Human noninsulin-dependent diabetes: identification of a defect in plasma cholesterol transport normalized in vivo by insulin and in vitro by selective immunoadsorption of apolipoprotein E

Lipid efflux mechanisms, relation to disease and potential therapeutic aspects

Lipids are hydrophobic and amphiphilic molecules involved in diverse functions such as membrane structure, energy metabolism, immunity, and signaling. However, altered intra-cellular lipid levels or composition can lead to metabolic and inflammatory dysfunction, as well as lipotoxicity. Thus, intra-cellular lipid homeostasis is tightly regulated by multiple mechanisms. Since most peripheral cells do not catabolize cholesterol, efflux (extra-cellular transport) of cholesterol is vital for lipid homeostasis. Defective efflux contributes to atherosclerotic plaque development, impaired β-cell insulin secretion, and neuropathology. Of these, defective lipid efflux in macrophages in the arterial walls leading to foam cell and atherosclerotic plaque formation has been the most well studied, likely because a leading global cause of death is cardiovascular disease. Circulating high density lipoprotein particles play critical roles as acceptors of effluxed cellular lipids, suggesting their importance in disease etiology. We review here mechanisms and pathways that modulate lipid efflux, the role of lipid efflux in disease etiology, and therapeutic options aimed at modulating this critical process.

Chemical profile and in vivo toxicity evaluation of unripe Citrus aurantifolia essential oil

Citrus aurantifolia (Christm.) Swingle (syn. C. MEDICA var. ACIDA Brandis) (family: Rutaceae) essential oil is one of the cheapest oils found in local markets. Although, it is generally accepted as non-toxic to vital organs and cells, majority of people are cynical about it usage. Herein, the present study reports the chemical composition and in vivo oral toxicity study of unripe C. aurantifolia essential oil found in Ghana. The toxicity of C. aurantifolia essential oil extract was investigated via oral administration using two methods: The acute toxicity single dose study (SDS) and the repeated dose method. The oil exhibited no acute toxicity but in the sub-chronic studies, the effects was dose and time-dependent. Chemical profile investigation of the oil showed 9 constituent of phytochemicals (Germacrene isomers (61.2%), Pineen (14%), Linalool dimmer (2.9%), Bornane (11%), Citral (2.9%), Anethole (1.5%), Anisole (1.1%), Safrole (0.3%) and Demitol (0.6%)). Histopathological studies revealed conditions such as necrosis, edema and inflammatory reaction in the liver, spleen and kidneys. Marginal upsurge of biochemical parameters above normal and elevated levels of lymphocytes (35.20-46.40 g/dL) demonstrated mild toxicity among the 100 mg/kg and 500 mg/kg dose groups at the sub-chronic stage. Low levels of hemoglobin (13.60 to 12.70 g/dL), MCV (34.20-24.0 fL), MCH (40.20-36.40 g/dL) along with high levels of liver enzymes confirmed the mild toxicity of the oil at sub-chronic stage. These results demonstrate that, despite consideration of lime essential oil as safe, it can have mild hematotoxic, nephrotoxic and hepatotoxic effects.

The metabolic syndrome: an emerging risk state for cardiovascular disease

The common clustering of glucose intolerance, insulin resistance, abdominal adiposity, elevated blood pressure, and low HDL cholesterol is referred to as metabolic syndrome. Individuals with this syndrome have an increased risk of developing cardiovascular disease (CVD). The World Health Organisation and the National Cholesterol Education Programme’s Adult Treatment Panel III (NCEP-ATP III) have outlined specific diagnostic criteria for the diagnosis of the metabolic syndrome to help in the Identification of this syndrome in clinical practice. While the WHO criteria were specifically developed for use in research, the NCEP criteria are useful in clinical diagnosis of the metabolic syndrome. The metabolic syndrome is amenable to lifestyle modifications such as increased physical activity, weight loss, and possibly intake of low-glycemic foods. Drug therapy may be used to treat individual components of the syndrome such as elevated blood pressure and dyslipidemia. To control elevated glucose levels (when there is failure of lifestyle modification), medications such as metformin, thiazolidinedione derivatives and alpha glucosidase inhibitors may be used.

Hypertension, lipoprotein subclasses and lipid transfer proteins in obese children and adolescents

BACKGROUND

Obesity-related childhood hypertension is associated with disturbances of serum lipids, but less is known about distribution of lipoprotein subclasses and activities of proteins involved in reverse cholesterol transport in hypertensive obese children. Our objective was to determine low-density lipoprotein (LDL) and high-density lipoprotein (HDL) subclasses distribution and activities of lecithin:cholesterol acyltransferase (LCAT) and cholesteryl ester transfer protein (CETP) in hypertensive and non-hypertensive obese children.

METHODS

A total of 40 hypertensive and 25 non-hypertensive obese children were enrolled. Lipoprotein subclasses were assessed by polyacrylamide gradient gel electrophoresis. LCAT and CETP activities were determined as a rate of formation and a rate of transfer of cholesteryl esters.

RESULTS

Despite of comparable values of serum lipid parameters, a shift toward smaller LDL and HDL subclasses was observed in hypertensive compared to normotensive obese children. Activities of LCAT were similar, but proatherogenic CETP activities were significantly higher in the hypertensive group (p = 0.036). LCAT/net CETP ratio inversely correlated with relative proportion of small, dense LDL particles (ρ = -0.423; p = 0.025) in the group with hypertension.

CONCLUSIONS

The results of our study demonstrated a tendency toward altered distribution of lipoprotein subclasses in favor of more proatherogenic particles in childhood hypertension. Also, hypertensive obese children had increased proatherogenic CETP activity.

Simvastatin and bezafibrate increase cholesterol efflux in men with type 2 diabetes

BACKGROUND

The importance of functional properties of high-density lipoproteins (HDL) for atheroprotection is increasingly recognized. We determined the impact of lipid-lowering therapy on 3 key HDL functionalities in Type 2 diabetes mellitus (T2DM).

MATERIALS AND METHODS

A placebo-controlled, randomized cross-over study (three 8-week treatment periods with simvastatin (40 mg daily), bezafibrate (400 mg daily), alone and in combination) was carried out in 14 men with T2DM. Cholesterol efflux was determined using human THP-1 monocyte-derived macrophages, HDL antioxidative capacity was measured as inhibition of low-density lipoprotein oxidation in vitro, and HDL anti-inflammatory capacity was assessed as suppression of thrombin-induced monocyte chemotactic protein 1 expression in human umbilical vein endothelial cells. Pre-β-HDL was assayed using crossed immunoelectrophoresis.

