Kinetic study of apo B100 containing lipoprotein metabolism using amino acids labeled with stable isotopes: methodological aspects

Use of water turnover method to measure mother's milk flow in a rat model: Application to dams receiving a low protein diet during gestation and lactation

Assessment of milk production is of utmost relevance for pediatricians and scientists interested in early life nutrition. The weight-suckle-weight (WSW) method, which consists of weighing babies before and after they suckle their mother, uses the difference in body weight as an estimate of milk intake. However, this is prone to many sources of error. In the current study, we used for the first time the water turnover method and compartmental analysis with deuterated water (D₂O) as a non-toxic tracer to quantify in vivo milk production in a rat model. We assessed the effect of a nutritional intervention presumed to affect milk production, a maternal dietary protein restriction during gestation and lactation, which results in the birth of pups with intrauterine growth restriction. The specific aim of this study was to determine milk production with the body water turnover method in rat dams receiving during gestation and lactation, either a control diet (NP) or an iso-caloric low-protein diet (LP). In NP dams, mass of dam’s total body water, output flow constant from dam to litter (K21) and median milk flow, calculated between days 11 to 14 after pup birth, were 282.1 g, 0.0122 h^-1 and 3.30 g/h for NP dams, respectively. Maternal dietary protein restriction (-59%) during perinatal period led to a 34% reduction in milk flow (NP versus LP). With the WSW method, milk flow varied from 1.96 g/h to 2.37 g/h between days 11 to 14 for NP dams. The main advantage of the D₂0 method compared to the WSW method stems from its higher precision, as attested by the narrowest range of measured values of milk flow ([2.90; 3.75] and [0.98; 6.85] g/h, respectively) for NP group. This method could be suitable for testing the effectiveness of candidate galactologue molecules presumed to enhance milk production in the lactating rat model.

The hyperenergetic-fed obese dog, a model of disturbance of apolipoprotein B-100 metabolism associated with insulin resistance: kinetic study using stable isotopes

The hyperenergetic-fed beagle dog model of obesity-associated insulin resistance has previously demonstrated lipoprotein abnormalities similar to those of obese insulin-resistant humans. The aim of this study was to check, in the insulin-resistant dog, the mechanism leading to abnormalities in the mass of apolipoprotein B-100 (apo B-100) containing lipoproteins. Six healthy male beagle dogs were overfed with a high-fat diet for 28 +/- 2.5 weeks. Obesity was associated with insulin resistance as assessed by the euglycemic hyperinsulinemic clamp technique. The kinetics of very low-density lipoprotein (VLDL) and low-density lipoprotein (LDL) apo B-100 were recorded in dogs, at healthy and insulin-resistant states, using a primed constant infusion of [5,5,5-D(3)]leucine. Isotopic enrichment was measured by gas chromatography-mass spectrometry (GC-MS). A multicompartmental model was used for the analysis of tracer kinetics data. Apolipoprotein B-100 concentration was higher in VLDL (2.8-fold, P < .05) but lower in LDL (2-fold, P < .05) in the insulin-resistant compared to the healthy state. Kinetic analysis showed a higher VLDL apo B-100 production (1.7-fold, P < .05). The fractional catabolic rate of VLDL did not change significantly, but the lipolysis was decreased significantly (3-fold, P < .05). The lower LDL apo B-100 level in insulin-resistant dogs was explained by a higher LDL fractional catabolic rate (2.5-fold, P < .05). The mechanisms leading to hypertriglyceridemia (higher production rate and lower lipolysis of VLDL) in insulin-resistant dogs were similar to those described in the insulin-resistant humans.

Lipoprotein transport in the metabolic syndrome: methodological aspects of stable isotope kinetic studies

The metabolic syndrome encapsulates visceral obesity, insulin resistance, diabetes, hypertension and dyslipidaemia. Dyslipidaemia is a cardinal feature of the metabolic syndrome that accelerates the risk of cardiovascular disease. It is usually characterized by high plasma concentrations of triacylglycerol (triglyceride)-rich and apoB (apolipoprotein B)-containing lipoproteins, with depressed concentrations of HDL (high-density lipoprotein). However, lipoprotein metabolism is complex and abnormal plasma concentrations can result from alterations in the rates of production and/or catabolism of these lipoprotein particles. Our in vivo understanding of kinetic defects in lipoprotein metabolism in the metabolic syndrome has been achieved chiefly by ongoing developments in the use of stable isotope tracers and mathematical modelling. This review deals with the methodological aspects of stable isotope kinetic studies. The design of in vivo turnover studies requires considerations related to stable isotope tracer administration, duration of sampling protocol and interpretation of tracer data, all of which are critically dependent on the kinetic properties of the lipoproteins under investigation. Such models provide novel insight that further understanding of metabolic disorders and effects of treatments. Future investigations of the pathophysiology and therapy of the dyslipoproteinaemia of the metabolic syndrome will require the development of novel kinetic methodologies. Specifically, new stable isotope techniques are required for investigating in vivo the turnover of the HDL subpopulation of particles, as well as the cellular efflux of cholesterol into the extracellular space and its subsequent transport in plasma and metabolic fate in the liver.

