R. P. F. Dullaart

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.

Higher plasma lipid transfer protein activities and unfavorable lipoprotein changes in cigarette-smoking men.

The mechanism responsible for the atherogenic lipoprotein changes associated with cigarette smoking are largely unknown. Lecithin: cholesterol acyltransferase (LCAT) and cholesteryl ester transfer protein (CETP) are key factors in the esterification of plasma cholesterol and the transfer of cholesteryl ester from high-density lipoproteins (HDLs) toward very-low- and low-density lipoproteins (VLDLs+LDLs). Another transfer factor, phospholipid transfer protein (PLTP), recently has been shown to be involved in the interconversion of HDL particles in vitro, but its physiological function is not yet clear. We measured the activities of LCAT, CETP (as cholesteryl ester exchange activity), and PLTP using exogenous substrate assays as well as lipoprotein profiles in the plasma of 21 normolipidemic cigarette-smoking men (total plasma cholesterol below 6.5 mmol/L and triglyceride below 2.5 mmol/L) and 21 individually matched nonsmoking control subjects. HDL cholesterol, HDL cholesteryl ester, and plasma apolipoprotein A-I levels were lower in the smokers than in the control subjects (P < or = .05 for all parameters). Median plasma CETP activity was 18% higher (P < .02) and median plasma PLTP activity was 8% higher (P < .05) in the smokers compared with the nonsmokers. LCAT activity was not different between the groups. HDL cholesteryl ester concentration was positively related to LCAT activity in control subjects but not in smokers. By contrast, there was an inverse relation of CETP activity with HDL cholesteryl ester in smokers but not in nonsmokers. Multiple regression analysis demonstrated that the lowering effect of smoking on HDL cholesteryl ester could be explained by its influence on CETP activity.(ABSTRACT TRUNCATED AT 250 WORDS)

Cyclic Cushing’s syndrome: a clinical challenge

Cyclic Cushings syndrome (CS) is a rare disorder, characterized by repeated episodes of cortisol excess interspersed by periods of normal cortisol secretion. The so-called cycles of hypercortisolism can occur regularly or irregularly with intercyclic phases ranging from days to years. To formally diagnose cyclic CS, three peaks and two troughs of cortisol production should be demonstrated. Our review of 65 reported cases demonstrates that cyclic CS originates in 54% of cases from a pituitary corticotroph adenoma, in 26% from an ectopic ACTH-producing tumour and in about 11% from an adrenal tumour, the remainder being unclassified. The pathophysiology of cyclic CS is largely unknown. The majority of patients with cyclic CS have clinical signs of CS, which can be either fluctuating or permanent. In a minority of patients, clinical signs of CS are absent. The fluctuating clinical picture and discrepant biochemical findings make cyclic CS extremely hard to diagnose. Clinicians should therefore be aware of this clinical entity and actively search for it in all patients with suspected CS but normal biochemistry or vice versa. Frequent measurements of urinary cortisol or salivary cortisol levels are a reliable and convenient screening tool for suspected cyclic CS. Cortisol stimulation or suppression tests may give spurious results owing to spontaneous falls or rises in serum cortisol at the time of testing. When cyclic CS is biochemically confirmed, further imaging and laboratory studies are guided by the presence or absence of ACTH dependency. In cases of suspected ectopic ACTH production, specific biochemical testing for carcinoids or neuroendocrine tumours is required, including measurements of serotonin in platelets and/or urine, chromogranin A and calcitonin.

CETP inhibition in cardiovascular risk management: a critical appraisal

In view of the cardioprotective effect of high‐density lipoproteins (HDL) and the limited effects of statin and fibrate therapy on HDL cholesterol, it is clinically relevant to test whether pharmacological treatment aimed at raising HDL lowers cardiovascular risk. Cholesteryl ester transfer protein (CETP) is a new therapeutic target, because the cholesteryl ester transfer process lowers HDL cholesterol and contributes to an atherogenic lipoprotein profile, particularly when plasma triglycerides are high. Clinical evidence suggests that coronary artery calcification as well as intima media thickness is positively related to plasma cholesteryl ester transfer, and that high plasma CETP concentration is associated with increased cardiovascular risk in hypertriglyceridaemia. However, CETP could also have anti‐atherogenic potential, since it provides a potentially beneficial route for delivery of HDL‐derived cholesteryl esters to the liver. In addition, CETP could also favourably stimulate peripheral cell cholesterol removal and enhance hepatic cholesterol uptake. Recent evidence suggests that a high CETP level may confer lower cardiovascular risk in the context of low triglycerides. At maximal doses, the CETP inhibitors JTT‐705 and torcetrapib elicit a marked rise in HDL cholesterol of up to 34% and 91–106%, respectively. The effectiveness of these drugs on (intermediate) clinical outcome measures is currently being tested in large‐scale phase III clinical trials, with torcetrapib being only evaluated in combination therapy with atorvastatin. When and how to use CETP inhibitors, e.g. in combination with a statin or a fibrate, is a major challenge. We propose that low HDL cholesterol in the context of high triglycerides, such as found in type 2 diabetes mellitus, could become an important indication area for this new class of drugs.

