Robin P. F. Dullaart

Effect of adiposity on plasma lipid transfer protein activities: a possible link between insulin resistance and high density lipoprotein metabolism

Abstract. The mechanisms responsible for the decreased high density lipoprotein (HDL) cholesterol levels associated with obesity and insulin resistance are not well understood. Lecithin: cholesterol acyltransferase (LCAT) and cholesterol ester transfer protein (CETP) are key factors in the esterification of cholesterol in HDL and the subsequent transfer of cholesteryl ester towards apolipoprotein B‐containing lipoproteins. Phospholipid transfer protein (PLTP) may be involved in the regulation of HDL particle size. We therefore measured the activities of LCAT, CETP and PLTP using exogenous substrate assays, as well as lipids, lipoproteins, insulin and C‐peptide in fasting plasma from eight healthy obese men (body mass index >27 kg m‐2) and 24 non‐obese subjects. The obese men had lower levels of HDL cholesterol (P<0·05) and higher levels of plasma triglycerides (P<0·05), insulin (P<0·05) and C‐peptide (P<0·01), as compared to the quartile of subjects with the lowest body mass index (BMI <22·4 kg m‐2). CETP and PLTP activities were elevated in the obese men by 35% (P<0·01) and by 15% (P<0·05), respectively. LCAT activity was comparable among the quartiles. Linear regression analysis showed that CETP activity was positively correlated with body mass index (P<0·02), fasting blood glucose (P7lt;0·05) and plasma C‐peptide (P<0·05). PLTP activity was positively related to body mass index (P<0·01), waist to hip circumference ratio (P<0·001), as well as to fasting blood glucose (P<0·05) and plasma C‐peptide (P<0·05)

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.

Plasma phospholipid transfer protein activity is related to insulin resistance : impaired acute lowering by insulin in obese Type II diabetic patients

Summary Cholesteryl ester transfer protein (CETP) and phospholipid transfer protein (PLTP) have important functions in high density lipoprotein (HDL) metabolism. We determined the association of plasma CETP and PLTP activities (measured with exogenous substrate assays) with insulin resistance, plasma triglycerides (TG) and non-esterified fatty acids (NEFA), and assessed the lipid transfer protein response to insulin during a 6–7 h hyperinsulinaemic euglycaemic clamp in non-obese and obese healthy subjects and patients with Type II (non-insulin-dependent) diabetes mellitus (n = 8 per group). Plasma PLTP activity was higher in obese healthy subjects and obese Type II diabetic patients compared with non-obese healthy subjects (p < 0.05 to 0.01) and was correlated with insulin resistance, plasma TG and NEFA (p = 0.02 to < 0.01). In non-obese healthy subjects, insulin decreased plasma TG and increased the HDL cholesteryl ester (CE)/TG ratio (p < 0.01 compared with saline infusion). Plasma PLTP activity fell by 14 % at the end of the clamp (p < 0.01 compared with saline) but CETP activity did not change. The decreases in plasma NEFA, TG and PLTP activity and the rise in HDL CE/TG were smaller in obese Type II diabetic patients than in non-obese healthy subjects (p < 0.01 for all). Baseline HDL CE/TG was negatively correlated with plasma TG (p < 0.001, n = 32) and PLTP activity (p < 0.01) but not with CETP activity. Likewise, the rise in HDL CE/TG during the clamp was related to the fall in plasma TG (p < 0.001) and in PLTP activity (p < 0.02). It is concluded that plasma PLTP, but not CETP, is regulated by insulin in an acute setting. High plasma PLTP activity is associated with insulin resistance in conjunction with altered NEFA and triglyceride metabolism. High plasma TG and PLTP activity have coordinate effects on HDL metabolism. [Diabetologia (1998) 41: 929–934]

Separating the Mechanism-Based and Off-Target Actions of Cholesteryl Ester Transfer Protein Inhibitors With CETP Gene Polymorphisms

