo A-I can modify lipoprotein function, since it results in a decreased binding of GLpA-I to HeLa cells and impaired cholesterol efflux from Fu5AH rat hepatoma cells.

Limited proteolysis of high density lipoprotein abolishes its interaction with cell-surface binding sites that promote cholesterol efflux

High-density lipoprotein (HDL) components remove cholesterol from cells by two independent mechanisms. Whereas HDL phospholipids pick up cholesterol that desorbs from the plasma membranes, HDL apolipoproteins appear to interact with cell-surface binding sites that target for removal pools of cellular cholesterol that feed into the cholesteryl ester cycle. Here we show that mild trypsin treatment of HDL almost completely abolishes this apolipoprotein-mediated cholesterol removal process. When HDL was treated with trypsin for various periods of time and then incubated with cholesterol-loaded fibroblasts, treatment for only 5 min reduced the ability of HDL to remove excess cholesterol from cellular pools that were accessible to esterification by the enzyme acyl CoA:cholesterol acyltransferase. This mild treatment digested less than 20% of HDL apolipoproteins and did not alter the lipid composition, size distribution, or electrophoretic mobility of the particles. Trypsin treatment of HDL for up to 1 h caused no further reduction in its ability to remove cellular cholesterol despite a greater than 2-fold increase in apolipoprotein digestion. Trypsin treatment of HDL also reduced its ability to deplete the cholesteryl ester content of sterol-laden macrophages. Promotion of cholesterol efflux from the plasma membrane by HDL phospholipids was unaffected by even extensive proteolysis. In parallel to the loss of cholesterol transport-stimulating activity, trypsin treatment of HDL for only 5 min nearly abolished its interaction with high-affinity binding sites on cholesterol-loaded fibroblasts. Reconstitution of trypsin-modified HDL with isolated apo A-I or apo A-II restored the cholesterol transport-stimulating activity of the particles. Thus a minor trypsin-labile fraction of HDL apolipoproteins is almost exclusively responsible for the apolipoprotein-dependent component of cholesterol efflux mediated by HDL particles.

The selective uptake of the cholesteryl esters of low density lipoproteins parallels the activity of protein kinase C

The analysis of the association of (125)I-LDL and [(3)H]cholesteryl ethers (CEt)-LDL with HepG2 cells revealed a selective uptake of cholesteryl esters (CE) of the LDL, as in the order of three-fold more CE were associated with the cells than LDL-proteins for an incubation of 4 h. To determine if a trans-signalling pathway is involved in this selective uptake, HepG2 cells were pre-treated for 2 h with either a Protein Kinase A activator [8-(4-chlorophenylthioadenosine 3'-5' cyclic monophosphate (CPT-cAMP)] or a Protein Kinase C activator [phorbol 12-myristate 13-acetate (PMA)]. We found that CPT-cAMP had a minimal effect, while PMA was able to significantly increase the selective uptake of the CE of LDL. Indeed, upon a 2 h pre-incubation of HepG2 cells with PMA at a concentration of 160 microM, an increase of more than 3-fold in CE selective uptake was registered and was shown to occur by the lipoprotein binding sites (LBS) of HepG2 cells. Also, an incubation of the cells with 100 nM calphostin C, an inhibitor of protein kinase C, decreased the selective uptake by 41%. The effect of PMA is not abolished by either cycloheximide or actinomycin D. However, cycloheximide was shown to potentiate the effect of PMA on the LBS activity, suggesting that a protein which synthesis is affected by cycloheximide is involved in maintaining the LBS activity low. Our results show that the HepG2 cell activity of CE selective uptake parallels the activity of Protein Kinase C and suggest that the LBS could be a G-protein linked receptor.

