Apolipoprotein-B subclasses as acceptors of cholesteryl esters transferred by CETP

BACKGROUND

Five apolipoprotein (apo)-defined apoB-containing lipoprotein (Lp) subclasses designated LpB, LpB:C, LpB:E, LpB:C:E and LpA-II:B:C:D:E are present in human plasma. This study was to determine whether these subclasses functioned equally as acceptors of cholesteryl esters (CE) transferred from high-density lipoproteins (HDL) by CE transfer protein in healthy subjects with normal and mildly increased plasma triglyceride (TG) levels.

MATERIALS AND METHODS

After 4 h incubation of plasma from 14 subjects at 37 degrees C, apoB-containing lipoproteins were separated from HDL by heparin-Mn++ precipitation and fractionated by immunochemical methods into these five subclasses. The neutral lipid (NL) composition for each subclass was measured by gas chromatography (GC) and compared between 0 h and 4 h. A subclass was considered to be a CE acceptor if its CE content increased more than 5% at 4 h and a non-acceptor if no change was observed.

RESULTS

Employing the above definition, TG-rich LpB:C and LpB:E + LpB:C:E functioned as CE acceptors and TG-poor LpB and LpA-II:B:C:D:E as non-acceptors. Both LpB:C and LpB:E + LpB:C:E could only actively accept CE as long as they retained their TG-rich character and displayed neutral lipid profiles similar to those of very low-density lipoproteins (VLDL) and intermediate density lipoproteins (IDL). When, as a result of lipolysis their TG content dropped below 25%, they ceased to function as CE acceptors. In subjects with elevated plasma TG, LpB:C was the dominant CE acceptor, a condition that may have pro-atherogenic consequences.

CONCLUSIONS

Among the apoB-containing particles, LpB:C and LpB:C:E + LpB:E functioned as CE acceptors while LpB and LpA-II:B:C:D:E did not.

Accelerated cholesteryl ester transfer in patients with essential hypertension and the effect of ramipril treatment

Although the transfer of cholesteryl ester (CE) from high-density lipoprotein (HDL) to the apolipoprotein B-containing lipoproteins (very-low-density lipoproteins + low-density lipoproteins) has been shown to be abnormally increased in a number of conditions associated with increased cardiovascular risk, it has not been studied in patients with essential hypertension (EH). To determine whether subjects with EH have increased CE transport, CE transfer (CET) was estimated isotopically and lipoprotein lipid and phospholipid composition determined in a group of 14 untreated normolipidemic (triglycerides 116+/-46, cholesterol 185+/-30, HDL 38+/-10 mg/dl) otherwise healthy ethnically diverse EH subjects. CET was significantly increased in EH subjects compared to a similar group of normotensive controls (EH: k = 0.27+/- 0.09 vs. control k = 0.11+/-0.02: P < 0.01). Lipoprotein concentration and composition were comparable in the two groups and closely resembled that of an age- and sex-matched reference group. The abnormal increase in CET persisted (k = 0.25+/-0.12) after 3 months of treatment with the angiotensin converting enzyme (ACE) inhibitor ramipril without a change in either plasma or lipoprotein lipids. Thus, CET is increased in normolipidemic subjects with EH and is not affected by the ACE inhibitor ramipril.

Effect of thyroid hormone on plasma apolipoproteins and apoA- and apoB-containing lipoprotein particles

METHODS

Apolipoprotein and apoB- and apoA-containing lipoprotein particle concentrations were determined in 10 athyreotic patients 4 weeks after withdrawal of Synthroid replacement therapy [T4, 0.96 +/- 0.66 microgram mL-1; thyroid-stimulating hormone (TSH), 62.7 +/- 22.8 muIU mL-1] and again 4 weeks after reinstitution of treatment.

RESULTS

Thyroid hormone replacement was associated with significant decreases in plasma cholesterol (TC), triglyceride (TG), apolipoprotein B and apolipoprotein C-III (P < 0.01). Both the cholesterol ester-rich LP-B particles and triglyceride-rich LP-Bc particles declined significantly in response to thyroid hormone (LP-B withdrawal 81.6 +/- 24.0 vs. replacement 65.1 +/- 22.0; LP-Bc withdrawal 14.3 +/- 6.0 vs. replacement 10.9 +/- 4.8 mg%, P < 0.01). ApoC-III also decreased in high-density lipoprotein (HDL) (apoC-III-HS), and in very low-density lipoprotein (VLDL) + low-density lipoprotein (LDL) (apoC-III-HP), but this reduction was proportionate so that the apo-C-III-HS/apoC-III-HP ratio, an indirect estimate of the efficiency of lipoprotein lipase (LPL), was unchanged. Apolipoprotein A-I concentrations also decreased significantly (withdrawal 140.7 +/- 27.0 vs. replacement 127.1 +/- 30.0 mg%, P < 0.01) in parallel with the changes in LP-A-I and LP-A-I:A-II particles (LP-A-I withdrawal 35.8 +/- 7.7 vs. replacement 31.5 +/- 6.3; LP-A-I:AII withdrawal 104.9 +/- 20.0 vs. replacement 95.5 +/- 26.0; P < 0.05).

CONCLUSION

These findings indicate that thyroid hormone influences the transport not only of both TG-rich and cholesterol-rich apoB-containing lipoprotein particles but also of those that contain apoAI.

Alterations in cholesteryl ester transfer, lipoprotein lipase, and lipoprotein composition after combined pancreas-kidney transplantation

IDDM patients treated with conventional subcutaneous insulin have an abnormal increase in cholesteryl ester transfer (CET), the proatherogenic step in reverse-cholesterol transport that results in the enrichment of the apolipoprotein (apo) B-containing lipoproteins (VLDL, LDL) with cholesteryl ester (CE). This disturbance is closely linked to iatrogenic hyperinsulinemia and the nonphysiologic stimulation of lipoprotein lipase (LpL), a physiologic activator of CET, because lowering systemic insulin levels by administering insulin through the intraperitoneal insulin route normalizes LpL and CET. Hyperinsulinemia persists in IDDM patients who undergo successful pancreas-kidney transplantation (PKT) when their allografts are placed in the pelvis and drain into the iliac vein. Therefore, to determine whether hyperinsulinemia promotes CET in this setting, we studied CET, LpL, and insulin levels in 14 euglycemic normolipidemic IDDM PKT patients with near-normal kidney function (creatinine 1.5 +/- 0.4 mg/dl). Consistent with our prediction, the net mass of CE transferred from HDL to VLDL + LDL was significantly increased in the PKT group (P < 0.001) compared with nondiabetic renal transplant patients receiving the same immunosuppressive drugs and healthy control subjects. Both basal and arginine-stimulated insulin levels were increased above the kidney transplant group's levels and correlated with the mass of CE transferred at 2 h (r = 0.71, P < 0.05; r = 0.66, P < 0.05, respectively). Total basal LpL activities, LpL and hepatic triacylglycerol lipase activities, and LpL mass all tended to be higher than levels in healthy control subjects. Consistent with these changes in lipase activity, VLDL particle size was significantly reduced (P < 0.025) compared with that of control subjects. These findings indicate that PKT patients with systemically draining allografts have a persisting profile of potentially atherogenic disturbances in insulin levels, LpL, and CET that resemble IDDM patients treated with conventional subcutaneous insulin injections.

Lack of effect of vitamin E on cholesteryl ester transfer and lipoprotein composition in cholesterol-fed rabbits

The concentration and activity of cholesteryl ester transfer protein (CETP) is increased in plasma in hypercholesterolemic humans and in experimental animals fed cholesterol. While the concentration of lipo-proteins appears to be the major determinant of CETP activity, we have found previously that dietary measures and pharmacologic agents that alter their lipid composition reduce the activity of CETP in plasma (CET). Since vitamin E is lipophilic and is incorporated into lipoproteins, we have examined the question of whether it too attenuates CET in cholesterol-fed New Zealand White rabbits prior to and 14 weeks after treatment with differing doses (5, 15, 30, 45 mg/kg) of vitamin E. Plasma triglycerides (TG), cholesterol (TC) and phospholipids (Lys, Sph, Lec, PI, PE) all increased significantly to a comparable degree in the rabbits fed cholesterol compared to those fed chow (p < 0.05; p < 0.01); the levels achieved were similar in the vitamin E-treated and untreated groups. As was observed with plasma lipids, cholesteryl ester transfer (CET) was accelerated to the same degree in each of the cholesterol-fed groups independent of whether they received vitamin E compared to chow-fed controls (p < 0.01) and the distribution of cholesterol in apo-B containing lipoproteins (VLDL, IDL, and LDL) was similar in the vitamin E-treated and untreated groups. These findings indicate that vitamin E has no discernible effect on CET when cholesterol levels are markedly elevated.