Background— Cholesteryl ester transfer protein (CETP) inhibitors raise high-density lipoprotein (HDL) cholesterol, but torcetrapib, the first-in-class inhibitor tested in a large outcome trial, caused an unexpected blood pressure elevation and increased cardiovascular events. Whether the hypertensive effect resulted from CETP inhibition or an off-target action of torcetrapib has been debated. We hypothesized that common single-nucleotide polymorphisms in the CETP gene could help distinguish mechanism-based from off-target actions of CETP inhibitors to inform on the validity of CETP as a therapeutic target. Methods and Results— We compared the effect of CETP single-nucleotide polymorphisms and torcetrapib treatment on lipid fractions, blood pressure, and electrolytes in up to 67 687 individuals from genetic studies and 17 911 from randomized trials. CETP single-nucleotide polymorphisms and torcetrapib treatment reduced CETP activity and had a directionally concordant effect on 8 lipid and lipoprotein traits (total, low-density lipoprotein, and HDL cholesterol; HDL2; HDL3; apolipoproteins A-I and B; and triglycerides), with the genetic effect on HDL cholesterol (0.13 mmol/L, 95% confidence interval [CI] 0.11 to 0.14 mmol/L) being consistent with that expected of a 10-mg dose of torcetrapib (0.13 mmol/L, 95% CI 0.10 to 0.15). In trials, 60 mg of torcetrapib elevated systolic and diastolic blood pressure by 4.47 mm Hg (95% CI 4.10 to 4.84 mm Hg) and 2.08 mm Hg (95% CI 1.84 to 2.31 mm Hg), respectively. However, the effect of CETP single-nucleotide polymorphisms on systolic blood pressure (0.16 mm Hg, 95% CI −0.28 to 0.60 mm Hg) and diastolic blood pressure (−0.04 mm Hg, 95% CI −0.36 to 0.28 mm Hg) was null and significantly different from that expected of 10 mg of torcetrapib. Conclusions— Discordance in the effects of CETP single-nucleotide polymorphisms and torcetrapib treatment on blood pressure despite the concordant effects on lipids indicates the hypertensive action of torcetrapib is unlikely to be due to CETP inhibition or shared by chemically dissimilar CETP inhibitors. Genetic studies could find a place in drug-development programs as a new source of randomized evidence for drug-target validation in humans.

Elevated plasma phospholipid transfer protein activity is a determinant of carotid intima-media thickness in type 2 diabetes mellitus

Aim/hypothesisThe plasma activity of phospholipid transfer protein (PLTP), which has putative pro- and anti-atherogenic roles in lipoprotein metabolism, is increased in type 2 diabetes mellitus. We analysed the relationship between carotid artery intima–media thickness (IMT), an established marker of atherosclerosis, and PLTP activity in diabetic patients and control subjects.MethodsThe IMT (mean of three segments in both carotid arteries by ultrasonography), clinical variables, plasma PLTP activity (phospholipid vesicle–HDL system), lipoproteins, C-reactive protein and insulin were measured in 87 non-smoking men and women, who had type 2 diabetes mellitus, no cardiovascular disease, and were not on insulin or lipid-lowering medication, and in 83 age-matched control subjects.ResultsIn diabetic patients, carotid IMT (p=0.02), pulse pressure (p=0.003), plasma PLTP activity (p<0.001), triglycerides (p=0.01), C-reactive protein (p<0.01) and insulin (p<0.001) were higher, whereas HDL cholesterol was lower (p<0.001) than in control subjects. Multiple stepwise linear regression analysis demonstrated that in type 2 diabetic patients IMT was independently associated with age (p<0.001), sex (p=0.001), pulse pressure (p=0.003), plasma PLTP activity (p=0.03) and HDL cholesterol (p=0.03), but not with very low density lipoprotein+LDL cholesterol, triglycerides, C-reactive protein and insulin (all p>0.20). The relationship between plasma PLTP activity and IMT was not significant in control subjects.Conclusions/interpretationPlasma PLTP activity is a positive determinant of IMT in type 2 diabetes mellitus, suggesting that high PLTP activity is involved in accelerated atherosclerosis in this disease.