RESULTS

While cholesterol efflux increased in response to simvastatin, bezafibrate and combination treatment (+12 to +23%; anova, P = 0.001), HDL antioxidative capacity (P = 0.23) and HDL anti-inflammatory capacity (P = 0.15) did not change significantly. Averaged changes in cellular cholesterol efflux during active treatment were correlated positively with changes in HDL cholesterol, apoA-I and pre-β-HDL (P < 0.05 to P < 0.001). There were no inter-relationships between changes in the three HDL functionalities during treatment (P > 0.10). Changes in HDL antioxidative capacity and anti-inflammatory capacity were also unrelated to changes in HDL cholesterol and apoA-I, while changes in HDL antioxidative capacity were related inversely to pre-β-HDL (P < 0.05).

CONCLUSION

Simvastatin and bezafibrate increase cholesterol efflux, parallel to HDL cholesterol and apoA-I responses. The antioxidative and anti-inflammatory properties of HDL are not to an important extent affected by these therapeutic interventions.

The effect of apoE genotype and sex on ApoE plasma concentration is determined by dietary fat in healthy subjects

The interindividual variation in ApoE plasma concentration is considerable, mainly determined by apoE genotype and sex. However, a large amount of variability remains unexplained by these factors. We have evaluated whether the quantity and quality of dietary fat interacts with the apoE genotype and sex modifying ApoE plasma levels in young healthy subjects. Eighty-four volunteers (sixty-six apoE3/3, eight apoE4/3 and ten apoE3/2) were subjected to three dietary periods, each lasting 4 weeks. The first was a SFA-enriched diet (38 % fat and 20 % SFA), which was followed by a carbohydrate (CHO)-rich diet (30 % fat, < 10 % SFA and 55 % carbohydrate) or a MUFA-rich diet (38 % fat and 22 % MUFA) following a randomised crossover design. apoE2 carriers have the highest ApoE levels, whereas apoE4 individuals show the lowest concentration after the SFA, CHO and MUFA diets. Women had significantly higher ApoE concentration than men only after the consumption of the SFA diet. The SFA diet increased the ApoE plasma concentration when compared with the CHO- and MUFA-rich diets in women, but not in men. In women, but not in men, the shift from the SFA- to CHO- or MUFA-rich diets significantly decreased the ApoE concentration in apoE3/2 and apoE3/3 subjects, whereas no differences were observed in women with the apoE4/3 genotype. Sex and apoE genotype determine ApoE plasma levels; however, this effect is dependent on dietary fat.

Increased cholesterol efflux from cultured fibroblasts to plasma from hypertriglyceridemic type 2 diabetic patients: roles of pre beta-HDL, phospholipid transfer protein and cholesterol esterification

We tested whether hypertriglyceridemia associated with type 2 diabetes mellitus is accompanied by alterations in pre beta-HDL, which are considered to be initial acceptors of cell-derived cholesterol, and by changes in the ability of plasma to promote cellular cholesterol efflux. In 28 hypertriglyceridemic and 56 normotriglyceridemic type 2 diabetic patients, and in 56 control subjects, we determined plasma lipids, HDL cholesterol and phospholipids, plasma pre beta-HDL and pre beta-HDL formation, phospholipid transfer protein (PLTP) activity, plasma cholesterol esterification (EST) and cholesteryl ester transfer (CET) and the ability of plasma to stimulate cholesterol efflux out of cultured human fibroblasts. HDL cholesterol and HDL phospholipids were lower, whereas plasma PLTP activity, EST and CET were higher in hypertriglyceridemic diabetic patients than in the other groups. Pre beta-HDL levels and pre beta-HDL formation were unaltered, although the relative amount of pre beta-HDL (expressed as % of total plasma apo A-I) was increased in hypertriglyeridemic diabetic patients. Cellular cholesterol efflux to plasma from hypertriglyceridemic diabetic patients was increased compared to efflux to normotriglyceridemic diabetic and control plasma, but efflux to normotriglyceridemic diabetic and control plasma did not differ. Multiple linear regression analysis demonstrated that cellular cholesterol efflux to plasma was positively and independently related to pre beta-HDL formation, PLTP activity and EST (multiple r=0.48), but not to the diabetic state. In conclusion, cholesterol efflux from fibroblasts to normotriglyceridemic diabetic plasma is unchanged. Efflux to hypertriglyceridemic diabetic plasma is enhanced, in association with increased plasma PLTP activity and cholesterol esterification. Unaltered pre beta-HDL formation in diabetic hypertriglyceridemia, despite low apo A-I, could contribute to maintenance of cholesterol efflux.

Lipoproteins in inflammation and sepsis. II. Clinical aspects

BACKGROUND

Systemic inflammation and sepsis are accompanied by severe metabolic alterations, including insulin resistance together with increased levels of triglycerides (TGs) and decreases in high- and low-density lipoproteins. Clinical studies have clearly established a link between lipid metabolism and systemic inflammation. Lipoproteins were shown to neutralize LPS and to exert direct anti-inflammatory actions. High- and low-density lipoproteins are thus thought to be important regulators of the host immune response during endotoxemia, which may also have the potential of improving the care of patients with Gram-negative sepsis.

DISCUSSION

Nutritional lipids supplied during critical illness have been shown to modulate the host response to inflammation. In particular, inclusion of omega-3 fatty acids seems to have beneficial effects on cellular immunity and helps to maintain the balance between pro- and anti-inflammatory cytokines thereby preventing hyperinflammatory complications. In addition to improvements in the profile of lipid mediators generated, omega-3 fatty acids act as activating ligands of peroxisome proliferator-activated receptors and directly inhibit nuclear factor kappaB mediated proinflammatory signaling. We present an overview on the alterations in the metabolism of serum lipoproteins during sepsis and present data from clinical studies and discuss the significance of nutritional lipids and their role in immunomodulation with special emphasis on omega-3 fatty acids.