Alterations in the physiochemical characteristics of low and high density lipoproteins after lipolysis with phospholipase A2. A spin-label study

Human low and high density lipoproteins (LDL and HDL, respectively) were treated with porcine pancreatic phospholipase A2 (PLA2) in the presence of albumin resulting in hydrolysis of 40-84% of the lipoproteins phospholipids. The resulting PLA2-treated LDL and HDL and concurrent control lipoproteins incubated without PLA2 were reisolated by ultracentrifugation and labelled with 5-doxyl- and 16-doxyl-stearic acid, and with a spin-labelled analogue of maleimide. Analysis of ESR spectra showed that phospholipid hydrolysis both of LDL and HDL resulted in an increase in order, micro-viscosity and polarity of lipid regions in the surface monolayer of the particles. In the temperature range from 3 degrees C to 50-60 degrees C, Arrhenius plots of a spectral parameter of LDL and HDL labelled with 5-doxyl-stearate exhibited alterations which suggest an increase in free cholesterol content near the surface of the lipoproteins after PLA2-treatment. ESR spectra of the maleimide analogue bound covalently to the protein moiety of the lipoproteins have demonstrated that, following phospholipid hydrolysis, the conformation of the apoproteins became more condensed, with more masked domains. The possible implications of the revealed alterations for enhanced delivery of LDL and HDL cholesterol to cells after phospholipolysis of the lipoproteins are discussed.

Changes in epitope exposition of apolipoprotein A-I on the surface of high density lipoproteins after phospholipase A2 treatment

The immunoreactivity of high density lipoprotein (HDL) modified by treatment with porcine pancreatic phospholipase A2 (PLA2) was studied in a competitive radioimmunoassay using 6 different monoclonal apolipoprotein (apo) A-I antibodies. The competition tests have shown that after PLA2 treatment the immunoreactivity of selected epitopes of apo A-I changed in different ways. While the binding behavior of two epitopes remained unchanged, three epitopes exhibited decreased immunoreactivities after phospholipids hydrolysis. In contrast to the latter epitopes, the immunoreactivity of an epitope located on the cyanogen bromide fragment 4 of apo A-I increased with the degree of lipolysis. A loss of apo A-I from HDL as a consequence of PLA2-treatment did not occur as shown by the determination of the apo A-I concentration in HDL before and after treatment with PLA2. Using overlapped synthetic decapeptides it could be shown that the epitope increasingly exposed on the particle surface of PLA2-modified HDL consists of the amino acid residues 162-173 and 212-229. These residues are characterized by high hydrophobic indices as determined by hydropathy analysis. Furthermore, these regions belong partially to the proposed receptor-binding domain of apo A-I. Thus, an increased exposition of this epitope might result in elevated cellular binding affinities of HDL occurring after modification of lipoproteins by PLA2-treatment.

Characterization of a novel protein regulated during the critical period for song learning in the zebra finch

A male zebra finch learns a song by listening to a tutor, but song learning is normally restricted to a critical period in juvenile development. Here we identify an RNA whose expression in the song control circuit is altered during this critical period. The RNA encodes a soluble presynaptic protein that forms a predicted amphipathic alpha helix typical of the lipid-binding domain in apolipoproteins. We show this protein, which we call synelfin, to be the homolog of the human non-A beta component (and its precursor) recently purified from Alzheimer's disease amyloid. We suggest this highly conserved protein may serve a novel function critical to the regulation of vertebrate neural plasticity.

Modulation of cholesterol efflux from Fu5AH hepatoma cells by the apolipoprotein content of high density lipoprotein particles. Particles containing various proportions of apolipoproteins A-I and A-II

The influence of apolipoproteins (apo) A-I and A-II on the ability of high density lipoproteins (HDL) to remove cholesterol from cultured Fu5AH rat hepatoma cells was studied independently on alterations in the overall structure and lipid composition of the lipoprotein particles. To this end, apoA-I was progressively replaced by apoA-II in ultracentrifugally isolated HDL3 without inducing changes in other remaining lipoprotein components. As apoA-II was progressively substituted for apoA-I in HDL3 (A-II:A-I+A-II percentage mass: 29.5, 47.6, 71.5, 97.4, and 98.9%), the rate of cholesterol efflux from Fu5AH hepatoma gradually and significantly decreased after 2 or 4 h of incubation at 37 degrees C (cholesterol efflux: 30.4 +/- 0.8, 24.1 +/- 1.0, 19.8 +/- 1.2, 15.7 +/- 1.4, and 13.4 +/- 1.3%/2h, respectively; 38.4 +/- 1.5, 29.2 +/- 0.9, 27.0 +/- 0.2, 20.4 +/- 0.4, and 17.5 +/- 1.0%/4h, respectively) (p < 0.01 with all A-II-enriched HDL3 fractions as compared with non-enriched homologues). In agreement with data obtained with total HDL3, increasing the A-II:A-I+A-II percentage mass in HDL3 particles containing initially only apoA-I (HDL3-A-I) progressively reduced cellular cholesterol efflux. After 2 h of incubation, cholesterol efflux correlated negatively with A-II:A-I+A-II percentage mass (r = -0.86; p < 0.0001; n = 20), but not with either free cholesterol:phospholipid ratio, A-I+A-II:total lipid ratio or mean size of HDL3. As determined by using Spearman rank correlation analysis, the A-II:A-I+A-II% mass ratio correlated negatively with the apparent maximal efflux (Vmax(efflux)) (rho = -0.68; p < 0.05, n = 10), but not with the HDL3 concentration required to obtain 50% of maximal efflux (Km(efflux)) (rho = -0.08; not significant, n = 10). It was concluded that the apoA-I and apoA-II content of HDL3 is one determinant of its ability to promote cholesterol efflux from Fu5AH rat hepatoma cells.