111In-Octreotide Is Superior to 123I-Metaiodobenzylguanidine for Scintigraphic Detection of Head and Neck Paragangliomas

In this study, we evaluated the diagnostic yield of somatostatin receptor scintigraphy (SRS), I-metaiodobenzylguanidine (MIBG) scintigraphy, and morphologic imaging (CT or MRI) in patients with head and neck paragangliomas. Methods: In a university hospital setting, patients considered to have head and neck paraganglioma were referred to the outpatient endocrinology department and underwent CT or MRI, SRS, and MIBG imaging. For validation, we used a composite reference standard consisting of clinical and histologic data and CT or MRI, with which SRS and MIBG imaging were compared. Urinary metanephrine and normetanephrine measurements were also obtained. Results: Twenty-nine consecutively referred patients (17 women and 12 men) were included and were found to have paraganglioma. Both morphologic and SRS were positive in 27 patients (sensitivity, 93%, and 95% confidence interval [CI], 77%–98%, compared with the composite reference standard), whereas MIBG was positive in only 13 patients (44%; 95% CI, 23%–61%) (P < 0.001, compared with SRS). On a lesion-based analysis, morphologic imaging detected 31 lesions (sensitivity, 82%; 95% CI, 65%–92%), SRS detected 34 (89%; 95% CI, 75%–97%), and MIBG detected 15 (42%; 95% CI, 26%–59%). SRS was superior to MIBG (P = 0.001). With SRS, a previously unknown carcinoid tumor was detected in 1 patient, and a carcinoid was suspected in another patient. MIBG detected an additional adrenal pheochromocytoma in 1 patient. Urinary metanephrine or normetanephrine excretion was elevated in 6 patients. The number of lesions on SRS and MIBG per patient correlated with the levels of abnormal metanephrine or normetanephrine excretion (P = 0.005 and P = 0.02, respectively). Conclusion: SRS was superior to MIBG in patients with highly suspected head and neck paraganglioma.

Better yield of (18)fluorodeoxyglucose-positron emission tomography in patients with metastatic differentiated thyroid carcinoma during thyrotropin stimulation

To determine whether (18)fluorodeoxyglucose-positron emission tomography (FDG-PET) for the detection of recurrences or metastases of differentiated thyroid carcinoma should be performed during thyrotropin (TSH) suppression or TSH stimulation, eight patients were studied sequentially. After the second FDG-PET scan, a therapeutic (131)I dose was administered with posttherapy scans obtained 10 days later. Both FDG-PET scans were compared with each other and with the (131)I posttherapy whole body scans by two independent observers. Findings were verified using other imaging modalities or biopsies. Median TSH was 0.04 mU/L during TSH suppression and 64 mU/L during TSH stimulation. The FDG-PET scans during TSH suppression showed abnormalities in four patients and the FDG-PET scan during TSH stimulation in five patients. One patient was only positive during TSH stimulation. In two other patients the FDG-PET scan during TSH stimulation clearly identified more lesions, and in all positive patients lesion contrast was better during TSH stimulation. In two patients FDG-PET findings during TSH stimulation led to a change in clinical management. Thus, the performance of FDG-PET during TSH stimulation was either superior or equal to FDG-PET during TSH suppression, but never inferior. To detect metastatic or recurrent differentiated thyroid carcinoma FDG-PET should be performed during hypothyroidism, leading to TSH stimulation.

Increased cholesterylester transfer activity in complicated Type 1 (insulin-dependent) diabetes mellitus — its relationship with serum lipids

SummaryIn Type 1 (insulin-dependent) diabetes mellitus, macrovascular complications and the increased risk for cardiovascular disease in patients with microvascular complications may be related to alterations in plasma cholesterylester transfer. The activity of cholesterylester transfer protein, which mediates cholesterylester transfer between lipoproteins and lipoprotein lipid levels, was assessed in 7 normolipidaemic control subjects, 7 Type 1 diabetic control subjects without complications, 11 Type 1 diabetic patients with microvascular complications (retinopathy, incipient nephropathy) and in 7 Type 1 diabetic patients with macrovascular atherosclerotic lesions.The cholesterylester transfer activity was 30% higher in the diabetic groups with macrovascular and microvascular lesions than in the 2 control groups. Very low + low density lipoprotein cholesterol was higher in the 3 diabetic groups than in the non-diabetic control group. High density lipoprotein cholesterol was not different. The cholesterylester transfer activity was correlated positively with HbA1, urinary albumin excretion rate, serum cholesterol, very low + low density lipoprotein cholesterol and apolipoprotein B. The high density lipoprotein over very low + low density lipoprotein cholesterylester molar ratio was lower in the diabetic groups with micro- and macrovascular complications. A role for cholesterylester transfer activity in the lipoprotein abnormalities found in complicated Type 1 diabetes is suggested. A high cholesterylester transfer activity might be indicative of mechanisms which promote atherogenesis.