Effects of short-term hormone replacement therapies on low-density lipoprotein metabolism in cynomologus monkeys

Estrogen replacement therapy reduces the risk of coronary heart disease in women and decreases the extent of atherosclerosis in monkeys. In our previous studies, estrogen treatment decreased arterial LDL degradation and accumulation, thus indicating one mechanism by which estrogen inhibits the progression of atherosclerosis. The influence of progestins on these processes remains nuclear. The objective of this study was to determine the effects of oral estrogen (conjugated equine estrogens) and progestin (medroxyprogesterone acetate) alone or in combination on arterial LDL metabolism after 12 weeks of atherogenic stimulus. This relatively short period of treatment was chosen to determine effects on arterial LDL metabolism before substantial subendothelial macrophage accumulation. In contrast to previous studies (16 to 18 weeks of treatment), when macrophages were present in the intima, neither estrogen nor progestin (nor their combination) had any effect on any index of arterial LDL metabolism. These results suggest that estrogen may preferentially reduce LDL metabolism in macrophages with little effect on cells of the normal artery. In contrast to arterial LDL metabolism, hepatic LDL uptake was significantly increased in animals treated with estrogen or estrogen plus progestin. Despite the increased LDL uptake by the liver, hepatic lipid content was significantly decreased by approximately 50% in both estrogen and estrogen-plus-progestin treatment compared with control and progestin-treated animals. The decrease in hepatic cholesterol content in hypothesized to be due to increased biliary secretion of cholesterol.

Effects of multiple daily insulin injections and intraperitoneal insulin therapy on cholesteryl ester transfer and lipoprotein lipase activities in NIDDM

Although the relationship between the actions of cholesteryl ester transfer protein (CETP) and atherosclerosis is complex, a strong body of evidence suggests that its activity (cholesteryl ester transfer [CET]) is proatherogenic. We have previously shown that CET is increased in IDDM patients receiving conventional subcutaneous insulin treatment and normalized when systemic insulin levels are lowered with intraperitoneal insulin delivery (IP). Since CET has been found by many observers to also be accelerated in NIDDM, we sought to determine whether the same salutary effect could be achieved in insulin-requiring NIDDM men before and 7 months after randomization to an intensive treatment regimen (Rx) of either IP (n = 9) or multiple daily insulin injections (MDI; n = 13). HbA1c improved to the same degree in both groups (MDI group: 9.4 +/- 1.1% pre-Rx vs. 7.2 +/- 0.7% post-Rx [P < 0.001]; IP group: 9.2 +/- 1.3% pre-Rx vs. 7.1 +/- 0.5% post-Rx [P < 0.001]). Compared with pre-Rx levels, plasma triglycerides were not significantly changed by either treatment (MDI group: 136 +/- 80 mg/dl pre-Rx vs. 139 +/- 87 mg/dl post-Rx; IP group: 157 +/- 63 mg/dl pre-Rx vs. 188 +/- 89 mg/dl post-Rx), though an upward trend followed IP. Before randomization, CET estimated with both mass and isotopic assays was greater in the NIDDM subjects than in nondiabetic control subjects (P < 0.001). With improved glycemic control, CE mass transfer declined in both groups, but only reached normal levels in the IP group (MDI group at 2 h: 49.0 +/- 13.7 [mean +/- SD] pg pre-Rx vs. 29.5 +/- 15.3 microg post-Rx [-39.7%, P < 0.01]; IP group at 2 h: 40.8 +/- 23.3 microg pre-Rx vs. 10.9 +/- 6.5 microg post-Rx [-73.2%, P < 0.05]) and remained abnormally increased (P < 0.005) in the subjects receiving MDI. Total lipolytic activity after intensive treatment was unchanged from pretreatment levels, which were similar to those of the reference group. Although directional changes in lipoprotein lipase (LpL) and hepatic triglyceride lipase (HTGL) similar to those found in IDDM after MDI and IP were observed, they were not statistically significant. Thus, while improved glycemic control alone achieved by either MDI or IP reduced the pathological increase in CET in these insulin-treated NIDDM men, normalization was only achieved in those treated with IP. Despite near-normal HbA1c levels, CET remained abnormally increased in NIDDM patients treated rigorously with conventional subcutaneous insulin delivery.

Improved lipoprotein surface and core lipid composition following intraperitoneal insulin delivery in insulin-dependent diabetes mellitus

To determine whether insulin delivered into portal circulation by an implanted pump reversed abnormalities in lipoprotein composition in insulin-dependent diabetes mellitus, 10 well-controlled normolipidaemic patients were studied after conventional intensive sub-cutaneous (ISC) insulin management and then 3 and 6 months after intraperitoneal pump implantation (IP). Glycated haemoglobin (ISC: 6.9 +/- 1.7% vs. IP-3 months 6.3 +/- 0.7; IP-6 months 6.3 +/- 0.8; mean +/- S.D.), plasma triglyceride and cholesterol levels, and the cholesterol content of HDL2 and HDL3 were normal and not significantly changed during these treatments. Fasting free insulin concentrations measured before and after 6 months of IP fell by more than half (ISC 8.22 +/- 6.5 vs IP 2.77 +/- 2.4 mU/ml; p < 0.025). The plasma-free cholesterol/lecithin ratio, a potential new cardiovascular risk factor, was increased during ISC, declined progressively after 3 months of IP, and approached normal by 6 months (ISC 0.96 +/- 0.37 mol/mol vs. IP-3 months 0.91 +/- 0.34; IP 6 months 0.86 +/- 0.10; reference group 0.83 +/- 0.33). In all lipoprotein fractions, sphingomyelin concentrations tended to fall, and lecithin concentrations to rise progressively during IP. As a result, the sphingomyelin/lecithin ratio, an index of the surface rigidity of lipoproteins, declined. The fact that some of the compositional modifications associated with ISC were reversed when insulin was administered intraperitoneally suggests that they may have been iatrogenic and resulted from high systemic insulin levels.

Differing effects of pancreas-kidney transplantation with systemic versus portal venous drainage on cholesteryl ester transfer in IDDM subjects

OBJECTIVE

Cholesteryl ester transfer (CET) is accelerated in patients with IDDM treated with conventional (subcutaneous) insulin therapy (CIT) and a number of other disorders associated with premature cardiovascular disease. We have shown that in IDDM this disturbance is closely linked to iatrogenic hyperinsulinemia (HI), because it was reversed when insulin was administered by the intraportal (i.p.) route. In this study, we sought to determine whether HI after successful pancreas-kidney transplantation (PKT) has the same adverse effect on CET.

RESEARCH DESIGN AND METHODS

CET was measured by both mass and isotopic assays and compared in two groups of euglycemic non-insulin-requiring IDDM PKT patients with either systemically draining allografts and persistent HI or grafts with portal vein anastomoses that were normoinsulinemic (PK-P). A third group of eight nondiabetic kidney transplant (KT) patients receiving the same immunosuppressive drugs served as control subjects.

RESULTS

CET in pancreas-kidney transplantation subjects with systemic venous drainage (PK-S) was increased (P < 0.001) to the same level we have reported previously in IDDM patients receiving CIT and was significantly higher (P < 0.001) than in those subjects with PK-P. CET in the PK-P group did not differ from that of the KT control patients.

CONCLUSIONS

CET is affected by variations in systemic insulin levels in pancreas transplant patients with allografts that have differing venous drainage. Because high systemic insulin levels are linked to the activation of (ET, euglycemic HI IDDM pancreas allograft recipients may continue to be at high risk for macrovascular complications.

Reduced cholesteryl ester transfer in plasma of patients with lipoprotein lipase deficiency

The net mass transfer of cholesteryl ester (CE) from high density lipoprotein (HDL) to the apolipoprotein (apo) B-containing lipoproteins, very low density lipoprotein (VLDL) and low density lipoprotein (LDL) in plasma (cholesteryl ester transfer (CET)) from three patients lacking lipoprotein lipase (LpL) activity was significantly lower (P < 0.001) than in plasma from fasting control subjects with comparable triglyceride levels. Chylomicrons isolated from LpL-deficient fasting plasma showed the same low level of CET activity as observed in the intact plasma when combined with HDL and cholesteryl ester transfer protein (CETP)-containing d 1.063 g/ml bottom fractions from control subjects. Preincubation of chylomicrons and large triglyceride-rich lipoproteins (Sf > 400) from LpL-deficient plasma with milk LpL, however, stimulated the capacity to engage in CET 4- to 5-fold to the same level as chylomicrons and VLDL from control subjects after a fat load. Consistent with these measurements of CET activity in plasma, chylomicrons obtained from the LpL-deficient subjects after a 14-h fast had higher TG/CE ratios than chylomicrons from controls 3 h after ingesting a fat load (LpL-deficient 26.3 +/- 9.0 vs. controls 6.9 +/- 2.1; mean +/- SD). The mass of CETP did not differ in LpL-deficient and control subjects (LpL-deficient 1.03 +/- 0.22 micrograms/ml vs. controls 1.58 +/- 0.58 micrograms/ml). These studies are consistent with earlier in vitro studies showing that the actions of lipoprotein lipase and its lipolytic products are essential, for maximal cholesteryl ester transfer protein activity.

Marine lipids normalize cholesteryl ester transfer in IDDM

Patients with insulin-dependent diabetes mellitus (IDDM) have a pathological increase in cholesteryl ester transfer (CET) that enriches the apolipoprotein B-containing lipoproteins with cholesteryl ester and increases their atherogenicity. Since we have shown earlier that omega-3 (n-3) fatty acids present in marine lipids normalize both CET and lipoprotein composition in non-diabetic patients with hypercholesterolaemia, we sought to determine whether the same beneficial effects could be achieved in nine normolipidaemic (triglycerides 1.10; cholesterol 4.94, high density lipoprotein 1.10 mmol/l) IDDM patients (fructosamine 424 +/- 156; normal 174-286 mumol/l) treated for 2 months with n-3 fatty acids (4.6 g/day). Before treatment, CET measured by both mass and isotopic assays was abnormally accelerated (p < 0.001). While marine lipids modestly decreased triglyceride levels (-14%; p < 0.05 ), CET fell dramatically in all subjects (mass assay: -97% at 1 h; isotopic assay: -58%; p < 0.001) to below control levels with no change in glycaemic control (fructosamine 408 +/- 103 mumol/l). The mass of cholesteryl ester transfer protein paradoxically increased significantly (pre-treatment: 2.04 +/- 0.86 vs post-treatment 2.48 +/- 0.97 micrograms/ml; p < 0.05). Since it is believed that accelerated CET promotes the formation of atherogenic cholesteryl ester-enriched apo B-containing lipoproteins, the capacity of marine lipids to reverse this functional abnormality without altering glycaemic control suggests that these agents may have an adjunctive role to play in the nutritional therapy of IDDM.