Cellular cholesterol efflux to plasma from moderately hypercholesterolaemic type 1 diabetic patients is enhanced, and is unaffected by simvastatin treatment

Aim/hypothesisCellular cholesterol efflux to plasma is important in reverse cholesterol transport and may be affected by simvastatin in type 1 diabetes mellitus.MethodsIn 14 moderately hypercholesterolaemic type 1 diabetic and 13 healthy men we determined plasma (apo)lipoproteins, pre-β HDL formation, cholesteryl ester transfer protein (CETP) activity, phospholipid transfer protein (PLTP) activity, cholesterol esterification, cholesteryl ester transfer and the capacity of plasma to induce cholesterol efflux out of Fu5AH cells and fibroblasts. After diet run-in, diabetic patients were randomly treated with simvastatin 10, 20, 40xa0mg and placebo, once daily each, for 6 weeks in a double-blind crossover design.ResultsPlasma very low density lipid protein (VLDL)+LDL cholesterol, LDL cholesterol, HDL phospholipids, apolipoprotein (apo) A-I, apo B, CETP activity, PLTP activity, cholesterol esterification, cholesteryl ester transfer and the capacity of plasma to induce cholesterol efflux from Fu5AH cells and fibroblasts were higher in diabetic patients. Pre-β HDL formation was unaltered. Simvastatin treatment decreased VLDL+LDL cholesterol, LDL cholesterol, triglycerides and apo B, CETP activity, cholesterol esterification and cholesteryl ester transfer. HDL cholesterol increased and its change was correlated with the change in cholesteryl ester transfer. The ability to promote cholesterol efflux from Fu5AH cells and fibroblasts did not change after simvastatin.Conclusions/interpretationThe capacity of plasma from moderately hypercholesterolaemic type 1 diabetic patients to induce cholesterol efflux out of Fu5AH cells and fibroblasts is enhanced, probably due to higher apo A-I, HDL phospholipids and PLTP activity. Simvastatin increases HDL cholesterol in type 1 diabetic patients via lowering of plasma cholesteryl ester transfer. The HDL changes after simvastatin do not increase cellular cholesterol efflux further.

Apolipoprotein M predicts pre-beta-HDL formation: studies in type 2 diabetic and nondiabetic subjects

Objective.u2002 Studies in mice suggest that plasma apoM is lowered in hyperinsulinaemic diabetes and that apoM stimulates formation of pre‐β‐HDL. Pre‐β‐HDL is an acceptor of cellular cholesterol and may be critical for reverse cholesterol transport. Herein, we examined whether patients with type 2 diabetes have reduced plasma apoM and whether apoM is associated with pre‐β‐HDL formation and cellular cholesterol efflux.

Apolipoprotein B/A-I and total cholesterol/high-density lipoprotein cholesterol ratios both predict cardiovascular events in the general population independently of nonlipid risk factors, albuminuria and C-reactive protein

Kappelle PJWH, Gansevoort RT, Hillege JL, Wolffenbuttel BHR, Dullaart RPF on behalf of the PREVEND study group (University Medical Center Groningen and University of Groningen, Groningen, The Netherlands). Apolipoprotein B/A‐I and total cholesterol/high‐density lipoprotein cholesterol ratios both predict cardiovascular events in the general population independently of nonlipid risk factors, albuminuria and C‐reactive protein. J Intern Med 2011; 269: 232–242.

Insulin decreases plasma cholesteryl ester transfer but not cholesterol esterification in healthy subjects as well as in normotriglyceridaemic patients with type 2 diabetes.

Plasma cholesterol esterification (EST) and subsequent cholesteryl ester transfer (CET) from high‐density lipoproteins (HDLs) towards apolipoprotein (apo) B‐containing lipoproteins are key steps in HDL metabolism.

Plasma cholesteryl ester transfer and hepatic lipase activity are related to high-density lipoprotein cholesterol in association with insulin resistance in type 2 diabetic and non-diabetic subjects