Plasma kinetics of free and esterified cholesterol in familial hypercholesterolemia: Effects of simvastatin

The objective of this study was to evaluate the kinetics of both free and esterified forms of cholesterol contained in a emulsion that binds to LDL receptors (LDE) in subjects with heterozygous familial hypercholesterolemia (FH), and the same subjects under the effects of high-dose simvastatin treatment, as compared with a control normolipidemic group (NL). Twenty-one FH patients (44.0 +/- 13.0 yr, 12 females, LDL cholesterol levels 6.93 +/- 1.60 mmol/L) and 22 normolipidemic patients (44.0 +/- 15.0, 10 females, LDL cholesterol levels 3.15 +/- 0.62 mmol/L) were injected intravenously with 14C-cholesteryl ester and 3H-cholesterol. FH patients were also evaluated after 2 mon of 40 or 80 mg/d simvastatin treatment, and plasma samples were collected over 24 h to determine the residence time (RT, in h) of both LDE labels, expressed as the median (25%; 75%). The RT of both 14C-cholesteryl ester and 3H-cholesterol were greater in FH than in NL [FH: 36.0 (20.5; 1191.0), NL: 17.0 (12.0-62.5), P = 0.015; and FH: 52.0 (30.0; 1515.0); NL 20.5 (14.0-30.0) P < 0.0001]. Treatment reduced LDL cholesterol by 36% (P < 0.0001), RT of 14C-cholesteryl ester by 49% (P = 0.0029 vs. baseline), and 3H-cholesterol RT by 44% (P = 0.019 vs. baseline). After treatment, the RT values of 14C-cholesteryl ester in the FH group approached the NL values (P = 0.58), but the RT of 3H-cholesterol was still greater than those for the NL group (P = 0.01). The removal of LDE cholesteryl esters and free cholesterol was delayed in FH patients. Treatment with a high dose of simvastatin normalized the removal of cholesterol esters but not the removal of free cholesterol.

Pharmacological modulation of cholesteryl ester transfer protein, a new therapeutic target in atherogenic dyslipidemia

In mediating the transfer of cholesteryl esters (CE) from antiatherogenic high density lipoprotein (HDL) to proatherogenic apolipoprotein (apo)-B-containing lipoprotein particles (including very low density lipoprotein [VLDL], VLDL remnants, intermediate density lipoprotein [IDL], and low density lipoprotein [LDL]), the CE transfer protein (CETP) plays a critical role not only in the reverse cholesterol transport (RCT) pathway but also in the intravascular remodeling and recycling of HDL particles. Dyslipidemic states associated with premature atherosclerotic disease and high cardiovascular risk are characterized by a disequilibrium due to an excess of circulating concentrations of atherogenic lipoproteins relative to those of atheroprotective HDL, thereby favoring arterial cholesterol deposition and enhanced atherogenesis. In such states, CETP activity is elevated and contributes significantly to the cholesterol burden in atherogenic apoB-containing lipoproteins. In reducing the numbers of acceptor particles for HDL-derived CE, both statins (VLDL, VLDL remnants, IDL, and LDL) and fibrates (primarily VLDL and VLDL remnants) act to attenuate potentially proatherogenic CETP activity in dyslipidemic states; simultaneously, CE are preferentially retained in HDL and thereby contribute to elevation in HDL-cholesterol content. Mutations in the CETP gene associated with CETP deficiency are characterized by high HDL-cholesterol levels (>60 mg/dL) and reduced cardiovascular risk. Such findings are consistent with studies of pharmacologically mediated inhibition of CETP in the rabbit, which argue strongly in favor of CETP inhibition as a valid therapeutic approach to delay atherogenesis. Consequently, new organic inhibitors of CETP are under development and present a potent tool for elevation of HDL in dyslipidemias involving low HDL levels and premature coronary artery disease, such as the dyslipidemia of type II diabetes and the metabolic syndrome. The results of clinical trials to evaluate the impact of CETP inhibition on premature atherosclerosis are eagerly awaited.

Alterations in high-density lipoprotein metabolism and reverse cholesterol transport in insulin resistance and type 2 diabetes mellitus: role of lipolytic enzymes, lecithin:cholesterol acyltransferase and lipid transfer proteins

Insulin resistance and type 2 diabetes mellitus are generally accompanied by low HDL cholesterol and high plasma triglycerides, which are major cardiovascular risk factors. This review describes abnormalities in HDL metabolism and reverse cholesterol transport, i.e. the transport of cholesterol from peripheral cells back to the liver for metabolism and biliary excretion, in insulin resistance and type 2 diabetes mellitus. Several enzymes including lipoprotein lipase (LPL), hepatic lipase (HL) and lecithin: cholesterol acyltransferase (LCAT), as well as cholesteryl ester transfer protein (CETP) and phospholipid transfer protein (PLTP), participate in HDL metabolism and remodelling. Lipoprotein lipase hydrolyses lipoprotein triglycerides, thus providing lipids for HDL formation. Hepatic lipase reduces HDL particle size by hydrolysing its triglycerides and phospholipids. A decreased postheparin plasma LPL/HL ratio is a determinant of low HDL2 cholesterol in insulin resistance. The esterification of free cholesterol by LCAT increases HDL particle size. Plasma cholesterol esterification is unaltered or increased in type 2 diabetes mellitus, probably depending on the extent of triglyceride elevation. Subsequent CETP action results in transfer of cholesteryl esters from HDL towards triglyceride-rich lipoproteins, and is involved in decreasing HDL size. An increased plasma cholesteryl ester transfer is frequently observed in insulin-resistant conditions, and is considered to be a determinant of low HDL cholesterol. Phospholipid transfer protein generates small pre beta-HDL particles that are initial acceptors of cell-derived cholesterol. Its activity in plasma is elevated in insulin resistance and type 2 diabetes mellitus in association with high plasma triglycerides and obesity. In insulin resistance, the ability of plasma to promote cellular cholesterol efflux may be maintained consequent to increases in PLTP activity and pre beta-HDL. However, cellular cholesterol efflux to diabetic plasma is probably impaired. Besides, cellular abnormalities that are in part related to impaired actions of ATP binding cassette transporter 1 and scavenger receptor class B type I are likely to result in diminished cellular cholesterol efflux in the diabetic state. Whether hepatic metabolism of HDL-derived cholesterol and subsequent hepatobiliary transport is altered in insulin resistance and type 2 diabetes mellitus is unknown. Specific CETP inhibitors have been developed that exert major HDL cholesterol-raising effects in humans and retard atherosclerosis in animals. As an increased CETP-mediated cholesteryl ester transfer represents a plausible metabolic intermediate between high triglycerides and low HDL cholesterol, studies are warranted to evaluate the effects of these agents in insulin resistance- and diabetes-associated dyslipidaemia.