Role of apolipoprotein A-I in cholesterol transfer between lipoproteins. Evidence for involvement of specific apoA-I domains

A series of monoclonal antibodies against epitopes spanning different domains of apoA-I have been tested for their effects on unesterified cholesterol transfer between low density lipoprotein (LDL) and well-defined homogenous lipoproteins reconstituted with phosphatidylcholine, cholesterol, and apoA-I (LpA-I). Antibodies 2G11 (reacting between residues 25 and 110), A05 (residues 25-82), A03 (residues 135-140), A44 and r5G9 (residues 149-186), and 4A12 (residues 173-205) significantly inhibit cholesterol transfer from LDL to Lp2A-I while they enhance transfer in the opposite direction, thus causing an increased net transfer to LDL. Most of these monoclonal antibodies (mAbs) also enhance phospholipid transfer to LDL but in a lesser and variable proportion relative to cholesterol. Their epitopes are mainly contained within domains that are predicted to be amphipathic alpha-helices. In contrast, mAbs 4H1 (residues 2-8), 3G10 (residues 96-121), and 5F6 (residues 116-141) have little or no effect on either cholesterol or phospholipid transfer, and the epitopes for these three mAbs have been shown in earlier studies to be structurally and functionally related. Their immunoreactivity responds similarly to variation in lipoprotein cholesterol content, and the antibodies binding to these sites compete with one another and have similar effects on the cholesterol esterification reaction. Thus, the current results are compatible with the hypothesis that they form an integrated domain with a common function in cholesterol metabolism, possibly as part of a hinge domain. Most mAbs were found to increase significantly the alpha-helicity of apoA-I in the Lp2A-I immunecomplexes, suggesting that they may increase the stability of the lipid-bound apoA-I. However, not unexpectedly, there is no correlation between the effects of mAbs on alpha-helicity and their effects on cholesterol or phospholipid transfer since each mAb has a discrete effect on these transfers. These studies demonstrate the specificity of LpA-I particles in cholesterol transport and document the existence of apoA-I domains with different functions in cholesterol transport.

Apolipoprotein A-I-mediated efflux of sterols from oxidized LDL-loaded macrophages

Although oxidized low-density lipoprotein (Ox-LDL) can accumulate in macrophages in vitro, generating cholesterol-loaded cells, little attention has been paid to the capacity of such macrophages loaded with OxLDL to export cholesterol and oxidized sterol moieties. In vitro lipid-loaded cells were generated by incubating primary cultures of mouse peritoneal macrophages with acetylated LDL (AcLDL) or OxLDL for 24 hours. The cellular content of native cholesterol, individual cholesteryl esters, and 7-ketocholesterol was determined by high-performance liquid chromatography. These cells were then incubated with medium containing apolipoprotein (apo) A-I and albumin or albumin alone for up to 24 hours; cholesterol and oxidized sterol efflux were measured both in terms of intracellular depletion and extracellular accumulation. Macrophages loaded with AcLDL accumulated cholesterol and large quantities of cholesteryl esters, whereas OxLDL-loaded cells accumulated cholesterol, a number of oxidized compounds (predominantly 7-ketocholesterol), and a relatively small quantity of cholesteryl esters. AcLDL-derived cells released approximately 50% of their total cholesterol (unesterified and esterified) to apo A-I-containing medium over 24 hours in the form of unesterified cholesterol, whereas OxLDL-derived cells released approximately 30% of their total cholesterol and 7% of their total content of 7-ketocholesterol over the same period. There was minimal efflux of any sterol in the absence of apo A-I. The proportions of cholesterol and 7-ketocholesterol released by either AcLDL- or OxLDL-loaded cells were not reduced by inhibiting cellular acyl-CoA:cholesterol acyl transferase using Sandoz 58-035, despite substantial alterations in the proportions of both free cholesterol and (in OxLDL-loaded cells) free 7-ketocholesterol in these cells.(ABSTRACT TRUNCATED AT 250 WORDS)