Increased glycation of plasma lipoproteins in diabetic cynomolgus monkeys

Hyperglycemia associated with diabetes mellitus increases glycation of hemoglobin and serum proteins in human and nonhuman primates. It also has been documented that numerous other circulating proteins may be glycated. In this study we found that most of the major subclasses of lipoproteins (low-density, very low-density, and high-density) from diabetic cynomolgus monkeys were significantly more glycated than were lipoproteins from age- and sex-matched controls. Correlations between levels of glycemic control and glycation of lipoproteins also were significant. Because glycation of lipoproteins has been shown to affect their normal metabolism, this animal model may be useful in determining the interaction between lipoproteins, diabetes mellitus, and the increased risk of atherosclerosis.

Effects of hydrochlorothiazide and captopril on lipoprotein lipid composition in patients with essential hypertension

OBJECTIVE

Effective antihypertensive agents may differ in their capacity to reduce cardiovascular risk because they induce potentially atherogenic alterations in lipoprotein composition.

PATIENTS

To assess this possibility, the effects of five months' treatment with either hydrochlorothiazide (HCTZ) or the converting enzyme inhibitor captopril (CAPT) on lipoprotein lipid composition were compared in thirty normolipidaemic patients with essential hypertension (EH).

RESULTS

The sixteen patients treated with HCTZ showed the expected directional alterations in plasma TG (+31%), HDL2-C (-16%), and CHOL (+7.6%); in contrast TG and CHOL were unchanged after captopril in fourteen patients and their HDL2-C declined (-16%). Neither drug altered lipoprotein core lipid composition, but small increases were observed in the HDL2 sphingomyelin/lecithin ratio after both agents. The plasma free (unesterified) cholesterol (FC) lecithin (L) ratio, a new index of cardiovascular risk, was abnormally increased before treatment and was not altered by either drug.

CONCLUSION

These findings indicate that HCTZ and CAPT treatment have small, but demonstrable effects on lipoprotein surface lipid composition in patients with EH that are confined to the HDL2 subfraction.

The effects of hypothyroidism and replacement therapy on cholesteryl ester transfer

To characterize further the impact of thyroid dysfunction on the transport of cholesterol in plasma, we studied plasma lipids and cholesteryl ester transfer (CET) in 10 hypothyroid women before and 3 months after thyroid replacement therapy. CET, estimated as the net mass transfer of CE from HDL to the apolipoprotein B-containing lipoproteins (very low density and low density lipoproteins) was significantly decreased at 4 h (P < 0.05) and 6 h (P < 0.001) when the patients were hypothyroid (T4, 2.01 +/- 1.4; TSH, 55.5 +/- 39.9 microIU/mL) and increased to normal levels after hormone replacement and restoration of eumetabolism. Plasma lipid levels in the hypothyroid state closely resembled those in a female reference group, although total plasma cholesterol fell significantly [pretreatment, 218 +/- 36 vs. posttreatment, 192 +/- 49 (P < 0.025); control, 218 +/- 28 mg/dL (mean +/- SD)] after treatment. Concentrations of cholesteryl ester transfer protein (CETP) were unchanged (pretreatment, 2.35 +/- 0.83 vs. posttreatment, 2.30 +/- 1.19 mg/dL). The results of recombination studies using different lipoprotein fractions suggest that decreases in CET during hypothyroidism may be secondary to acceptor lipoprotein (low density and very low density lipoprotein) changes in the hypothyroid state and not to changes in the concentration of CETP itself.

Changes in high density lipoprotein subfraction lipids during neutral lipid transfer in healthy subjects and in patients with insulin-dependent diabetes mellitus

While it is known that the transfer of cholesteryl ester (CE) from high density lipoprotein (HDL) to the apo B-containing lipoproteins is increased in patients with diabetes, the extent to which the various lipoprotein fractions engage in neutral lipid exchange and the magnitude to which triglyceride (TG) is translocated is not known. To examine in greater detail neutral lipid net mass transfer in diabetes, the HDL subfractions and the apo B-containing lipoproteins were separated, and the net mass transfer of CE and TG was compared to that of control subjects. In both groups, bidirectional transfer of CE from HDL3 to very low density lipoprotein (VLDL) + low density lipoprotein (LDL) and of TG from VLDL + LDL to HDL3, took place, but this process was significantly greater (P < .01) in insulin-dependent diabetes mellitus (IDDM). In contrast, CE and TG accumulated in HDL2 to a similar degree in normal and IDDM subjects. In recombination experiments with each of the apo B-containing lipoproteins, IDDM VLDL had a greater capacity to facilitate the exchange of core lipids from both IDDM and control HDL3: on the other hand, LDL from IDDM and control subjects both donated TG and CE to HDL2 and affected little change in HDL3. These findings indicate that all the major plasma fractions normally participate in the trafficking of CE and TG among the lipoproteins during neutral lipid transfer and show that the principal perturbation in cholesteryl ester transfer in IDDM involves altered interaction between VLDL and the HDL3 subfraction.

Neutral lipid mass transfer among lipoproteins in plasma from normolipidemic subjects is not an equimolar heteroexchange

To further characterize the cholesteryl ester transfer protein (CETP)-mediated distribution of neutral lipids that occurs among lipoproteins in plasma, the net mass transfer of core lipids between donor and acceptor lipoproteins in intact plasma was measured in ten healthy normolipidemic subjects. The rate of loss of cholesteryl ester (CE) from high density lipoprotein-3 (HDL3) (19.5 +/- 8.8 nmol/ml per h) was linear and increased significantly (P < 0.01) during the 6-h incubation. Approximately 50% of the CE transferred from HDL3 (118.7 +/- 54.3 nmol/ml) went to very low density lipoprotein (VLDL); the remainder was distributed to low density lipoprotein (LDL) (approximately 30%) and HDL2 (approximately 20%). The rate of loss of triglyceride (TG) from VLDL (14.5 +/- 6.6 nmol/ml per h) to the HDL subfractions and LDL also was linear and increased significantly with time (P < 0.01). About 50% of the TG mass lost from VLDL (85.2 +/- 38.4 nmol/ml) was transferred to LDL and the remainder was recovered in HDL2 (approximately 10%) and HDL3 (approximately 40%). As the number of nmoles of CE lost from HDL3 was almost three times greater than the nmoles of TG it acquired, these findings indicate that the exchange of core lipids in plasma that result from the interaction between CETP-VLDL-HDL3 is not equimolar. Even in the absence of VLDL, HDL3 continued to donate CE to LDL and HDL2 to almost the same degree as in intact plasma (plasma minus VLDL: 17.5 +/- 5.9 nmol/ml per h vs. intact plasma: 20.2 +/- 7.5 nmol/ml per h) without accepting any TG. Our findings demonstrate that independent pathways exist for the transfer of CE and TG among the plasma lipoproteins and, contrary to what is generally believed, a heteroexchange of TG for CE during cholesteryl ester transfer is not obligatory.

Abnormal lipoprotein phospholipid composition in patients with essential hypertension

Vascular cell membranes from patients with essential hypertension (EH) and animals with genetic forms of hypertension have been found to have alterations in the content of free cholesterol and negatively charged phospholipids that may modify their function. Since membrane and lipoprotein lipids exchange freely, the lipid composition of lipoproteins may be an indirect measure of the content of vascular and other cells. To determine whether abnormalities are present in the lipid and phospholipid composition of lipoproteins from patients with EH, 30 EH (11 women; 19 men) and 20 normotensive control subjects were studied. Since significant gender differences were present in a number of parameters of lipoprotein composition, male and female data were examined separately. The EH group of both sexes tended to have higher plasma TG and VLDL + LDL and HDL2 lipid levels than their respective controls. Not only were the calcium-binding phospholipids phosphatidylinositol (PI) + phosphatidylserine (PS), and the membrane fluidizer phosphatidylethanolamine (PE) were significantly reduced in their VLDL + LDL, but all phospholipids (L, sphingomyelin (SPH), PI + PS, and PE) were significantly reduced in their neutral lipid content in both the HDL2 and HDL3 subfractions. These directional changes in lipoprotein FC and phospholipid in the EH women significantly increased the EH FC/PC (mol/mol) ratio in their plasma, a new cardiovascular risk factor, (EH 1.08 +/- 0.22 vs. control 0.86 +/0 0.08; P < 0.01) and lowered the SPH/PC ratio HDL2 and HDL3 in EH patients of both sexes. These findings showed that lipoproteins in normolipidemic EH women are relatively enriched in FC and in EH patients of both sexes depleted in certain phospholipids lacking in lipoproteins, their functional properties could be altered and vascular tone increased.