Atherogenic role of elevated CE transfer from HDL to VLDL(1) and dense LDL in type 2 diabetes : impact of the degree of triglyceridemia

Plasma cholesteryl ester transfer protein (CETP) facilitates intravascular lipoprotein remodeling by promoting the heteroexchange of neutral lipids. To determine whether the degree of triglyceridemia may influence the CETP-mediated redistribution of HDL CE between atherogenic plasma lipoprotein particles in type 2 diabetes, we evaluated CE mass transfer from HDL to apoB-containing lipoprotein acceptors in the plasma of type 2 diabetes subjects (n=38). In parallel, we investigated the potential relationship between CE transfer and the appearance of an atherogenic dense LDL profile. The diabetic population was divided into 3 subgroups according to fasting plasma triglyceride (TG) levels: group 1 (G1), TG<100 mg/dL; group 2 (G2), 100<TG<200 mg/dL; and group 3 (G3), TG>200 mg/dL. Type 2 diabetes patients displayed an asymmetrical LDL profile in which the dense LDL subfractions predominated. Plasma levels of dense LDL subfractions were strongly positively correlated with those of plasma triglyceride (TG) (r=0.471; P:=0.0003). The rate of CE mass transfer from HDL to apoB-containing lipoproteins was significantly enhanced in G3 compared with G2 or G1 (46.2+/-8.1, 33.6+/-5.3, and 28.2+/-2.7 microg CE transferred. h(-1). mL(-1) in G3, G2, and G1, respectively; P:<0.0001 G3 versus G1, P:=0.0001 G2 versus G1, and P:=0.02 G2 versus G3). The relative capacities of VLDL and LDL to act as acceptors of CE from HDL were distinct between type 2 diabetes subgroups. LDL particles represented the preferential CE acceptor in G1 and accounted for 74% of total CE transferred from HDL. By contrast, in G2 and G3, TG-rich lipoprotein subfractions accounted for 47% and 72% of total CE transferred from HDL, respectively. Moreover, the relative proportion of CE transferred from HDL to VLDL(1) in type 2 diabetes patients increased progressively with increase in plasma TG levels. The VLDL(1) subfraction accounted for 34%, 43%, and 52% of total CE transferred from HDL to TG-rich lipoproteins in patients from G1, G2, and G3, respectively. Finally, dense LDL acquired an average of 45% of total CE transferred from HDL to LDL in type 2 diabetes patients. In conclusion, CETP contributes significantly to the formation of small dense LDL particles in type 2 diabetes by a preferential CE transfer from HDL to small dense LDL, as well as through an indirect mechanism involving an enhanced CE transfer from HDL to VLDL(1), the specific precursors of small dense LDL particles in plasma.

Pathogenesis of vascular disease

Only recently are we beginning to understand the complex interplay of factors involved in vascular disease and diabetes. Insulin resistance provides a starting point to explain the many factors that lead to the more severe vascular disease characteristic of diabetes. Insulin resistance syndrome comprises insulin resistance and compensatory hyperinsulinaemia as well as hypertension, dyslipidaemia, macrovascular disease, and increased plasminogen activator inhibitor-1 activity. The development of type 2 diabetes may be viewed as the inability of the pancreas to continue to overcome insulin resistance, even with excessive insulin production.

Diminished rate of mouse peritoneal macrophage cholesterol efflux is not related to the degree of HDL glycation in diabetes mellitus

The efflux of (14)C-cholesterol from mouse peritoneal macrophages mediated by in vivo and in vitro glycation of intact HDL(3) and by HDL(3) apolipoproteins was investigated. Cholesterol-laden cells were incubated a long time with HDL(3) from control subjects (C), poorly controlled diabetes mellitus patients (D) and with HDL C submitted to in vitro glycation (G), as well as with all their respectively isolated apolipoproteins. A diminished cholesterol efflux rate occurred in incubations with intact HDL(3) D but not with intact HDL(3)G or with apoHDL(3)C, G or D. The specific binding of (125)I-HDL(3)G to the cell receptor, obtained upon incubation in the absence and in the presence of excess unlabelled HDL(3), was lower than the control. The role of apoE secretion by cholesterol-laden macrophages on cholesterol efflux was analyzed by incubating apoE knockout and control mice macrophages with HDL C or HDL G: a lower cholesterol efflux was observed from apoE knockout macrophages but glycation of HDL(3) did not influence this process either. The diminished capacity to remove cholesterol by the HDL drawn from diabetic subjects must be attributed to other modifications of the lipoproteins, except for non enzymatic glycation. Thus, events that impair the cell cholesterol removal in diabetes mellitus are multifaceted.

Reverse cholesterol transport in diabetes mellitus

There are epidemiological data and experimental animal models relating the development of premature atherosclerosis with defects of the reverse cholesterol transport (RCT) system. In this regard, the plasma concentrations of the high density lipoprotein (HDL) subfractions, of cholesteryl ester transfer protein (CETP), as well as the activity of the enzyme lecithin-cholesterol acyl transferase (LCAT) play critical roles. However, there has been plenty of evidence that atherosclerosis in diabetes mellitus (DM) is ascribed to a greater arterial wall cell uptake of modified apoB-containing lipoproteins whereas a primary or predominant defect of the RCT system is still a subject of debate. In other words, in spite of the fact that in DM the composition and rates of metabolism of the HDL particles are greatly altered and display a diminished in vitro efficiency to remove cell cholesterol, definitive in vivo demonstration of the importance of this fact in atherogenesis is lacking. Furthermore, the roles played by LCAT and CETP in RCT in DM are difficult to interpret because the in vitro procedures of measurement utilized have either been inadequate, or inappropriately interpreted. Knock-out or transgenic mice are much needed models to investigate the roles of LCAT, CETP, phospholipid transfer protein (PLTP), and of a CETP inhibitor in the development of atherosclerosis of experimental DM.