Analysis of the selective uptake of the cholesteryl ester of human intermediate density lipoproteins by HepG2 cells

We have recently shown by the analysis of the association to HepG2 cells of human intermediate density lipoproteins (IDL) either labeled in cholesteryl ester (CE) with [3H]cholesteryl ethers (CEt) or in proteins by iodine-125 that the CE of IDL are selectively taken up by these cells. Our results also revealed that the addition of a sufficient quantity of HDL3 abolishes the binding of IDL to the 'Lipoprotein binding site' (LBS) and also the CE-selective uptake. This suggested that the LBS mediates this uptake (Brissette and Falstrault (1992) Biochim. Biophys. Acta. 1165, 84-92). This study was undertaken to analyze further the mechanism of the selective uptake of the CE of IDL by HepG2 cells to determine if the LBS is directly or indirectly involved. We show that the different labeling had no effect on the binding affinity of IDL to HepG2 cells. To verify that the apolipoprotein moiety of HDL3 was responsible for the abolishment of the CE selective uptake, we have studied the effect of free apoA-I and apoA-II on the association of IDL. Our results demonstrate that apoA-I and apoA-II are approximately 10-times better than HDL3 or apoA-I liposomes in abolishing the selective uptake of the CE from IDL. We also show that this correlates with a more efficient reduction of the binding of 125I-IDL to HepG2 cells by free apoA-I compared to apoA-I associated with lipids. Thus free apoA-I interfere with the binding of IDL to the LBS and free apolipoproteins have a better capacity to saturate the LBS than lipoproteins. Also, we found no evidence for the transfer of CE from the labeled IDL to HDL3 or to apolipoproteins used to abolish the interaction of IDL to the LBS. Thus our results indicate that the LBS is directly responsible for the selective uptake of CE.

Structural domain of apolipoprotein A-I involved in its interaction with cells

Apolipoprotein A-I (apo A-I) is the major protein constituent of high-density lipoprotein (HDL), the lipoprotein fraction which mediates the reverse cholesterol transport. This apolipoprotein plays an important role in the binding of HDL to cells and participates in the efflux of cellular cholesterol. We have recently compared six different genetic variants of apo A-I and found that the apo A-I (Pro 165-->Arg) mutant is defective in promoting cellular cholesterol efflux from murine adipocytes and peritoneal macrophages and we have proposed that this region of apo A-I may be involved in their interaction with cells. To confirm this hypothesis, four monoclonal antibodies (mAbs) specific for apo A-I were used to study the inhibition of the interaction of palmitoyloleoylphosphatidylcholine (POPC): apoA-I complexes with HeLa cells and adipocytes. Among these antibodies, the apo A-I epitope recognized by the A44 mAb lies in the COOH terminal region (amino acid residues 149-186) including the proposed region. The antibodies A05, and A03 react with residues 25-82, 135-140, respectively and the A11 mAb corresponds to a discontinuous epitope at residues 99-105 and 126-132. Our results show clearly that the A44 and A05 mAbs reduce both the binding to HeLa cells and the cholesterol efflux from adipocytes. The inhibition of POPC: apoA-I complexes binding to both cell types is more strictly observed with the Fab fragments of monoclonal antibodies A44 and A05. Partial cotitration curves of these mAbs in a solid phase assay (RIA), indicated partial competition between these two antibodies. We propose a structural model for the POPC: apoA-I complexes where the N-terminal domain of one apo A-I molecule is in close spatial relationship with the C-terminal domain of the adjacent apo A-I molecule. We therefore suggest that the domain around amino acid 165 of apo A-I and which is recognized by mAb A44 (149-186) forms or contains some specific regions which mediate selectively the interaction with the binding site of cells and is involved in the efflux of cellular cholesterol.