Probucol normalizes cholesteryl ester transfer in IDDM

OBJECTIVE

Cholesteryl ester transfer (CET) is pathologically increased in insulin-dependent diabetes mellitus (IDDM), and the resulting enrichment of the apolipoprotein B-containing lipoproteins with cholesteryl ester (CE) is believed to increase their atherogenicity. Because we have shown previously that treatment with the lipid-modifying antioxidant probucol normalizes CET in nondiabetic patients with hypercholesterolemia, we sought to determine whether the same beneficial effects could be achieved in IDDM.

RESEARCH DESIGN AND METHODS

CET was measured by both mass and isotopic assay in eight normolipidemic (triglyceride, 102; cholesterol, 192; high-density lipoprotein [HDL] cholesterol, 45 mg/dl) IDDM patients (fructosamine 495 +/- 146 mumol/l; normal 174-286) before and after 2 months of treatment with probucol (1.0 g/day).

RESULTS

Before treatment, CET was accelerated abnormally (P < 0.001). As expected, probucol decreased plasma (-13%; P < 0.025) and HDL2 cholesterol levels (-52%; P < 0.025) and the concentration of lipoprotein A-I particles (P < 0.025). In conjunction with these changes, CET fell dramatically in all subjects (mass assay: -94%; isotopic assay: -22%, P < 0.001) with no change in the mass of cholesteryl ester transfer protein (CETP) (pretreatment 2.91 +/- 0.97 vs. posttreatment 3.21 +/- 1.03 micrograms/ml). Glycemic control, however, improved significantly (fructosamine 409 +/- 85 mumol/l, P < 0.025).

CONCLUSIONS

Because it is believed that accelerated CET promotes the formation of apolipoprotein B-containing lipoproteins enriched with atherogenic CE, the capacity of probucol to reverse this functional abnormality without adversely affecting glycemic control suggests that it has a place in the therapy of IDDM.

Specificity of lecithin:cholesterol acyltransferase and atherogenic risk: comparative studies on the plasma composition and in vitro synthesis of cholesteryl esters in 14 vertebrate species

To determine whether the specificity of lecithin: cholesterol acyltransferase (LCAT) influences the susceptibility to atherosclerosis, we compared the composition and in vitro synthesis of cholesteryl ester (CE) in the plasmas of 14 vertebrate species with varying predisposition to atherosclerosis. The susceptible species (Group I) had significantly higher ratios of 16:0 CE/20:4 CE in their plasma than the resistant species (Group II). The in vitro formation of labeled CE species in native plasma from labeled cholesterol correlated highly with the mass composition, showing that the LCAT reaction is the predominant source of plasma CE in all the animal species examined. Isolated LCATs from Group I species also synthesized CE with higher ratios of 16:0/20:4 than LCATs from Group II when egg phosphatidylcholine (PC) was used as the acyl donor. In addition, the Group I LCATs exhibited lower specificity towards sn-2-20:4 and sn-2-22:6 PCs, and higher specificity towards sn-2-18:2 PC species than Group II LCATs. With 16:0-20:4 PC as the substrate, all Group I LCATs synthesized more 16:0 CE than 20:4 CE, whereas all Group II LCATs, with the exception of dog enzyme, synthesized predominantly 20:4 CE, showing that the two types of LCAT have different positional specificities towards this PC. These results suggest that there are two classes of LCAT in nature that differ from each other in their substrate and positional specificities, possibly because of differences in their active-site architectures. We propose that the presence of one type of LCAT, which cannot efficiently transfer certain long chain polyunsaturated acyl groups and which consequently synthesizes more saturated CE, may increase the risk of atherosclerosis.

Accelerated cholesteryl ester transfer and altered lipoprotein composition in diabetic cynomolgus monkeys

To determine whether nonhuman primates demonstrate the same alterations in transport of cholesteryl ester (CE) in plasma observed in diabetic humans, cholesteryl ester transfer (CET) was measured in cynomolgus monkeys with chronic spontaneous diabetes mellitus (glycated hemoglobin: diabetics 10.7 +/- 4.1%; controls 3.8 +/- 0.8%, P < 0.005). Among the plasma lipids, only triglycerides were significantly increased in diabetic monkeys (diabetics 303 +/- 294 mg/dl; controls 85 +/- 34 mg/dl; P < 0.05); total plasma, LDL, HDL2, and HDL3 cholesterol concentrations did not differ significantly from those of control animals. Similar to human beings with insulin-dependent and non-insulin-dependent diabetes mellitus, CET estimated both as the mass of cholesteryl ester transferred from HDL to the apoB-containing lipoproteins (VLDL + LDL) and as the loss of radiolabeled cholesteryl ester from HDL was significantly greater (P < 0.001) in diabetic compared to control monkeys. Glycated hemoglobin levels in the combined control and diabetic groups correlated directly with both the mass of cholesteryl ester transferred at 2 h (r = 0.75; P < 0.001) and the isotopic transfer (k) (r = 0.64; P < 0.005). The mass of cholesteryl ester transfer protein (CETP) tended to be higher in the diabetic animals (diabetic 4.06 +/- 0.73 microgram/ml versus control 3.05 +/- 0.93; P < 0.1).(ABSTRACT TRUNCATED AT 250 WORDS)

Accelerated cholesteryl ester transfer in baboons with insulin-requiring diabetes mellitus

Cholesteryl ester transfer (CET), plasma, lipoprotein lipid and phospholipid composition were studied in insulin-treated baboons with chronic streptozotocin-induced diabetes. In these diabetic animals, CET measured both as the mass (p < 0.001) and isotopic transfer (p < 0.05) of CE from HDL to the apo B-containing lipoproteins (VLDL+LDL) were significantly accelerated compared to controls and the response closely resembled that recently reported in diabetic humans. No significant differences were present in plasma triglyceride, cholesterol, or HDL-C or in lipoprotein core or surface lipid composition. Thus, despite the fact that they did not display the same spectrum of abnormalities in lipoprotein composition, these insulin-treated diabetic baboons demonstrated an abnormality in CET identical to that described in humans. These findings suggest that this non-human primate may provide a suitable diabetic animal model in which to better characterize the mechanisms that underlie this potentially atherogenic disturbance in lipoprotein transport.

Intraperitoneal insulin therapy corrects abnormalities in cholesteryl ester transfer and lipoprotein lipase activities in insulin-dependent diabetes mellitus

Patients with insulin-dependent diabetes mellitus (IDDM) have proatherogenic disturbances in cholesteryl ester transfer (CET) despite intensive subcutaneous insulin therapy (ISC). Since CET is activated by insulin-sensitive lipoprotein lipase (LPL), which normally increases postprandially, we queried whether iatrogenic hyperinsulinism from ISC stimulated LPL and CET by studying well-controlled IDDM patients after ISC and then 6 months after lowering systemic insulin levels by intraperitoneal (IP) insulin delivery. Although glycemic control (HbA1c IDDM, 6.9 +/- 1.7%; control, 4.5% to 8%) was excellent during ISC, CET was accelerated (P < .001) and both systemic insulin levels and LPL specific activity were increased (P < .05). Following IP, basal systemic insulin levels declined by more than one half (ISC, 8.22 +/- 6.5 versus IP, 2.77 +/- 2.4 microU/mL; mean +/- SD; P < .025), and both LPL and CET activities returned to normal. Plasma triglyceride, cholesterol, high-density lipoprotein-2 (HDL2) cholesterol, HDL3 cholesterol, cholesteryl ester transfer protein mass, and glycemic control (HbA1c, 6.3 +/- 0.8%) were unchanged and remained normal. These findings indicate that ISC is associated with high levels of basal CET and LPL. These alterations both appear to be closely linked to iatrogenic hyperinsulinemia resulting from ISC. The fact that they are both reversed when systemic insulin levels are reduced by IP suggests that insulin, acting through LPL, influences the nature of the interaction of the lipoproteins engaged in CET.(ABSTRACT TRUNCATED AT 250 WORDS)

Contribution of glycaemic control, endogenous lipoproteins and cholesteryl ester transfer protein to accelerated cholesteryl ester transfer in IDDM

In an earlier study we demonstrated that the transfer of cholesteryl ester (CET) estimated as the net mass of CE lost from HDL to the apoB-containing lipoproteins (VLDL + LDL) during incubation of plasma is accelerated in normolipidaemic patients with insulin-dependent diabetes mellitus (IDDM). Recombination experiments with isolated lipoprotein fractions employing this same mass transfer assay indicated that this disturbance resulted from dysfunction of VLDL and not from changes in the activity of CE transfer protein (CETP). In this study, we sought first to determine whether CET estimated with an isotopic method that measures the transfer of radiolabelled CE from exogenous HDL from non-diabetic controls to endogenous VLDL + LDL was also increased in IDDM and, if so, the extent to which this disturbance was affected by glycaemic control, VLDL and CETP. As observed with the mass transfer assay, the rate of transfer of the HDL-CE label to VLDL + LDL was also significantly accelerated in IDDM plasma (IDDM: k = 0.256 +/- 0.07; control: k = 0.092 +/- 0.05; mean +/- SD; P < 0.001). Fasting glucose and fructosamine correlated with both isotopic transfer (k) (r = 0.54, P = 0.009; r = 0.57, P = 0.005, respectively) and the mass of CE transferred at 2 h (r = 0.55, P = 0.006; r = 0.59, P = 0.004, respectively). Recombination experiments revealed that isotopic CET was accelerated when: (a) IDDM VLDL were combined with controls HDL and d > 1.21 fractions; and (b) IDDM d > 1.21 plasma fractions containing CETP were combined with controls VLDL + LDL and HDL.(ABSTRACT TRUNCATED AT 250 WORDS)