Genetic Variation at the Phospholipid Transfer Protein Locus Affects Its Activity and High-Density Lipoprotein Size and Is a Novel Marker of Cardiovascular Disease Susceptibility

Background— In contrast to clear associations between variants in genes participating in low-density lipoprotein metabolism and cardiovascular disease risk, such associations for high-density lipoprotein (HDL)–related genes are not well supported by recent large studies. We aimed to determine whether genetic variants at the locus encoding phospholipid transfer protein (PLTP), a protein involved in HDL remodeling, underlie altered PLTP activity, HDL particle concentration and size, and cardiovascular disease risk. Methods and Results— We assessed associations between 6 PLTP tagging single nucleotide polymorphisms and PLTP activity in 2 studies (combined n=384) and identified 2 variants that show reproducible associations with altered plasma PLTP activity. A gene score based on these variants is associated with lower hepatic PLTP transcription (P=3.2×10−18) in a third study (n=957) and with an increased number of HDL particles of smaller size (P=3.4×10−17) in a fourth study (n=3375). In a combination of 5 cardiovascular disease case-control studies (n=4658 cases and 11 459 controls), a higher gene score was associated with a lower cardiovascular disease risk (per-allele odds ratio, 0.94; 95% confidence interval, 0.90 to 0.98; P=1.2×10−3; odds ratio for highest versus lowest gene score, 0.69; 95% confidence interval, 0.55 to 0.86; P=1.0×10−3). Conclusions— A gene score based on 2 PLTP single nucleotide polymorphisms is associated with lower PLTP transcription and activity, an increased number of HDL particles, smaller HDL size, and decreased risk of cardiovascular disease. These findings indicate that PLTP is a proatherogenic entity and suggest that modulation of specific elements of HDL metabolism may offer cardiovascular benefit.

Effect of the composition of very low and low density lipoproteins on the rate of cholesterylester transfer from high density lipoproteins in man, studied in vitro

Abstract. The process of cholesterylester (CE) transfer is supposed to be a regulatory factor in the distribution of CE between lipoproteins. In addition to the activity of CE transfer protein, this process may be affected by acceptor lipoprotein characteristics. In this study the effect of the composition of different very low density lipoproteins (VLDL) and low density lipoproteins (LDL) on the ability to accept CE from HDL in vitro was investigated. [3H]‐CE high density lipoprotein (HDL) (100 nmol CE) from one batch was incubated with VLDL (75 nmol CE), isolated from fifteen subjects for 4 h and separately with LDL (250 nmol CE), isolated from thirteen subjects for 16 h, both in the presece of lipoprotein‐free plasma providing a source of cholesterylester transfer protein. The CE transfer rate to VLDL (range 1·34–2·84% [3H]‐CE transferred h‐1) was correlated to the triacylglycerol (TG):CE molar ratio (r: 0·63, P < 0·05), to the phospholipid (PL):CE molar ratio (r: 0·75, P < 0·01), to the protein (Pr):CE ratio (expressed in g nmol‐1) (r: 0·72, P < 0·01) and to the free cholesterol (FC):CE molar ratio (r: 0·69, P < 0·01), but not to the FC:PL molar ratio (r: ‐0·08, NS). The CE transfer rate to LDL (range 1·18–3·59 nmol CE h‐1) was correlated to the Pr:CE ratio (r: 0·72, P < 0·01) and inversely to the FRC:PL molar ratio (r: ‐0·88, P < 0·001), but not to the TG:CE molar ratio (r: 0·40, NS), nor to the FC:CE molar ratio (r: ‐0·37, NS). The correlation between CE transfer rate and PL:CE molar ratio almost reached significance (r: 0·54, 0·05 < P < 0·10). It is concluded that the ability of VLDL to accept CE is enhanced when it contains more TG and more surface constituent, relative to CE, while in LDL surface characteristics are involved in the process of CE transfer. An influence of VLDL and LDL composition and CE transport in human plasma is suggested.