Fractional efflux and net change in cellular cholesterol content mediated by sera from mice expressing both human apolipoprotein AI and human lecithin:cholesterol acyltransferase genes

Human lecithin:cholesterol acyltransferase (LCAT) is a key enzyme in the metabolism of cholesterol and is postulated to participate in the physiological process called reverse cholesterol transport. We have used transgenic mice (Tgm) expressing either both human apolipoprotein AI (apo AI) and human LCAT genes or only the human apo AI gene (HuAILCAT or HuAI Tgm, respectively) to assess the consequences of LCAT overexpression on serum lipid and lipoprotein profiles and on the ability of each serum to promote bidirectional flux of cholesterol between serum and Fu5AH hepatoma cells. Mean serum LCAT activity of HuAILCAT Tgm was 2-fold increased compared to the HuAI group (48+/-9 vs. 24+/-5 nmol/ml per h, P<0.01 for HuAILCAT and HuAI Tgm, respectively) and the cholesterol esterification rates were not significantly different between the two groups of animals (66+/-11 vs. 74+/-18 nmol/ml per h for HuAILCAT and HuAI Tgm, respectively). HuAILCAT Tgm exhibited higher total cholesterol serum values (2.3-fold) due to an increase in both HDL-cholesterol (1. 9-fold) and non-HDL-cholesterol (3-fold). The HDL particles from HuAILCAT Tgm were relatively phospholipid depleted and cholesterol enriched compared to HuAI mice. When cells were incubated for six hours with the mouse serum, the fractional efflux of radiolabeled cholesterol was slightly increased with the HuAILCAT Tgm (1.2-fold) but the increase in intracellular cholesterol content was also 2-fold higher than with the HuAI Tgm. Fu5AH can be viewed as a model for the evaluation of bidirectional flux of cholesterol in SR-BI-rich cells. In this model LCAT overexpression in mice, by increasing both HDL and non-HDL-cholesterol, mostly enhances the uptake of cholesterol by the cells, which would be of benefit for the last step of reverse cholesterol transport in hepatocytes.

Elevated plasma cholesteryl ester transfer in NIDDM: relationships with apolipoprotein B-containing lipoproteins and phospholipid transfer protein

Lecithin:cholesteryl acyl transferase (LCAT) and cholesteryl ester transfer protein (CETP) are key factors in the esterification of cholesterol and the subsequent transfer of cholesteryl ester from high density lipoproteins (HDL) towards very low and low density lipoproteins (VLDL + LDL). Phospholipid transfer protein (PLTP), lipoprotein lipase (LPL) and hepatic lipase (HL) are involved in plasma phospholipid and triglyceride metabolism and also affect HDL. Equivocal changes in plasma cholesteryl ester transfer have been reported in non-insulin-dependent diabetes mellitus (NIDDM). In 16 NIDDM men with plasma triglycerides < or = 4.5 mmol/l and cholesterol < or = 8.0 mmol/l. plasma cholesteryl ester transfer (CET), cholesterol esterification rate, LCAT and PLTP activity levels were higher (P < 0.05 to P < 0.02) in conjunction with higher plasma triglycerides (P < 0.01) and lower HDL cholesterol and cholesteryl ester levels (P < 0.05) compared to 16 matched healthy men. Multiple stepwise regression analysis demonstrated that CET was positively related to VLDL + LDL cholesterol (P < 0.001), triglycerides (P = 0.001), PLTP activity (P = 0.007) and CETP activity (P = 0.008, multiple r = 0.94). NIDDM had no effect on CET, independently from these parameters. HDL cholesteryl ester was negatively related to CET (P= 0.017), HL activity (P = 0.033) and NIDDM (P = 0.047) and positively to LCAT activity levels (P = 0.034, multiple r = 0.68). It is concluded that the elevated CET in plasma from NIDDM patients is associated with higher plasma triglycerides and PLTP activity levels. Furthermore, our data suggest that in normo- and moderately dyslipidaemic subjects PLTP and CETP activity levels per se may influence the rate of cholesteryl ester transfer in plasma. Plasma cholesteryl ester transfer appears to be a determinant of HDL cholesteryl ester, but other factors are likely to contribute to lower HDL cholesteryl ester levels in NIDDM.

Lipoproteins and cellular cholesterol homeostasis

Cholesterol homeostasis in peripheral cells involves a balance between the influx and efflux processes. The acquisition of cholesterol by such cells is mediated by a variety of receptor and non-receptor processes involving both normal and modified lipoproteins. The offsetting efflux process is mediated by HDL and especially particles containing only apo A-I. An efficient reverse cholesterol transport by HDL of cholesterol from peripheral cells to the liver protects against the development of atherosclerosis. In cells that do not contain excess cholesterol, the cholesterol is distributed as unesterified cholesterol molecules between the plasma membrane and the membranes of the intracellular organelles. In cholesterol-loaded cells such as macrophage foam cells, the membranes became enriched in unesterified cholesterol and, in addition, cytoplasmic CE droplets and lysosomal cholesterol crystals can form. The ways in which cholesterol molecules move between intracellular sites and the plasma membrane to become available for efflux to extracellular acceptor particles are becoming known. Cholesterol molecules in the plasma membrane can desorb and diffuse through the aqueous phase and be sequestered by HDL particles. The cell cholesterol available for efflux can exist in different kinetic pools, and these pools, such as those in various domains in the plasma membrane, require further definition. The cholesterol molecules present in intracellular pools also efflux with different kinetics and by different pathways. Thus, newly synthesized cholesterol is actively transported by a vesicle system from the ER to the plasma membrane, whereas lysosomal cholesterol seems to be transported to the plasma membrane by a protein-mediated, diffusional process. Clearance of cytoplasmic CE is dependent upon the rate of turnover of the CE cycle and the magnitude of the cholesterol gradient between the plasma membrane and the extracellular acceptor particle. It can be expected that the interdependence of the pathways and the molecular mechanisms underlying the intracellular trafficking of cholesterol will be elucidated in the near future.