Accelerated cholesteryl ester transfer in noninsulin-dependent diabetes mellitus

Alterations in core lipid composition of lipoproteins in noninsulin-dependent diabetes mellitus (NIDDM) patients have suggested that the heteroexchange of neutral lipids between HDL and the apo B-containing lipoproteins may be enhanced. For this reason, we studied cholesteryl ester transfer (CET) in ten sulfonylurea-treated patients with stable NIDDM. CET measured in all NIDDM subjects with an assay of mass transfer was significantly greater than that of controls at 1 and 2 h (P < 0.001); the transfer of radiolabeled CE also was increased in a subset of four of the NIDDM group (NIDDM k = 0.21 +/- 0.04 vs. control k = 0.10 +/- 0.05; P < 0.05). A weak correlation was demonstrable between the mass of CE transferred at 1 h and diabetic control expressed as plasma fructosamine (r = 0.58, P < 0.09). To characterize this disturbance in CET further, the donor (HDL + VHDL) and acceptor (VLDL + LDL) lipoprotein fractions were isolated by ultracentrifugation at d 1.063 g/ml from NIDDM and control plasma and a series of recombination experiments were performed. Combining NIDDM acceptor with control donor fractions that contained HDL and CETP and not the combination of NIDDM donor and control acceptor lipoproteins resulted in an accelerated CET response identical to that observed in NIDDM whole plasma. This observation indicated that the abnormality in CET in NIDDM was associated with the VLDL + LDL fraction.(ABSTRACT TRUNCATED AT 250 WORDS)

Incorporation of dietary n-3 fatty acids into molecular species of phosphatidyl choline and cholesteryl ester in normal human plasma

To understand the differences in the antiatherogenic actions of eicosapentaenoic acid (EPA, 20:5) and docosahexaenoic acid (DHA, 22:6), we determined their incorporation into molecular species of phosphatidylcholine (PC) and cholesteryl ester (CE) after feeding 12 g marine lipid concentrate/d to six normolipidemic males for 28 d. The time course of incorporation of EPA into plasma PC and CE showed a precursor-product relationship. In contrast, the DHA concentration of CE was markedly lower than that in PC, and the EPA-DHA ratio was 2-6-fold higher in CE than in PC at all time intervals. Three PC species--16:0-20:5, 16:0-22:6, and 18:0-20:5--increased, whereas 18:1-18:2, 18:0-18:2, and 16:0-20:3 decreased. In vitro formation of CE species in plasma by lecithin-cholesterol acyltransferase (LCAT) showed an increased formation of 20:5 CE but not 22:6 CE, indicating that DHA is a poor substrate for LCAT. These results demonstrate a differential incorporation of EPA and DHA into plasma lipids, which may be related to the differences in their biological effects.

Effects of insulin treatment on lipoprotein composition and function in patients with IDDM

Normolipidemic patients of both sexes with insulin-dependent diabetes mellitus have the same pervasive changes in lipoprotein surface and core lipid composition. The disproportionate increase observed in their lipoprotein free (unesterified) cholesterol relative to the predominant surface phospholipid lecithin (phosphatidylcholine) is reflected by elevation of the FC-L ratio in their whole plasma, VLDL, HDL2, and HDL3. As a possible consequence of this qualitative disturbance, cholesteryl ester transfer is pathologically increased and the mass of cholesteryl ester transferred from HDL to VLDL + LDL is significantly greater in IDDM patients than in control subjects at 1, 2, and 4 hr (P less than 0.001). Consistent with accelerated CET in vivo, the TG-CE core lipid ratio was decreased in VLDL from six subjects (IDDM 9.5 +/- 0.8 vs. control 12.9 +/- 3.4; P less than 0.01) and increased in their HDL (diabetic 0.55 +/- 0.11 vs. control 0.42 +/- 0.04: P less than 0.025). These abnormalities in lipoprotein composition and CET do not correlate with glycemic control and persist after intensive management with s.c. insulin. They may be related to the peripheral hyperinsulinemia that is an unavoidable consequence of conventional s.c. insulin administration because preliminary studies indicate that these disturbances in lipoprotein composition and function are reversed when systemic insulin levels are lowered and insulin is delivered into the portal circulation from an i.p. catheter connected to an implanted programmable s.c. insulin pump.

Effect of marine lipids on cholesteryl ester transfer and lipoprotein composition in patients with hypercholesterolemia

While the effects of omega-3 (n-3) fatty acids present in marine lipids on plasma lipoprotein levels have been intensively studied, less is known about their impact on reverse cholesterol transport. For this reason, for a 3-month period we studied the effects of the administration of n-3 fatty acids (6 g/day) as a dietary supplement on cholesteryl ester transfer (CET), a key step in this process, and lipoprotein composition in 12 outpatients with genetically heterogeneous forms of hypercholesterolemia. Before treatment, CET in hypercholesterolemic patients, estimated as the mass of cholesteryl ester (CE) transferred from high density lipoprotein (HDL) to very low density lipoprotein (VLDL) plus low density lipoprotein (LDL), was markedly accelerated, peaking after only 1-2 hours of incubation of whole plasma; this response differed significantly (p < 0.001) from the initial delayed curvilinear response of control subjects. Consistent with the accelerated CET occurring in vivo, their triglyceride to esterified cholesterol core lipid ratio before treatment was reduced in the intact VLDL fraction and VLDL1 but not in VLDL2 or VLDL3 and was reciprocally increased in HDL. In addition, the free (unesterified) cholesterol to lecithin ratio of VLDL1 was abnormally increased. Recombination experiments performed with individual lipoprotein fractions revealed that accelerated CET was specifically associated with the VLDL1 subfraction and not LDL, HDL, and cholesteryl ester transfer protein (CETP), although pretreatment levels of CETP were significantly increased (p < 0.01).(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of apolipoprotein activators on the specificity of lecithin:cholesterol acyltransferase: determination of cholesteryl esters formed in A-I/C-III deficiency

Although it is known that plasma lecithin:cholesterol acyltransferase (LCAT) is activated by several apolipoproteins (apo) including A-I, C-I, D, A-IV, and E, it is not clear what the physiological importance of having different apolipoprotein activators is. One possible explanation is that the activation by different apolipoproteins may result in the utilization of different species of phosphatidylcholine (PC), leading to the formation of different species of cholesteryl esters (CE). In order to determine this possibility, we analyzed the molecular species composition of PC and CE in two patients with familial deficiency of apoA-I and apoC-III. The LCAT activity, assayed by three different procedures, was found to be 36-63% of the control value. The lower LCAT activity, however, was due to deficiency of the enzyme rather than the absence of apoA-I. The patients' plasma was relatively enriched with sn-2 18:2 PC species reflecting the partial deficiency of LCAT activity. The fatty acid composition of plasma CE was not significantly different from that of controls. HPLC analysis of labeled CE formed after incubation of plasma with [C14]cholesterol showed no significant difference in the species of CE synthesized by the LCAT reaction. The transfer of pre-existing as well as newly formed CE from HDL to the apoB-containing lipoproteins was accelerated compared to control plasma. These results show that the absence of apoA-I does not significantly affect either the activity or the specificity of LCAT, and that the other apolipoprotein activators can substitute adequately for it.

Apparent beneficial effects of tamoxifen on bone mineral content in patients with breast cancer: preliminary study

Eight postmenopausal women undergoing adjuvant treatment with tamoxifen (20 mg/day) for breast cancer had baseline pretreatment, and 6- and 12-month post-treatment dual-photon determinations of vertebral bone mineral content (BMC). BMC measured at 6 and 12 months increased over baseline in all subjects. These observations imply that women treated with tamoxifen may retain rather than surrender the protective effect of estrogen against osteoporosis.

Lipoprotein composition and HDL particle size distribution in women with non-insulin-dependent diabetes mellitus and the effects of probucol treatment

To further characterize the spectrum of potentially atherogenic disturbances in lipoprotein composition in non-insulin-dependent diabetes mellitus (NIDDM), we have studied a subset of women with NIDDM before and after treatment with the lipophilic lipid-lowering drug probucol (1 gm day), which we have shown corrects certain compositional abnormalities these women share with subjects who have hypercholesterolemia. Before treatment, the NIDDM group had a somewhat higher plasma triglyceride level (154 +/- 58.3 mg/dl, vs control, 80.0 +/- 21 mg/dl [mean +/- SD]; p less than 0.025) than controls but their cholesterol and high-density lipoprotein cholesterol (HDL-C) levels did not differ from control levels. A number of significant disturbances, however, were present in the surface and core lipid composition of their lipoproteins. Although the cholesterol content of NIDDM low-density lipoprotein (LDL) was similar to that of controls, its content of sphingomyelin and phosphatidylinositol plus phosphatidylserine and sphingomyelin-to-lecithin ratio all were significantly reduced. Moreover, their very-low-density lipoprotein (VLDL) and HDL2 tended to have reduced amounts of free (unesterified) cholesterol (FC) relative to lecithin, and their HDL2 and HDL3 tended to be triglyceride enriched. Probucol therapy resulted in significant decreases in total plasma cholesterol (-15%), FC (-28%), HDL-C (-22%), and triglyceride (-16%) and in apoproteins A-I, B, and E (apo A-I, B, and E), without changing diabetic control (before probucol: hemoglobin A1, cholesterol, 10.7% +/- 2.7%; after probucol: 10.9% +/- 3.0%).(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of continuous insulin infusion therapy on lipoprotein surface and core lipid composition in insulin-dependent diabetes mellitus