Metabolism of cholesterol ester of apolipoprotein B100-containing lipoproteins in dogs: evidence for disregarding cholesterol ester transfer

BACKGROUND

It has been shown that dogs exhibit no cholesterol ester transfer protein (CETP) activity in vitro, in contrast to humans. The aim of our study was to determine modalities of in vivo plasma cholesterol ester turnover in this species, using a kinetic approach with stable isotopes.

MATERIALS AND METHODS

Kinetics of very low-density lipoprotein (VLDL) and low-density lipoprotein (LDL) were studied in seven adult male Beagle dogs using a dual isotope approach through endogenous labelling of both their cholesterol moiety and their protein moiety. A primed constant infusion of both [1,2(13)C]acetate and [5,5,5-2H3]leucine enabled us to obtain measurable deuterium enrichments by gas chromatography-mass spectrometry for plasma leucine and apoB100, as well as measurable 13C enrichment by gas chromatography-combustion-isotopic ratio mass spectrometry for unesterified cholesterol and cholesterol ester in the VLDL and LDL. Two identical multicompartmental models (SAAM II) were used together for the analysis of tracer kinetics' data of proteins and cholesterol.

RESULTS

Characterization of the apoB100-containing lipoprotein cholesterol ester model allowed determination of kinetic parameters of VLDL and LDL cholesterol ester metabolism. We succeeded in modelling VLDL and LDL cholesterol ester metabolism and apoB100 metabolism simultaneously. Fractional catabolic rate (FCR) of apoB100 and CE had the same values. Introducing cholesterol ester transfer between lipoproteins in the model did not significantly improve the fit. Total VLDL FCR was 2.97 +/- 01.47 h(-1). Approximately one-quarter corresponded to the direct removal of VLDL (0.81 +/- 00.34 h(-1)) and the remaining three-quarters corresponded to the fraction of VLDL converted to LDL, which represented a conversion of VLDL into LDL of 2.16 +/- 01.16 h(-1). Low-density lipoproteins were produced exclusively from VLDL conversion and were then removed (0.031 +/- 0.004 h(-1)) from plasma.

CONCLUSION

These kinetic data showed that VLDL cholesterol ester and LDL cholesterol ester metabolism followed VLDL and LDL apoB100 metabolism, and that consequently there is no in vivo transfer of cholesterol ester in dogs.

Apolipoprotein B100 metabolism in autosomal-dominant hypercholesterolemia related to mutations in PCSK9

OBJECTIVE

We have reported further heterogeneity in familial autosomal-dominant hypercholesterolemia (FH) related to mutation in proprotein convertase subtilisin/kexin type 9 (PCSK9) gene previously named neural apoptosis regulated convertase 1 (Narc-1). Our aim was to define the metabolic bases of this new form of hypercholesterolemia.

METHODS AND RESULTS

In vivo kinetics of apolipoprotein B100-containing lipoproteins using a 14-hour primed constant infusion of [2H3] leucine was conducted in 2 subjects carrying the mutation S127R in PCSK9, controls subjects, and FH subjects with known mutations on the low-density lipoprotein (LDL) receptor gene (LDL-R). Apo B100 production, catabolism, and transfer rates were estimated from very LDL (VLDL), intermediate-density lipoprotein (IDL), and LDL tracer enrichments by compartmental analysis. PCSK9 mutation dramatically increased the production rate of apolipoprotein B100 (3-fold) compared with controls or LDL-R mutated subjects, related to direct overproduction of VLDL (3-fold), IDL (3-fold), and LDL (5-fold). The 2 subjects also showed a decrease in VLDL and IDL conversion (10% to 30% of the controls). LDL fractional catabolic rate was slightly decreased (by 30%) compared with controls but still higher than LDL-R-mutated subjects.

CONCLUSIONS

These results showed that the effect of the S127R mutation of PCSK9 on plasma cholesterol homeostasis is mainly related to an overproduction of apolipoprotein B100.