Abnormal reverse cholesterol transport in controlled type II diabetic patients. Studies on fasting and postprandial LpA-I particles

The high incidence and prevalence of coronary heart disease in diabetes mellitus is clearly established. The usual lipid pattern found in type II diabetic patients is a moderate increase in fasting triglyceride levels associated with low HDL cholesterol levels. These abnormalities are further amplified in the postprandial state. To study the effect of these alterations on reverse cholesterol transport, we isolated lipoprotein containing apoA-I but not apoA-II (LpA-I) particles by immunoaffinity chromatography from the plasma of well-controlled type II diabetic patients and nondiabetic matched control subjects. Different parameters involved in this antiatherogenic pathway were measured in both fasting and postprandial states. Diabetic patients had reduced levels of LpA-I particles that were protein enriched and phospholipid depleted. Gradient gel electrophoresis showed that control LpA-I particles had five distinct populations, whereas diabetic particles lacked the largest one. LpA-I isolated from diabetic plasma exhibited a decreased capacity to induce cholesterol efflux from Ob 1771 adipose cells both in fasting (15.1 +/- 10.0% versus 7.5 +/- 2.7%, P < .05) and postprandial (17.7 +/- 11.2% versus 7.7 +/- 3.9%, P < .05) states, whereas only control particles showed significantly higher ability to promote cholesterol efflux after the test meal (P = .02). Lecithin:cholesterol acyltransferase activity measured with an exogenous substrate showed a 54% increase and an 18% decrease postprandially for control subjects and patients, respectively. Thus, the different abnormalities found in the fasting state were further amplified in the postprandial situation. This resulted in LpA-I particles with aberrant size and composition and decreased ability to accomplish their antiatherogenic role in type II diabetic patients.

A cell culture system for screening human serum for ability to promote cellular cholesterol efflux. Relations between serum components and efflux, esterification, and transfer

A cell culture system was employed to test a large number of samples of human serum for the ability to stimulate the efflux of cell cholesterol. The extent of efflux obtained with each specimen was correlated with the serum concentrations of cholesterol, triglycerides, apoprotein (apo) B, apo A-I, apo A-II, and lipoprotein subfractions (ie, high-density lipoprotein2 [HDL2], HDL3, lipoprotein [Lp] A-I, and LpA-I:A-II). In addition, the subsequent esterification of the released cholesterol and the distribution of the synthesized exogenous cholesteryl esters between HDL and low-density lipoprotein/very-low-density lipoprotein provided estimates of the lecithin:cholesterol acyltransferase (LCAT) and cholesteryl ester transfer protein (CETP) activities of each serum. The values for these activities were analyzed for correlations with cell efflux and the various serum parameters. Cell cholesterol efflux best correlated with serum total HDL cholesterol values. HDL2 and HDL3 correlated about equally well with efflux, whereas LpA-I demonstrated a much greater association with efflux than did LpA-I:A-II. Analysis of the data by partial correlation analysis indicated that HDL3 and LpA-I were the HDL subfractions most closely associated with efflux. Esterification of the released radiolabeled cholesterol was strongly and positively correlated with serum triglyceride concentrations and negatively related to the serum concentrations of HDL2. There was no relation between esterification values, which reflect LCAT activity, and efflux. The transfer of the labeled cholesteryl esters between HDL and apoB-containing lipoproteins was used as a measure of CETP activity and demonstrated a pattern in which all apoB-related parameters were positively correlated to transfer of esterified cholesterol, and all HDL associated parameters, particularly HDL3, were negatively related to transfer. No relations were observed between efflux, esterification, and transfer.

Lipoprotein disorders in diabetes mellitus

In IDDM or NIDDM, the total plasma cholesterol and triglycerides are usually within normal limits when the blood glucose is controlled. Marked hypertriglyceridemia can develop with loss of glycemic control and is often due to superimposed genetic abnormalities in lipoprotein metabolism. Tight control in IDDM usually reduces LDL and VLDL to normal levels and may raise HDL above the normal range. Low HDL cholesterol and mild to moderate elevations of VLDL triglyceride are common in NIDDM if obesity or proteinuria is also present. Both HDL and LDL may be smaller and more dense and may be enriched with triglyceride as compared with cholesterol. These abnormalities may require weight loss for control. The increased incidence of cardiovascular disease in diabetes is unexplained but is amplified by the well-defined cardiovascular risk factors. The new American Diabetes Association guidelines call for treatment of high triglycerides and LDL cholesterol to be aggressively reduced. Triglycerides should be under 200 mg/dL, are considered borderline high between 200 and 400 mg/dL, and high when above 400 mg/dL. Low HDL is defined as less than 35 mg/dL. Control of obesity with diet and exercise and reduced intake of saturated fat and cholesterol are important first steps. If needed, drug therapy is appropriate to reduce LDL to levels below 130 mg/dL in all adult diabetics and below 100 mg/dL in those with cardiovascular disease.

Decreased ability of high density lipoproteins to transfer cholesterol esters in non-insulin-dependent diabetes mellitus

Esterified cholesterol transfer (ECT) from high density lipoproteins (HDL) to very low (VLDL) and low density lipoproteins (LDL) may be abnormal in situations at high risk for atherosclerosis. It has been shown to increase in insulin-dependent diabetes and to decrease in non-insulin-dependent diabetes (NIDD). Since the net transfer of esterified cholesterol (EC) results from a bidirectional exchange between HDL and VLDL/LDL, we developed a transfer assay specifically designed to measure the unidirectional transfer of EC from HDL to lipid emulsions according to first-order kinetics. Our results show that in NIDD the rate constant of HDL-dependent ECT is decreased by 30% by comparison with control subjects. Analysis of HDL composition revealed that, in both groups, HDL-dependent ECT was positively correlated with the free cholesterol/phospholipid ratio (r = 0.94; P < 0.001) and negatively correlated with the triglyceride/EC ratio (r = -0.85; P < 0.001). It is concluded that, besides the known defect of acceptor lipoproteins, the abnormality of ECT in NIDD is also caused by a decreased ability of HDL to act as an EC donor, presumably because of a change in composition. In addition, our work shows that the amount of EC lost by HDL during the reaction transfer is counterbalanced by a reciprocal equimolar transfer of triglycerides.

Decreased activation of lecithin:cholesterol acyltransferase by glycated apolipoprotein A-I

Non-enzymatic glycation of plasma proteins may contribute to the excess risk of developing atherosclerosis in patients with diabetes mellitus. Glycated apolipoprotein A-I isolated from diabetic subjects was tested in vitro for its ability to activate lecithin:cholesterol acyltransferase, the principal cholesterol-esterifying enzyme in plasma. Activation by glycated apolipoprotein A-I was significantly lower at all concentrations than the activation by normal apolipoprotein A-I. Linear regression analysis of the kinetic data shows that the ratio app Vmax/app Km was significantly lower (p < 0.01) for glycated apolipoprotein A-I (0.29 nmol.l/h.mumol) than for normal apolipoprotein A-I (0.78 nmol.l/h.mumol). Because lecithin:cholesterol acyltransferase provides a driving force in reverse cholesterol transport by esterifying the cellular cholesterol removed by HDL, it is tempting to postulate that this abnormal activation may be associated with a reduction in reverse cholesterol transport and associated with the accelerated development of atherosclerosis in diabetic patients.