To determine whether intensive insulin therapy has the same beneficial effects on lipoprotein composition that it has been shown to have in insulin-dependent diabetes mellitus (IDDM) on the routinely measured plasma lipids, we studied 10 patients after 6 months of conventional therapy (CIT) and again after 6 months of therapy with continuous subcutaneous insulin infusion (CSII). While the mean of home blood glucose levels (8.1 +/- 0.5 v 7.9 +/- 0.5 mmol/L) decreased no further, plasma triglycerides (TG) (CIT, 102.7 +/- 25.0; CSII, 89.6 +/- 27.1 mg/dL; P less than .001) decreased after CSII, and high-density lipoprotein cholesterol (HDL-C) increased significantly, primarily as a consequence of an increase in HDL2 (CIT, 12.2 +/- 6.0; CSII, 18.1 +/- 6.3 mg/dL; P less than .02). Low-density lipoprotein cholesterol (LDL-C) was unchanged (CIT, 82.2 +/- 32; CSII, 84.0 +/- 27.8 mg/dL). After CIT, two indices of lipoprotein surface composition were altered: (1) the free cholesterol (FC) to lecithin ratio, which is a new cardiovascular risk factor, was abnormally increased in plasma, very-low-density lipoprotein (VLDL) + LDL, and HDL, and (2) the sphingomyelin to lecithin ratio, an index of the surface rigidity of lipoproteins, was increased in the HDL subfractions. While CSII treatment resulted in favorable changes in whole plasma lipids, it failed to correct these disturbances in composition. Since the participation of lipoproteins in certain steps in reverse cholesterol transport appears to be impaired when their surface constituents are altered, persistence of these disturbances may sustain the increased cardiovascular risk of IDDM patients, even when their clinical control is very good and their plasma lipids are normal.

Accelerated cholesteryl ester transfer in patients with insulin-dependent diabetes mellitus

Abnormalities in cholesteryl ester transfer (CET) may play a role in the development of diabetic arterial vascular complications. To assess this important step systematically in reverse cholesterol transport, we have studied 20 treated, clinically stable, normolipidaemic patients. Contrary to the impairment in CET described previously in NIDDM, the mass of CE transferred from HDL to VLDL + LDL was significantly greater in IDDM patients than in controls at 1,2, and 4 h (P less than 0.001). When the d less than 1.063 plasma fractions from IDDM subjects were combined with controls d less than 1.063 fractions, an accelerated CET response was observed which was identical to that found in intact IDDM plasma. This finding, which indicates that this disturbance in CET was associated with the acceptor lipoproteins, was confirmed when we found that it was reproduced by the addition of IDDM VLDL and not LDL to control d greater than 1.063 fractions. Changes observed in lipoprotein core lipid composition were consistent with accelerated CET occurring in IDDM in vivo: the TG/CE core lipid ratio was decreased in VLDL from six subjects (diabetic 9.5 +/- 0.8 vs control 12.9 +/- 3.4; P less than 0.1) and increased in their HDL (diabetic 0.55 +/- 0.11 vs control 0.42 +/- 0.04; P less than 0.025). No correlation was demonstrable between estimates of diabetic control (glycoalbumin, fasting glucose) and CET. These data indicate that CET may be abnormally increased in normolipidaemic IDDM patients. A defect of this type may be atherogenic because it increases the number of lipoprotein particles in plasma which resemble cholesteryl ester-enriched chylomicron and VLDL remnants but whose normal receptor-mediated catabolism may be altered.

Accelerated cholesteryl ester transfer in plasma of patients with hypercholesterolemia

To discern the mechanism(s) that underlie abnormal cholesteryl ester transfer (CET) in patients with hypercholesterolemia, we have studied this dysfunctional step in reverse cholesterol transport in 13 subjects with genetically heterogeneous forms of hypercholesterolemia (HC). In all HC patients, the mass of CE transferred in whole plasma from HDL to VLDL and LDL increased rapidly initially and was significantly greater than in controls at 1, 2, and 4 h (P less than 0.005). To further characterize this disturbance, we performed a series of recombination experiments. Combining HC d less than 1.063 containing acceptor VLDL + LDL with the d greater than 1.063 fraction from controls containing donor HDL + CE-transfer protein (CETP) and not the converse combination showed the same characteristics of accelerated CET noted with intact HC plasma, indicating that abnormal transfer was associated with the HC acceptor lipoproteins. When HC VLDL and its subfractions and LDL were isolated separately and then combined with control d greater than 1.063 fractions, accelerated CET was only associated with VLDL1. Consistent with an acceleration of the neutral lipid transfer reaction occurring between HDL and VLDL1 in HC in vivo, we found that the triglyceride/CE ratio was decreased in HC VLDL1 (P less than 0.001), and increased in HDL (P less than 0.25). CETP mass was significantly increased in HC plasma (HC 2.3 +/- 4 micrograms/ml vs. control 1.3 +/- 0.3 micrograms/ml; mean +/- SD; P less than 0.025). This series of observations demonstrate that CET is accelerated in the plasma of HC patients, and this disturbance results from dysfunction of the VLDL1 subfraction rather than an elevation of CETP levels. Since an abnormality of this type in vivo can lead to the accumulation of potentially atherogenic CE-enriched apoB-containing lipoproteins in plasma, it may be an additional previously unrecognized factor that increases cardiovascular risk in HC patients.

Effects of estropipate treatment on plasma lipids and lipoprotein lipid composition in postmenopausal women

It is generally believed that the cardioprotective benefit of long term treatment of postmenopausal women with estrogen results in part from its capacity to increase high density lipoprotein (HDL) and lower low density lipoprotein (LDL) concentrations. The extent to which the various estrogens employed in replacement treatment affect the composition of lipoproteins, however, is not known. For this reason, we have examined the impact of one such preparation, the synthetic estrone estropipate (1.25 mg/day), on lipoprotein levels and composition in six postmenopausal women. After 6 months of treatment, whole plasma triglyceride (pretreatment, 135 +/- 63; posttreatment, 143 +/- 56 mg/dL), cholesterol (pretreatment, 232 +/- 14; posttreatment, 216 +/- 29 mg/dL), and HDL-C (pretreatment, 57.8 +/- 14.8; posttreatment, 55.6 +/- 13.2) were unchanged. However, plasma free (unesterified) cholesterol (FC) fell (pretreatment, 73.4 +/- 6.2; posttreatment, 53.7 +/- 9.3 mg/dL; P less than 0.05) and lecithin (L) rose significantly (pretreatment, 2.12 +/- 0.29; posttreatment, 2.47 +/- 0.34 mumol/mL; P less than 0.01). The consequence of these changes was a significant decline in the plasma FC/L ratio (pretreatment, 0.91 +/- 0.17; posttreatment, 0.68 +/- 0.12; P less than 0.01) to levels observed in healthy menstruating women. The calculated lipoprotein particle size was unchanged in very low density lipoproteins and increased significantly (P less than 0.05) in LDL after estropipate therapy. Since qualitatively altered lipoproteins enriched in FC and an increased FC/L ratio in plasma are both associated with increased coronary risk, the improvement noted in these parameters after estropipate therapy indicates that its use may be beneficial despite the lack of change in whole plasma lipids.

A high-performance liquid chromatographic procedure for the determination of disaturated phosphatidylcholine in human plasma

Plasma disaturated phosphatidylcholine (DSPC) concentration has been implicated as a risk factor for atherosclerosis. However, suitable methods for the estimation of these compounds in plasma are not available. In this paper, a method for the estimation of DSPC using argentation thin-layer chromatography and high-performance liquid chromatography is described. It is quantitative for the measurement of individual and total DSPC species and is not dependent on fatty acid chain length. The method employs hydrolysis of total plasma phosphatidyl choline by phospholipase C, followed by benzoylation of the diacylglycerols. The benzoates are then fractionated on silver nitrate-impregnated silica gel thin-layer chromatography plates, and the disaturated species separated and quantitated by high-performance liquid chromatography. The method is sensitive and reproducible and allows many samples to be done at once. With this method, the amounts of DSPC were found to be significantly higher in a group of normolipidemic diabetic subjects, compared to age-matched controls.

Probucol treatment in hypercholesterolemic patients: effects on lipoprotein composition, HDL particle size, and cholesteryl ester transfer

Despite probucol's capacity to induce regression of tendinous xanthomata and reduce whole plasma and LDL cholesterol (LDL-C) in patients with hypercholesterolemia, its therapeutic use in the United States has been limited because of concern about its HDL-lowering effects. To assess the possibility that probucol might facilitate mobilization of tissue cholesterol in the presence of low HDL levels as a consequence of favorable changes in lipoprotein composition and function, we have analyzed lipoproteins and studied cholesteryl ester transfer (CET) in hypercholesterolemic patients before and after treatment. Prior to treatment, the free cholesterol (FC)/lecithin (L) ratio in plasma, a new index of cardiovascular risk, and the mass of cholesteryl ester transferred from HDL to the apo B-containing lipoproteins (CET) both were significantly increased (P less than 0.001). As previously shown, plasma cholesterol, LDL-C, HDL-C, HDL2, and apolipoproteins A-I, A-II, and B all fell significantly following probucol treatment. The FC/L ratio in plasma (P less than 0.01) and HDL2 (P less than 0.01) both fell significantly also, as did the sphingomyelin/lecithin ratio in VLDL + LDL (P less than 0.001) which is typically increased in untreated patients with hypercholesterolemia. Nondenaturing gradient gel electrophoresis in 6 patients revealed that the quantitative changes in HDL were associated with a redistribution of particles characterized by a decrease in the prevalence of the largest (HDL2b) and a relative increase in the number of the smallest (HDL3b) particles. Moreover, CET following probucol therapy returned to levels which were indistinguishable from those of normolipidemic controls. These results indicate that untreated patients with hypercholesterolemia have abnormalities in (1) lipoprotein composition which have been shown to retard the movement of cholesterol from tissues to HDL, and in (2) CET which is accelerated and can potentially lead to the formation in plasma of atherogenic CE-enriched apo B-containing lipoproteins. Probucol's capacity to reverse these specific alterations suggests that it may have beneficial effects on cholesterol transport in patients with hypercholesterolemia.

