A novel method for measuring intestinal and hepatic triacylglycerol kinetics

Abstract

Hovorka R, Umpleby AM. A novel method for measuring intestinal and hepatic triacylglycerol kinetics. Am J Physiol Endocrinol Metab 305: E1041-E1047, 2013. First published April 16, 2013; doi:10.1152/ajpendo.00105.2013.-This study aimed to 1) develop a method that completely separated hepatic (VLDL1, VLDL2) and intestinal [chylomicron (CM)] lipoproteins and 2) use the method to measure triacylglycerol (TAG) kinetics in these lipoproteins in the fed and fasting state in healthy subjects, using intravenous [ 2 H5]glycerol as the tracer. An immunoaffinity method that completely separated hepatic and intestinal particles using sequential binding to three antibodies to apolipoprotein B-100 (apoB-100) was established and validated. Six healthy volunteers were studied in a fasted and continuous feeding study (study 1). Five additional healthy volunteers were studied in a continuous feeding study that included an oral [ 13 C3]glycerol tripalmitin tracer (study 2). In both studies, an intravenous bolus of [ 2 H5]glycerol was administered to label TAG in hepatic and intestinal lipoproteins. In both feeding studies there was sufficient incorporation of the [ 2 H5]glycerol tracer into the exogenous lipoproteins to enable isotopic enrichment to be measured. In study 2, the oral tracer enrichment in VLDL1 was Ͻ5% of CM enrichment 150 min after tracer administration, demonstrating negligible contamination of VLDL1 with apoB-48. Western blotting showed no detectable apoB-100 in CMs. VLDL1 and VLDL2 TAG fractional catabolic rate (FCR) did not differ between feeding and fasting (study 1). There was no difference between CM and VLDL1 TAG FCR in both fed studies. In fed study 2, 47% of the total TAG production rate (CM ϩ VLDL1) was from CM. This methodology may be a useful tool for understanding the abnormalities in postprandial TAG kinetics in metabolic syndrome and type 2 diabetes.very-low-density lipoprotein; chylomicron; stable isotopes; immunoaffinity; postprandial THERE IS CONSIDERABLE EVIDENCE that increased fasting plasma triacylglycerol (TAG) levels are an independent risk factor for cardiovascular disease (4) and play a role in the development of atherosclerosis (17). However, postprandial hypertriglyceridemia may be a more important risk factor for atherosclerosis than fasting hypertriglyceridemia (37), especially on a Western diet, since TAG levels are elevated for most of the day due to the frequent consumption of food (35).Hypertriglyceridemia is due to the presence of excess TAGrich lipoproteins (TRL), which consist of very low-density lipoproteins (VLDL) synthesized by the liver and chylomicrons synthesized in the small intestine in response to an intake of dietary fat. Measurement of VLDL flux using isotopic tracers has provided an invaluable insight into the abnormalities of VLDL metabolism in various diseases (3, 10). During fasting, VLDL metabolism can be measured with a fatty acid (18) or glycerol tracer to measure TAG kinetics (24) and in fasting and feeding by labeling the main protein component apolipoprotei...

Prandial Hypertriglyceridemia in Metabolic Syndrome Is Due to an Overproduction of Both Chylomicron and VLDL Triacylglycerol

“…A number of studies have shown that VLDL apoB100 and VLDL TAG production rate is increased by insulin resistance ( 12 , 13 ) in the fasted state, but no studies have measured VLDL TAG production rate quantitatively, in the fed state, in MetS. We have recently demonstrated that intravenously administered 2 H 5 -glycerol is incorporated into CM TAG and can be used to measure both CM and VLDL TAG kinetics in a frequent feeding study ( 14 ). In the current study, we have used this methodology to investigate whether the greater increase in postprandial TAG in MetS compared with lean healthy subjects is due to an increase in CM and/or VLDL TAG production rate or a decrease in clearance rate.…”

New insights into the pathophysiology of dyslipidemia in type 2 diabetes

“…228 Sun et al developed an immunoaffinity method that completely separated hepatic and intestinal lipoproteins. 229 Using a continuous feeding protocol and an intravenous glycerol tracer in healthy volunteers the authors showed that the FCR of VLDL 1 and VLDL 2 in the fasted state did not differ from the equivalent fraction in a fed state in healthy subjects. The study also measured VLDL and chylomicron TG production rate for the first time in a separate study during a continuous feeding protocol and showed that 47% of total TG production in the Sf > 60 fraction was from chylomicrons.…”

Role of the Enterocyte in Fructose-Induced Hypertriglyceridaemia

“…Therefore, the separation of the CM-TG component within the S f > 60 fraction (TRL) using immunoaffinity chromatography provides a greater specificity when assessing the enterocyte contribution to circulating TG levels. Sun et al (2013) used this technique to develop a novel stable isotope protocol to enable CM-TG production to be measured in humans in vivo [91]. Shojaee-Moradie et al (2013) [19] utilised the protocol to demonstrate an increased CM-TG concentration in men with metabolic syndrome, compared to lean individuals, which was due to an enhanced CM-TG production rate, with no difference in the CM-TG clearance rate.…”