Interaction of apolipoprotein AII with the putative high-density lipoprotein receptor

There is strong evidence to indicate that binding of HDL by cells is due to recognition of apoproteins residing on the surface of the lipoprotein by the putative HDL receptor(s). Although both of the major HDL apoproteins, AI and AII, are recognized by the putative receptor, the nature of the binding interaction and the domains of the apoproteins involved are largely unknown. Previous data from this laboratory led to the proposal of a model to explain how HDL particles containing AII interacted with the HDL receptor in a different manner as compared to HDL particles which contain apoAI but not apoAII [Vadiveloo, P. K., & Fidge, N. H. (1992) Biochem. J. 284, 145-151]. The model predicted that each chain of the apoAII homodimer contained a binding domain capable of interacting with the HDL receptor. This model was tested in the current study by preparing apoAII monomers, complexing them with phospholipid, and determining the ability of these complexes to bind to putative HDL receptors in rat liver plasma membranes (RLPM) and bovine aortic endothelial cell membranes (BAECM) by ligand blotting. The data showed that these complexes were bound by HB1 and HB2 from RLPM, and to the 110-kDa HDL binding protein from BAECM, providing critical evidence to support the model. Further investigation into the binding interaction revealed that apoAII complexed with phospholipid (apoAII-PC) bound more than delipidated apoAII, which bound more than delipidated apoAII monomers. Thus, optimum binding required the presence of lipid.(ABSTRACT TRUNCATED AT 250 WORDS)

Analysis of the binding and association of human intermediate density lipoproteins to HepG2 cells

The binding of human intermediate density lipoproteins (IDL) to HepG2 cells was studied. We found that human 125I-IDL interact with a binding site of high-affinity (Kd 0.74 micrograms/ml, Bmax 0.049 micrograms/mg cell protein) and a binding site of lower affinity (Kd 86.8 micrograms/ml; Bmax 0.53 micrograms/mg cell protein). The high-affinity binding sites show characteristics of LDL-receptors since they interact with IDL and low-density lipoproteins (LDL) and are calcium dependent. The low-affinity binding sites are calcium-independent and interact with IDL, LDL, high density lipoproteins-3 (HDL3), apolipoprotein (apo) E-liposomes, apoCs-liposomes, apoA-I-liposomes but not with liposomes containing albumin or erythrocyte membrane proteins. Therefore, HepG2 cells have on their surface a binding site that resembles or is identical to the lipoprotein binding site (LBS) that we found on rat liver membranes (Brissette and Noël (1986) J. Biol. Chem. 261, 6847-6852). Internalization, degradation and cholesterol ester selective uptake were determined in the presence or in the absence of a sufficient amount of human HDL3 to abolish the interaction of IDL to the LBS in order to obtain information on the function of this site. Our results suggest that the LBS participates in the internalization of IDL but not in their degradation and that it is responsible for the selective uptake of cholesterol esters of IDL.

Probucol increases the selective uptake of HDL cholesterol esters by Hep G2 human hepatoma cells

A previous study in rats showed that even though probucol substantially lowers high density lipoprotein (HDL) levels, near-normal mass transport of HDL cholesterol esters (CE) to the liver is maintained by the induction of "selective" (direct) uptake of HDL CE. The present study describes a parallel result in cultured Hep G2 human hepatoma cells. Cells were preincubated in the presence or absence of probucol before measuring the uptake of doubly labeled HDL3 in the absence of probucol. Preincubation with probucol decreased the uptake of HDL3 particles (iodine-125-labeled N-methyltyramine cellobiose-apolipoprotein [125I-NMTC-apo] A-I uptake) but increased the uptake of [3H]cholesteryl oleyl ether in excess of 125I-NMTC-apo A-I (i.e., selective uptake) in a dose-dependent fashion. The reversibly cell-associated pool of CE tracer, a precursor for selective uptake, enlarged on probucol treatment, but the increase was not in proportion to the increase in selective uptake. HDL3 particle uptake decreased on probucol treatment. The decrease was evident after less than 20 minutes of probucol exposure and was maximal after 6 hours; in contrast, HDL3 CE selective uptake increased only after greater than 13 hours and had not reached a plateau after 20 hours. Thus, effects on particle uptake and selective uptake were dissociated in time.