Persistent abnormalities in lipoprotein composition and cholesteryl ester transfer following lovastatin treatment

Optimally effective lipid-lowering agents should not only restore plasma lipids to normal levels but also correct potentially atherogenic alterations in lipoprotein composition and function often present in hyperlipidemic patients. Lovastatin, a competitive inhibitor of cholesterol biosynthesis, clearly lowers plasma cholesterol levels. Its effects on lipoprotein composition and cholesteryl ester transfer (CET), a key step in reverse cholesterol transport, however, are not known. Since abnormalities in CET and lipoprotein composition are present in patients with hypercholesterolemia, we studied these parameters of plasma lipoprotein transport in twelve hypercholesterolemic (HC; Type IIa) subjects (six male, six female) before and 2 months after lovastatin treatment (20 mg qd). Before lovastatin, the free cholesterol (FC)/lecithin (L) ratio in plasma, a new index of cardiovascular risk that reflects lipoprotein surface composition, was abnormally increased (1.18 +/- 0.26 vs controls 0.83 +/- 0.14; P less than 0.001) in very low density lipoproteins (VLDL) and high density lipoprotein-3 (HDL3), and remained so after treatment despite significant declines in whole plasma cholesterol (311.7 +/- 68.2 vs 215.6 +/- 27.2 mg/dl; P less than 0.001), low density lipoprotein (LDL)-cholesterol (206.3 +/- 47.9 vs 146.8 +/- 29.4; P less than 0.001), and apolipoprotein B (149 +/- 30 vs 110 +/- 17; P less than 0.005).(ABSTRACT TRUNCATED AT 250 WORDS)

Molecular species of cholesteryl esters formed in abetalipoproteinemia: effect of apoprotein B-containing lipoproteins

In order to study the effects of very low density (VLDL) and low density (LDL) lipoproteins on the activity and specificity of lecithin:cholesterol acyltransferase (LCAT), we determined the molecular species of cholesteryl esters (CE) synthesized in the plasma from three abetalipoproteinemic (ABL) patients, before and after supplementation with normal VLDL or LDL. The patients' plasma had significantly lower concentration of 18:2 CE and higher concentrations of 16:0 CE and 18:1 CE compared to normal plasma. Incubation of ABL plasma with [4-14C]cholesterol at 37 degrees C and the subsequent analysis of labeled CE formed by high performance liquid chromatography revealed that the major species formed was 16:0 CE (34% of total label), whereas similar incubation of the d greater than 1.063 g/ml fraction of normal plasma resulted in the formation of predominantly 18:2 CE (45% of total label). Addition of normal VLDL or LDL to ABL plasma stimulated the total LCAT activity by 30-80% and normalized the CE species synthesized. The LCAT activity of a normal d greater than 1.063 g/ml fraction also was stimulated by the normal VLDL or LDL, but there was no alteration in the species of CE formed. Most of the CE synthesized was found in the added VLDL or LDL with both ABL and normal plasma, indicating that the CE transfer (CET) activity was not affected in ABL plasma. These results suggest that while the VLDL and LDL are required for the maximal activity of LCAT, the species of CE formed are primarily determined by the molecular species composition of phosphatidylcholine in the plasma.

Effects of tamoxifen treatment on plasma lipids and lipoprotein lipid composition

Concern has been raised that long term treatment with the antiestrogen tamoxifen might predispose women to the rapid development of cardiovascular disease. Since estrogen-induced changes in plasma lipids confer protection to females from coronary heart disease, we have examined the impact of tamoxifen on lipoprotein levels and composition on eight posmenopausal women. After 3 months of tamoxifen treatment (10 mg, twice daily), no significant changes were observed in either whole plasma triglyceride (pre-Rx, 137 +/- 59; post-Rx, 157 +/- 110 mg/dL) or cholesterol (pre-Rx, 193 +/- 23; post-Rx, 204 +/- 14 mg/dL); plasma free (unesterified) cholesterol (FC), however, fell significantly (pre-Rx, 66.5 +/- 6.5; post-Rx, 59.6 +/- 4.6 mg/dL; P less than 0.05). Since plasma lecithin (L) was unchanged, the FC/L ratio declined significantly to levels observed in healthy menstruating women (pre-Rx, 95 +/- 0.16; post-Rx, 0.74 +/- 0.12 molar ratio; P less than 0.025). In low density lipoprotein (LDL), the concentrations of cholesterol and FC and the FC/L ratio all fell significantly (P less than 0.025, P less than 0.05, and P less than 0.025, respectively). Despite a tendency for high density lipoprotein2 cholesterol (HDL2-C) to increase (pre-Rx, 9.7 +/- 3.6; post-Rx, and 14.4 +/- 13.3 mg/dL; P less than 0.4) and phosphoinositol to fall, there were few clear-cut alterations in either HDL2 or HDL3 surface or core lipid composition. The combination of reduced HDL3 lysolecithin (P less than 0.025) associated with a posttreatment trend toward increased triglyceride/cholesterol esters ratios in both HDL subfractions are findings consistent with tamoxifen-induced inhibition of hepatic lipase. These changes in lipoprotein composition together with the fall in LDL cholesterol and increase in sex hormone-binding globulin (P less than 0.005) indicate that tamoxifen acts as an estrogen agonist on the liver. Since elevated LDL cholesterol levels and qualitatively altered lipoproteins enriched in FC are both associated with increased coronary risk, the improvement noted in these parameters after tamoxifen should allay to some degree anxiety about its use with regard to cardiovascular risk.

Effects of omega-3 fish oils on plasma lipids, lipoprotein composition, and postheparin lipoprotein lipase in women with IDDM

Because the apparent reduction in cardiovascular risk noted in nondiabetic populations that ingest diets rich in marine lipids containing omega-3 fatty acids is believed to result in part from their capacity to modify the composition and physicochemical behavior of lipoproteins, we sought to determine whether dietary supplementation with marine lipids might favorably affect lipoprotein composition in insulin-dependent diabetes mellitus (IDDM). Eight normolipidemic IDDM women (mean +/- SD age 29.8 +/- 4.7 yr) were studied before and 3 mo after receiving a marine-lipid concentrate (Super-EPA) containing 6 g omega-3 fatty acids and a total of 12 mg of cholesterol daily. Weight, insulin requirements, and glycosylated hemoglobin remained stable. After treatment, mean +/- SD plasma triglyceride (TG) levels fell (before, 81.7 +/- 22 mg/dl; after, 69.19 +/- 17; P less than 0.025). High-density lipoprotein2 (HDL2) cholesterol (before, 10.98 +/- 5.45 mg/dl; after, 18.43 +/- 7.93; P less than 0.01), its major apolipoprotein A-I (apoAI), and the major phospholipids (sphingomyelin and lecithin) all rose significantly. ApoB and plasma and low-density lipoprotein cholesterol levels and HDL3 composition were unchanged. Postheparin hepatic and lipoprotein lipase activities were unaffected by marine lipids. These data indicate that women with IDDM experience apparently beneficial effects on TG and HDL2 from dietary supplementation with omega-3 fatty acids administered in a low-cholesterol-containing oil without adversely affecting overall diabetes management. If these changes in lipoprotein concentration and composition prove to have antiatherogenic consequences and are free of long-term toxicity, these agents may have a role in the therapy of IDDM patients.

Persistent abnormalities in lipoprotein composition in noninsulin-dependent diabetes after intensive insulin therapy

To determine whether rigorous insulin therapy, which normalized the routinely measured plasma lipids, also reversed qualitative abnormalities in the composition of lipoproteins in noninsulin-dependent diabetes mellitus (NIDDM), we studied 18 NIDDM patients (eight men and 10 women) before and 2 months after intensive insulin therapy. Glycosylated hemoglobin levels (11.7% vs. 8.7%), plasma triglyceride (TG) (250 +/- 91 vs. 164 +/- 56 mg/dl, p less than 0.001), and cholesterol (214 +/- 43 vs. 198 +/- 31 mg/dl, p less than 0.025) all fell, and both HDL2 cholesterol and HDL3 cholesterol increased (59.1% and 10.9%, respectively, p less than 0.001). However, abnormalities in two indices of lipoprotein surface constituents, which were present before insulin therapy, remained so thereafter. The first of these, the new cardiovascular risk factor, the plasma free cholesterol/lecithin ratio, which was increased before treatment, fell only slightly after therapy (pre-therapy 1.02 +/- 0.29 vs. post-therapy 0.90 +/- 0.17, p less than 0.4; reference group, 0.83 +/- 0.14), and remained elevated in very low density lipoprotein (VLDL) and low density lipoprotein (LDL). Secondly, the sphingomyelin/lecithin ratio, an index of the surface rigidity of lipoproteins, was abnormal before treatment in VLDL, HDL2, and HDL3, and this alteration persisted after insulin therapy in HDL3 (p less than 0.001). Lipoprotein core lipid abnormalities were also present before treatment: the TG/cholesteryl ester ratio was reduced in VLDL and increased in LDL, HDL2, and HDL3. Rigorous insulin therapy improved, but failed to fully correct, this disturbance.(ABSTRACT TRUNCATED AT 250 WORDS)

