Sitosterol and cholesterol in chylomicrons of type 2 diabetic and non-diabetic subjects: the relationship with ATP binding cassette proteins G5 and G8 and Niemann-Pick C1-like 1 mRNA.

To the Editor: The dyslipidaemia of diabetes is in part related to a disturbance in genes regulating cholesterol absorption and excretion [1]. Sitosterolaemia is associated with increased atherosclerosis [2]. The higher the absorption of cholesterol, the more plant sterol is found in the atherosclerotic plaque [3]. The intestinal cholesterol transporter, Niemann–Pick C1-like 1 (NPC1L1), regulates cholesterol absorption and NPC1L1 mRNA is increased in type 2 diabetic patients [4]. ATP binding cassette proteins (ABC) G5 and G8 regulate cholesterol homeostasis by reexcreting some of the cholesterol and most of the plant sterols from the enterocyte back into the intestinal lumen [2]. Patients with diabetes have decreased ABCG5 and G8 mRNA expression [1]. In diabetes, chylomicron cholesterol is increased. Absorbed plant sterols that enter the circulation are rapidly re-excreted directly into the bile rather than being converted to bile salts. Thus abnormalities in plant sterol absorption may not be detected by examining serum in the fasting state. Foods enriched with plant sterol are being promoted to prevent atherosclerosis. However, the amount needed is more than five times the normal dietary plant sterol intake (0.3–0.4 g/day) and there are no endpoint studies to confirm benefit. Our aim was to examine the effect of a sitosterol-enriched diet on chylomicron composition and to examine the relationship between chylomicron sitosterol and intestinal ABCG5 and G8 and NPC1L1 gene expression. Six type 2 diabetic patients (two male, four female; age 63.8±3.4 years, BMI 29.5±5.6 kg/m, HbA1c 6.6±1.1% [means±SD]) and six control subjects of similar age (three male, three female, age 63.3±4.8, BMI 26.8±2.0 kg/m), none of whom were on statin treatment, were studied. All subjects gave informed consent and the study was approved by the hospital ethics committee. Two of the diabetic patients were treated with diet alone, two with metformin, one with metformin and rosiglitazone, and one with gliclazide. All subjects had previously had duodenal biopsies when undergoing routine gastroscopy for non-specific abdominal symptoms. Subjects with cancer, inflammatory bowel disease and coeliac disease or previous gastric operations were excluded. All subjects were given 1 week of a sitosterol-enriched diet (2.5 g/day), in which sitosterolenriched margarine and milk or yogurt were substituted for their normal dairy products. On the day of the study all subjects had fasting blood taken followed by a 4.60 MJ (1,100 kcal) high-fat test meal containing 2.0 g sitosterol. Blood was sampled at 4 and 6 h following the meal. Chylomicrons were isolated from plasma by ultracentrifugation. Apolipoprotein (Apo) B48 and Apo B100 were determined by gradient gel electrophoresis and quantified by densitometry [2]. Chylomicron cholesterol and sitosterol were determined by gas chromatography and identified using mass spectrometry. NPC1L1, and ABCG5 and ABCG8 mRNA were determined by TaqMan assay and quantified (arbitrary units, AU) by reference to GAPDH [2]. Fasting serum cholesterol for diabetic and control subjects was 5.0±0.9 and 4.7±1.1 mmol/l, respectively, and mean plasma triacylglycerol was 1.6±0.7 vs 2.1±1.3 mmol/l, Diabetologia (2007) 50:217–219 DOI 10.1007/s00125-006-0504-0