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.

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)

Hyperinsulinemia is characterized by jointly disturbed plasma VLDL, LDL, and HDL levels. A population-based study

Plasma very low density lipoprotein (VLDL) cholesterol and triglyceride, low density lipoprotein (LDL) cholesterol and triglyceridea, high density lipoprotein (HDL) cholesterol, glucose and insulin response (sums of 1- and 2-hour postload oral glucose levels), body mass index (BMI), and blood pressure were determined in a representative sample (n = 542) of the adult Israeli Jewish population. Persons with diabetes or on antihypertensive medications were excluded. Total VLDL and LDL fractions were estimated from their cholesterol and triglyceride subfraction levels that were standardized relative to the mean of the reference group (participants free of glucose intolerance, obesity, and hypertension--the GOH conditions). Hyperinsulinemia and disturbed levels of VLDL and LDL were defined as levels equal to or greater than the 75th percentile and those of HDL, equal to or less than the 25th percentile of their respective reference group distributions. When VLDL was disturbed jointly with LDL and HDL, the mean insulin response adjusted for age, gender, glucose response, BMI, blood pressure, and smoking was high compared to the reference group (166.0 vs. 122.5, p less than 0.001). With isolated disturbed VLDL, or disturbed LDL and HDL but normal VLDL, the mean insulin response resembled the reference group. The adjusted risk ratio for this jointly disturbed lipoprotein profile among hyperinsulinemic individuals was 3.4 (95% confidence limits 2.6 to 4.4, p less than 0.001) with no further association with the GOH conditions. We conclude that hyperinsulinemia is characterized by an atherogenic lipoprotein profile.

Short- and longer-term regulation of very-low-density lipoprotein secretion by insulin, dexamethasone and lipogenic substrates in cultured hepatocytes. A biphasic effect of insulin

1. The precise effects of insulin, dexamethasone and lipogenic precursors on the secretion of very-low-density lipoprotein (VLDL) cholesterol and triacylglycerol were dependent on the age of the culture and the duration of treatment. 2. The rates of secretion of triacylglycerol and cholesterol gradually declined with the age of the culture, although there was no detectable decrease within a given 24 h period. 3. Between 4 h and 24 h after cell preparation, insulin inhibited VLDL secretion. Inhibition was maximal between 6 and 12 h after addition of insulin. Longer-term treatment (24-48 h) with insulin resulted in a stimulation of VLDL secretion. This effect was less apparent when dexamethasone was simultaneously present. The secretion of triacylglycerol and cholesteryl ester was more sensitive to insulin than was that of non-esterified cholesterol. 4. Dexamethasone alone stimulated the secretion of VLDL to an extent which increased with the age of the culture. In young cultures (up to 24 h old) dexamethasone protected against inhibition by insulin, but was ineffective in older cultures. 5. In young cultures the stimulatory effect of lipogenic precursors (lactate and pyruvate) on the secretion of triacylglycerol and cholesterol was more pronounced in the presence of dexamethasone. In cultures older than 24 h, the secretion of these components was less sensitive to short-term stimulation by lactate and pyruvate.

Regulation of hepatic very-low-density lipoprotein secretion in rats fed on a diet high in unsaturated fat

Rats were fed ad libitum on either a standard, high-carbohydrate, chow diet or a similar diet supplemented with 15% unsaturated fat (corn oil). Hepatocytes were prepared either during the dark phase (D6-hepatocytes) or during the light phase (L2-hepatocytes) of the diurnal cycle. In hepatocytes from rats fed on the unsaturated-fat-containing diet, secretion of very-low-density lipoprotein (VLDL) triacylglycerol was inhibited to a greater extent in the D6- than in the L2-hepatocytes. Plasma non-esterified fatty acid concentrations were elevated to the same extent at both D6 and L2 in the unsaturated-fat-fed animals. The secretion of VLDL esterified and non-esterified cholesterol was relatively insensitive to changes in the unsaturated-fat content of the diet. This resulted in proportionate increases in the content of these lipid constituents compared with that of triacylglycerol in the nascent VLDL. There was also an increase in the ratio of esterified to non-esterified cholesterol in the nascent VLDL produced by hepatocytes of the unsaturated-fat-fed animals. In the D6-hepatocytes from the unsaturated-fat-fed animals, the decrease in the secretion of VLDL triacylglycerol could not be reversed by addition of exogenous oleate (0.7 mM) to the incubation medium. In contrast, addition of a mixture of lactate (10 mM) and pyruvate (1 mM) stimulated both fatty acid synthesis de novo and the rate of VLDL triacylglycerol secretion. Secretion of esterified and non-esterified cholesterol also increased under these conditions. Insulin suppressed the secretion of VLDL triacylglycerol and cholesteryl ester under a wide range of conditions in all types of hepatocyte preparations. Non-esterified cholesterol secretion was unaffected. In hepatocytes prepared from the fat-fed animals, these effects of insulin were more pronounced at D6 than at L2. Glucagon also inhibited VLDL lipid secretion in all types of hepatocyte preparations. The decrease in cholesterol secretion was due equally to decreases in the rates of secretion of both esterified and non-esterified cholesterol.

Very low density lipoprotein metabolism in diabetes mellitus

The concentration of VLDL and their major lipid, triglyceride, are regulated at many levels from the initial availability of the substrates needed for their synthesis all the way to the function of the enzymes and receptors involved in their removal from plasma. It should be clear from this review that in diabetes mellitus metabolic derangements resulting from the absolute lack of insulin or from resistance to the actions of insulin can affect VLDL triglyceride metabolism at any or all of these regulatory points. The outcome of this interplay between diabetes and VLDL metabolism is the common occurrence of elevated plasma VLDL and triglyceride concentrations in individuals with both Type 1 and Type 2 diabetes mellitus. Mildly elevated plasma levels of triglycerides are nearly universal in diabetics; more significant hypertriglyceridemia can be the consequence of either metabolic decompensation or the concomitant inheritance of a familial pattern of hyperlipoproteinemia. The combination of the latter two situations usually presents with as severe hypertriglyceridemia. Although deregulation can occur at many points, the most common abnormality associated with hypertriglyceridemia in human diabetes appears to be overproduction of VLDL triglycerides. Increased rates of synthesis of VLDL apoB may also be a common consequence of diabetes. The basis for this belief is the accumulated data from kinetic studies in humans and in experimental models of diabetes in rats. Although the latter may also demonstrate defects in catabolism when insulin deficiency is severe, catabolic abnormalities appear to be uncommon as the primary force in the development of hypertriglyceridemia in humans. Finally, despite the complexity of the systems regulating VLDL metabolism and the many metabolic abnormalities that may be present in diabetic subjects, it appears that reduction of the hyperglycemia by means of dietary or pharmacologic interventions is associated with normalization of the rates of synthesis and catabolism of the VLDL and their triglycerides. In view of the probable atherogenecity of VLDL, particularly in individuals with diabetes, such intervention must be aimed at both plasma glucose and lipid concentrations.