Molecular species of phosphatidylcholine in abetalipoproteinemia: effect of lecithin:cholesterol acyltransferase and lysolecithin acyltransferase

In order to study the role of very low density lipoproteins (VLDL) and low density lipoproteins (LDL) in determining the molecular species composition of phosphatidylcholine (PC) and the specificity of lecithin:cholesterol acyltransferase (LCAT) in human plasma, we studied the PC species composition in plasma from abetalipoproteinemic (ABL) and control subjects before and after incubation at 37 degrees C. The ABL plasma contained significantly higher percentages of sn-2-18:1 species (16:0-18:1, 18:0-18:1, and 18:1-18:1) and lower percentages of sn-2-18:2 species (16:0-18:2, 18:0-18:2, and 18:1-18:2) as well as sn-2-20:4 species (16:0-20:4, 18:0-20:4, and 18:1-20:4). Similar abnormalities were found in the PC of ABL erythrocytes, while the PE of the erythrocytes was less affected. The relative contribution of various PC species towards LCAT reaction in ABL plasma was significantly different from that found in normal plasma. Thus, while 16:0-18:2 and 16:0-18:1 contributed, respectively, 43.8% and 15.9% of the total acyl groups used for cholesterol esterification in normal plasma, they contributed, respectively, 21.5% and 37.9% in ABL plasma. The relative contribution of 16:0-20:4 was also significantly lower in ABL plasma (4.7% vs. 9.0% in normal), while that of 16:0-16:0 was higher (6.4% vs. 0.5%). However, the selectivity factors of various species (percent contribution/percent concentration) were not significantly different between ABL and normal plasma, indicating that the substrate specificity of LCAT is not altered in the absence of VLDL and LDL. Incubation of ABL plasma in the presence of normal VLDL or LDL resulted in normalization of its molecular species composition and in the stimulation of its LCAT activity. Addition of LDL, but not VLDL, also resulted in the activation of lysolecithin acyltransferase (LAT) activity. The incorporation of [1-14C]palmitoyl lysoPC into various PC species in the presence of LDL was similar to that observed in normal plasma, with the 16:0-16:0 species having the highest specific activity. These results indicate that the absence of apoB-containing lipoproteins significantly affects the molecular species composition of plasma PC as well as its metabolism by LCAT and LAT reactions.

Effects of dietary supplementation with marine lipid concentrate on the plasma lipoprotein composition of hypercholesterolemic patients

Although the triglyceride-lowering actions of n-3 fatty acids of marine lipids are now well-recognized, their effects on plasma lipoproteins have not been studied systematically in patients with hypercholesterolemia. To address this question, we supplemented the Phase 1 American Heart Association diets of 14 hypercholesterolemic ambulatory outpatients with a commercially available preparation of marine lipid concentrate (SuperEPA) containing 7.5 g n-3 fatty acids per day and studied their plasma lipids and lipoproteins before and after 30 days of treatment. Both plasma triglyceride and cholesterol levels fell uniformly in all patients while the mean VLDL- and LDL-cholesterol decreased by 58% (P less than 0.005) and 13% (P less than 0.025) respectively. The decrease in whole plasma cholesterol was significantly correlated with the fall in LDL-cholesterol (r = 0.85, P less than 0.01), and not VLDL-cholesterol (r = 0.39, NS). Among the other potentially beneficial actions observed was an increase in HDL2 in all patients (mean increment 41%, P less than 0.005), and an increase in the HDL2/HDL3 ratio (+46%, P less than 0.001) and decreases in the LDL/HDL ratio (-14%, P less than 0.005) and in the unesterified cholesterol/lecithin ratio (-17%; P less than 0.001) in plasma. The increase in the unesterified cholesterol/esterified cholesterol ratio in VLDL and HDL3 suggested that marine lipid therapy resulted in a reduction in the size of lipoprotein particles in these fractions. Since these changes may reduce cardiovascular risk, these findings suggest that marine lipids may prove useful in the treatment of certain patients with hypercholesterolemia.

Whole-plasma and high-density lipoprotein subfraction surface lipid composition in IDDM men

To determine whether compositional abnormalities are present in high-density lipoprotein (HDL) in patients with insulin-dependent diabetes mellitus (IDDM) that might negate its putatively protective cardiovascular effects, we studied the plasma lipoproteins of 12 men with varying degrees of clinical control (mean fasting glucose 193 +/- 10 mg/dl, mean glycoalbumin greater than 73% above control mean). The diabetic patients' basal plasma triglyceride, total- and free- (unesterified) cholesterol, HDL cholesterol (HDL-chol), and apolipoprotein AI, AII, and B concentrations were similar to those of control subjects, but the free-cholesterol-to-lecithin ratio, a new index of cardiovascular disease risk, was significantly increased in their plasma (0.97 +/- 0.14 vs. 0.88 +/- 0.07, P less than .02) and their very-low-density lipoprotein (VLDL)-low-density lipoprotein (LDL) subfraction (1.50 +/- 0.51 vs. 1.08 +/- 0.15, P less than .005). Although HDL2-chol was similar in diabetic and control groups, the HDL2-chol-to-free-cholesterol ratio (diabetic vs. control, 4.64 +/- 1.7 vs. 1.96 +/- 1.0 mumol/ml, P less than .025) and the sphingomyelin-to-lecithin ratio (0.23 +/- 0.08 vs. 0.20 +/- 0.09, P less than .025) were both significantly increased in the IDDM group. HDL3-chol was higher in the IDDM than in the control subjects (diabetic vs. control, 38.6 +/- 5.2 vs. 32.7 +/- 2.7 mg/dl, P less than .005). In contrast to whole plasma and the VLDL + LDL subfraction, the free-cholesterol-to-lecithin ratio of IDDM and control HDL subfractions were similar.(ABSTRACT TRUNCATED AT 250 WORDS)

Abnormal high-density lipoprotein composition in women with insulin-dependent diabetes

To determine whether alterations in lipoprotein phospholipid composition might be an unrecognized factor that contributes to the unexplained acceleration of atherogenesis and the loss of sex-related protection from the development of coronary heart disease in women with insulin-dependent diabetes mellitus, we have estimated levels of neutral lipids, apolipoproteins (A-I, A-II, B), and free cholesterol (FC) in plasma and the four major phospholipid constituents of the very low-density lipoprotein + low-density lipoprotein and high-density lipoprotein (HDL) fractions in 12 ambulatory female patients with varying degrees of diabetic control. Although levels of triglyceride, cholesterol, HDL-cholesterol, and lipoprotein phospholipids in whole plasma of the patients with diabetes were similar to those in controls, their FC levels and FC/lecithin ratio, a recently described index of cardiovascular risk, both were abnormally increased (p less than 0.01). In the HDL-containing plasma fraction, concentrations of sphingomyelin, lecithin, and lysolecithin all were significantly reduced (p less than 0.05; p less than 0.01, and p less than 0.02, respectively). These compositional changes may be potentially atherogenic, because a reduction in the phospholipid content of HDL may impair its capacity to promote the efflux of cholesterol from cells, and the transfer of cholesterol ester from HDL to the larger apo-B-containing lipoproteins is inhibited when their content of FC is increased relative to phospholipid. These previously unrecognized qualitative defects, which are inapparent in the routine estimation of plasma lipids, may compromise reverse cholesterol transport and thereby promote atherogenesis in women with insulin-dependent diabetes mellitus.

Influence of low estrogen-containing oral contraceptives on lipoprotein phospholipid composition and mononuclear cell membrane fluidity

Since the effects of the new low estrogen-containing oral contraceptive (OC) preparations on lipoprotein phospholipid (PL) composition are unknown, we studied 3 groups of 10 young women before and after 6 months of use of 3 commonly prescribed agents containing almost identical amounts of ethinyl estradiol (0.30-0.35 microgram) and differing progestogens, and correlated these changes with their estrogen to progestin (E/P) ratio. The directional changes in both plasma neutral lipid and PL concentrations tended to correlate with the E/P ratio, with plasma HDL-cholesterol (HDL-C) falling slightly and the low density lipoprotein-cholesterol (LDL-C)/HDL-C ratio increasing in the women taking the OC with the lowest E/P ratio; in contrast, plasma HDL-C increased and the LDL-C/HDL-C ratio fell in those receiving the preparation with the highest E/P ratio. In HDL, the ratio of the 2 principal PL, sphingomyelin and lecithin, an index of lipid fluidity, tended to increase, suggesting that the surface of this lipoprotein class had become more rigid. This change was most apparent in women receiving the agent in which the progestin was predominant; in women receiving the preparations with the higher E/P ratios the sphingomyelin/lecithin ratio actually declined. The membrane fluidity of mononuclear cells obtained from five women taking an OC with a relatively high E/P ratio, however, was significantly increased (P less than 0.007) compared to that in normal women. These findings demonstrate that, even with substantial reductions in their estrogen content, the use of these newer OC is associated with quantitative and qualitative changes in lipoprotein PL composition that parallel their E/P balance and are associated with altered fluidity of mononuclear cell membranes.