We evaluated the hypothesis that plasma cholesteryl ester transfer (CET) and lipase activities are influenced by insulin sensitivity and contribute to the low high-density lipoprotein (HDL) cholesterol observed in type 2 diabetic patients and insulin-resistant non-diabetic subjects. Sixteen type 2 diabetic and 16 non-diabetic subjects participated. Diabetic and non-diabetic subjects were divided in equal groups of eight subjects with low or high insulin sensitivity, which was documented as the glucose infusion rate (M-value) during the last hour of a 3-h euglycaemic hyperinsulinaemic clamp (150 mU kg(-1) h(-1), blood glucose target 4.6 mmol L(-1)). Post-heparin plasma lipoprotein lipase (LPL) and hepatic lipase (HL) activities were measured in samples obtained 1-2 weeks before the clamp. Plasma CET was measured by a radioisotope method. Compared to non-diabetic men with high insulin sensitivity (n = 8) HDL cholesterol was lower in type 2 diabetic men (n=8, p<0.01) and non-diabetic men (n=8, p <0.05) with low insulin sensitivity, and the HDL cholesterylester content was lower in type 2 diabetic men with high insulin sensitivity (n=8, p<0.05). In non-diabetic subjects with high insulin sensitivity, plasma CET was lower than in the other groups (p<0.05 for all). Multiple regression analysis showed that plasma CET (p=0.001) and HL activity (p=0.02) were independently and negatively associated with the M-value. No association between the M-value and LPL activity was observed. Independent negative relationships of HDL cholesterol with plasma CET (p = 0.04) and HL activity (p=0.03) were observed. This study supports the hypothesis that a low HDL cholesterol associated with insulin resistance in type 2 diabetic and non-diabetic subjects is related to a high plasma CET and a high HL activity.We evaluated the hypothesis that plasma cholesteryl ester transfer (CET) and lipase activities are influenced by insulin sensitivity and contribute to the low high-density lipoprotein (HDL) cholesterol observed in type 2 diabetic patients and insulin-resistant non-diabetic subjects. Sixteen type 2 diabetic and 16 nondiabetic subjects participated. Diabetic and non-diabetic subjects were divided in equal groups of eight subjects with low or high insulin sensitivity, which was documented as the glucose infusion rate (M-value) during the last hour of a 3-h euglycaemic hyperinsulinaemic clamp (150 mU kg

Role of phospholipid transfer protein and pre beta-high density lipoproteins in maintaining cholesterol efflux from Fu5AH cells to plasma from insulin-resistant subjects

Plasma phospholipid transfer protein (PLTP) enhances the generation of prebeta-high density lipoproteins (HDL) that may act as initial acceptors of cellular cholesterol, and are likely to play an important role in the antiatherogenic process of reverse cholesterol transport. We examined the interrelationships between insulin resistance, the ability of plasma to stimulate cellular cholesterol efflux, HDL cholesterol, plasma PLTP activity and prebeta-HDL in 12 non-diabetic, non-smoking, normotriglyceridaemic men. Cholesterol efflux from Fu5AH cells to plasma, plasma lipoproteins, PLTP activity and prebeta-HDL formation as measured in incubated plasma were determined after a 12-h fast. Insulin sensitivity was assessed by a euglycaemic, hyperinsulinaemic clamp (M-value). HDL cholesterol was positively correlated with the M-value (r=0.65, p< 0.05), whereas plasma PLTP activity (r= -0.59, p <0.05) and prebeta-HDL in incubated plasma (r= -0.66, p<0.05) were negatively correlated with the M-value. Thus, the lower the insulin sensitivity, the lower was HDL cholesterol and the higher were plasma PLTP activity and prebeta-HDL. Cellular cholesterol efflux tended to be correlated with HDL cholesterol (r=0.55, p < 0.10) as well as with plasma PLTP activity (r=0.56, p<0.10) and was positively correlated with prebeta-HDL in incubated plasma (r=0.74, p<0.01). No positive correlation between the M-value and cellular cholesterol efflux was found (r= -0.34, ns). These preliminary results support the hypothesis that, despite a lower HDL cholesterol, the ability of plasma from insulin-resistant subjects to promote cellular cholesterol efflux is not impaired, as a consequence of a higher plasma PLTP activity and enhanced prebeta-HDL formation.Plasma phospholipid transfer protein (PLTP) enhances the generation of prebhigh density lipoproteins (HDL) that may act as initial acceptors of cellular cholesterol, and are likely to play an important role in the antiatherogenic process of reverse cholesterol transport. We examined the interrelationships between insulin resistance, the ability of plasma to stimulate cellular cholesterol ef ̄ ux, HDL cholesterol, plasma PLTP activity and preb-HDL in 12 nondiabetic, non-smoking, normotriglyceridaemic men. Cholesterol ef ̄ ux from Fu5AH cells to plasma, plasma lipoproteins, PLTP activity and preb-HDL formation as measured in incubated plasma were determined after a 12-h fast. Insulin sensitivity was assessed by a euglycaemic, hyperinsulinaemic clamp (M-value). HDL cholesterol was positively correlated with the M-value (r= 0.65, po 0.05), whereas plasma PLTP activity (r= ± 0.59, po 0.05) and preb-HDL in incubated plasma (r= ± 0.66, po 0.05) were negatively correlated with the M-value. Thus, the lower the insulin sensitivity, the lower was HDL cholesterol and the higher were plasma PLTP activity and preb-HDL. Cellular cholesterol ef ̄ ux tended to be correlated with HDL cholesterol (r= 0.55, po 0.10) as well as with plasma PLTP activity (r= 0.56, po 0.10) and was positively correlated with preb-HDL in incubated plasma (r= 0.74, po 0.01). No positive correlation between the M-value and cellular cholesterol ef ̄ ux was found (r= ± 0.34, ns). These preliminary results support the hypothesis that, despite a lower HDL cholesterol, the ability of plasma from insulin-resistant subjects to promote cellular cholesterol ef ̄ ux is not impaired, as a consequence of a higher plasma PLTP activity and enhanced preb-HDL formation.