Type 1 (insulin-dependent) versus type 2 (non-insulin-dependent) diabetes mellitus: characterization of serum lipoprotein alterations

Serum lipoprotein lipids and apolipoproteins A-I, B, and E were investigated in Type 1 (insulin-dependent) diabetics, Type 2 (non-insulin-dependent) diabetics, and two control groups, twenty subjects each. Lipoproteins were separated and analysed by common methods, apolipoproteins were measured by endpoint immunonephelometry. Compared with controls, Type 2 diabetics had increased serum apolipoprotein E levels (0.116 +/- 0.020 vs. 0.079 +/- 0.014 g 1-1, P less than 0.01) together with an increased content of cholesteryl ester-enriched very low-density lipoproteins. Furthermore, Type 2 diabetics had higher apolipoprotein B concentrations (1.06 +/- 0.21 vs. 0.85 +/- 0.21 g l-1 P less than 0.01), but lower high-density lipoprotein cholesterol concentrations than the controls. Conversely, Type 1 diabetics had elevated serum apolipoprotein A-I values vs. controls and Type 2 diabetics (1.70 +/- 0.33 vs. 1.49 +/- 0.22 and 1.43 +/- 0.21 g 1-1, P less than 0.01). It is concluded that Type 2 diabetics, like other groups at risk for atherosclerotic diseases, are characterized by an increased concentration of partly catabolized very low-density lipoproteins. Sufficiently insulinized Type 1 diabetics have, on the other hand, an increased number of high-density lipoprotein particles.

Plasma cholesterol metabolism in end-stage renal disease. Difference between treatment by hemodialysis or peritoneal dialysis

Plasma cholesterol metabolism was investigated in normotriglyceridemic patients with end-stage renal disease treated by hemo- or continuous ambulatory peritoneal dialysis (CAPD), and compared with that in a control group with normal renal function. A reversed net transport of free cholesterol from plasma to cultured fibroblasts, as well as greatly reduced levels of plasma cholesterol esterification and cholesterol ester transfer rates to low and very low density lipoproteins (LDL and VLDL), was found in the hemodialysis group compared to the controls. The LDL and VLDL contained increased amounts of free cholesterol and inhibited cholesterol ester transfer when recombined with control plasma. The LDL triglyceride content was doubled in the hemodialysis group, whereas cholesterol esters were decreased. Patients treated by CAPD, in marked contrast, had cholesterol metabolic rates that were within the normal range, as well as normal lipoprotein composition.

Diabetes, lipoproteins, and atherosclerosis

Certain lipoproteins are deemed to be atherogenic because (a) they are found in plasmas of patients with various dyslipoproteinemias who also have atherosclerosis, (b) they appear in plasmas and arterial walls of animals during the experimental induction of atherosclerosis, and (c) they convert cultured macrophages and arterial smooth muscle cells into lipid-laden foam cells such as are found in atherosclerotic lesions. These lipoproteins are low-density lipoproteins (LDL), very low density lipoproteins (VLDL), and the alpha-VLDL of hypertriglyceridemic patients. Low levels or the absence of high-density lipoproteins (HDL) in patients is also associated with atherosclerosis. Poorly controlled diabetics may have higher levels of VLDL and/or LDL and lower levels of HDL than are found in well-controlled diabetics. These quantitative changes place poorly controlled patients into higher coronary risk categories. Several qualitative alterations of lipoprotein also may be present that may make them more atherogenic, among these, glycosylation of apoprotein could be very important.

Effects of postprandial lipemia on plasma cholesterol metabolism

Cholesterol net transport, esterification, and cholesteryl ester transfer have been determined in plasma during fasting, and postprandially, after a high fat-cholesterol meal. Significant rises in plasma triglyceride, phospholipid, and free cholesterol were associated with increases in cholesterol net transport, esterification, and transfer (all P less than 0.005), which were well correlated in individual subjects (r greater than 0.60). Essentially, the whole of free cholesterol required for such increased esterification was derived from cell membranes, when cultured fibroblasts were present, despite the increased level of free cholesterol in postprandial plasma; most of the additional cholesteryl ester generated was transferred to the low and very low density lipoproteins (LDL and VLDL) of plasma. Postprandial LDL (the major carrier of free and ester cholesterol and phospholipids among the acceptor lipoproteins) contained significantly decreased ratios of free cholesterol to phospholipid (P less than 0.001), which may modulate the increased transfer of cholesteryl ester to VLDL and LDL. These data suggest that the presence of postprandial acceptor lipoproteins in plasma may play an important role in stimulating the "reverse" transport of cholesterol from peripheral cells for hepatic degradation, which is effective even after the ingestion of dietary cholesterol.

Increased free cholesterol in plasma low and very low density lipoproteins in non-insulin-dependent diabetes mellitus: its role in the inhibition of cholesteryl ester transfer

Recombination of low and very low density lipoproteins (VLDL and LDL) from normal subjects with plasma from patients with non-insulin-dependent diabetes mellitus significantly increased the reduced rate of transfer of cholesteryl ester to these lipoproteins, which is characteristic of diabetic plasma, whereas diabetic VLDL and LDL reduced cholesteryl ester transfer rates in normal plasma. VLDL and LDL from diabetic plasma had an increased ratio of free cholesterol to phospholipid compared to normal, and unlike normal VLDL and LDL spontaneously lost free cholesterol to high density lipoprotein. These data suggest that the block to cholesteryl ester transfer to these lipoproteins in non-insulin-dependent diabetes is mediated by their increased free cholesterol content and may be related to the increased risk of these patients for developing atherosclerosis.